libstdc++: Ensure active union member is correctly set
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Commit Message
On Sat, Sep 23, 2023 at 07:40:48AM +0100, Jonathan Wakely wrote:
> On Sat, 23 Sept 2023, 01:39 Nathaniel Shead via Libstdc++, <
> libstdc++@gcc.gnu.org> wrote:
>
> > Now that bootstrap has finished, I have gotten regressions in the
> > following libstdc++ tests:
> >
> > Running libstdc++:libstdc++-dg/conformance.exp ...
> > FAIL: 20_util/bitset/access/constexpr.cc -std=gnu++23 (test for excess
> > errors)
> > FAIL: 20_util/bitset/access/constexpr.cc -std=gnu++26 (test for excess
> > errors)
> > FAIL: 20_util/variant/constexpr.cc -std=gnu++20 (test for excess errors)
> > FAIL: 20_util/variant/constexpr.cc -std=gnu++26 (test for excess errors)
> > FAIL: 21_strings/basic_string/cons/char/constexpr.cc -std=gnu++20 (test
> > for excess errors)
> > FAIL: 21_strings/basic_string/cons/char/constexpr.cc -std=gnu++26 (test
> > for excess errors)
> > FAIL: 21_strings/basic_string/cons/wchar_t/constexpr.cc -std=gnu++20 (test
> > for excess errors)
> > FAIL: 21_strings/basic_string/cons/wchar_t/constexpr.cc -std=gnu++26 (test
> > for excess errors)
> > FAIL: 21_strings/basic_string/modifiers/swap/constexpr-wchar_t.cc
> > -std=gnu++20 (test for excess errors)
> > FAIL: 21_strings/basic_string/modifiers/swap/constexpr-wchar_t.cc
> > -std=gnu++26 (test for excess errors)
> > FAIL: 21_strings/basic_string/modifiers/swap/constexpr.cc -std=gnu++20
> > (test for excess errors)
> > FAIL: 21_strings/basic_string/modifiers/swap/constexpr.cc -std=gnu++26
> > (test for excess errors)
> > FAIL: std/ranges/adaptors/join_with/1.cc -std=gnu++23 (test for excess
> > errors)
> > UNRESOLVED: std/ranges/adaptors/join_with/1.cc -std=gnu++23 compilation
> > failed to produce executable
> > FAIL: std/ranges/adaptors/join_with/1.cc -std=gnu++26 (test for excess
> > errors)
> > UNRESOLVED: std/ranges/adaptors/join_with/1.cc -std=gnu++26 compilation
> > failed to produce executable
> >
> > On investigation though it looks like the issue might be with libstdc++
> > rather than the patch itself; running the failing tests using clang with
> > libstdc++ also produces similar errors, and my reading of the code
> > suggests that this is correct.
> >
> > What's the way forward here? Should I look at creating a patch to fix
> > the libstdc++ issues before resubmitting this patch for the C++
> > frontend? Or should I submit a version of this patch without the
> > `std::construct_at` changes and wait till libstdc++ gets fixed for that?
> >
>
> I think we should fix libstdc++. There are probably only a few places that
> need a fix, which cause all those failures.
>
> I can help with those fixes. I'll look into it after the weekend.
>
Thanks. I did end up getting a chance to look at it earlier today, and
with the following patch I had no regressions when applying the frontend
changes. Bootstrapped and regtested on x86_64-pc-linux-gnu.
-- >8 --
This patch ensures that the union members for std::string and
std::variant are always properly set when a change occurs.
libstdc++-v3/ChangeLog:
* include/bits/basic_string.h: (basic_string(basic_string&&)):
Activate _M_local_buf when needed.
(basic_string(basic_string&&, const _Alloc&)): Likewise.
* include/bits/basic_string.tcc: (basic_string::swap): Likewise.
* include/std/variant: (__detail::__variant::__construct_n): New.
(__detail::_variant::__emplace): Use __construct_n.
Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
---
libstdc++-v3/include/bits/basic_string.h | 7 +++--
libstdc++-v3/include/bits/basic_string.tcc | 8 +++---
libstdc++-v3/include/std/variant | 32 ++++++++++++++++++++--
3 files changed, 38 insertions(+), 9 deletions(-)
Comments
On Sat, 23 Sept 2023, 08:30 Nathaniel Shead, <nathanieloshead@gmail.com>
wrote:
> On Sat, Sep 23, 2023 at 07:40:48AM +0100, Jonathan Wakely wrote:
> > On Sat, 23 Sept 2023, 01:39 Nathaniel Shead via Libstdc++, <
> > libstdc++@gcc.gnu.org> wrote:
> >
> > > Now that bootstrap has finished, I have gotten regressions in the
> > > following libstdc++ tests:
> > >
> > > Running libstdc++:libstdc++-dg/conformance.exp ...
> > > FAIL: 20_util/bitset/access/constexpr.cc -std=gnu++23 (test for excess
> > > errors)
> > > FAIL: 20_util/bitset/access/constexpr.cc -std=gnu++26 (test for excess
> > > errors)
> > > FAIL: 20_util/variant/constexpr.cc -std=gnu++20 (test for excess
> errors)
> > > FAIL: 20_util/variant/constexpr.cc -std=gnu++26 (test for excess
> errors)
> > > FAIL: 21_strings/basic_string/cons/char/constexpr.cc -std=gnu++20 (test
> > > for excess errors)
> > > FAIL: 21_strings/basic_string/cons/char/constexpr.cc -std=gnu++26 (test
> > > for excess errors)
> > > FAIL: 21_strings/basic_string/cons/wchar_t/constexpr.cc -std=gnu++20
> (test
> > > for excess errors)
> > > FAIL: 21_strings/basic_string/cons/wchar_t/constexpr.cc -std=gnu++26
> (test
> > > for excess errors)
> > > FAIL: 21_strings/basic_string/modifiers/swap/constexpr-wchar_t.cc
> > > -std=gnu++20 (test for excess errors)
> > > FAIL: 21_strings/basic_string/modifiers/swap/constexpr-wchar_t.cc
> > > -std=gnu++26 (test for excess errors)
> > > FAIL: 21_strings/basic_string/modifiers/swap/constexpr.cc -std=gnu++20
> > > (test for excess errors)
> > > FAIL: 21_strings/basic_string/modifiers/swap/constexpr.cc -std=gnu++26
> > > (test for excess errors)
> > > FAIL: std/ranges/adaptors/join_with/1.cc -std=gnu++23 (test for excess
> > > errors)
> > > UNRESOLVED: std/ranges/adaptors/join_with/1.cc -std=gnu++23 compilation
> > > failed to produce executable
> > > FAIL: std/ranges/adaptors/join_with/1.cc -std=gnu++26 (test for excess
> > > errors)
> > > UNRESOLVED: std/ranges/adaptors/join_with/1.cc -std=gnu++26 compilation
> > > failed to produce executable
> > >
> > > On investigation though it looks like the issue might be with libstdc++
> > > rather than the patch itself; running the failing tests using clang
> with
> > > libstdc++ also produces similar errors, and my reading of the code
> > > suggests that this is correct.
> > >
> > > What's the way forward here? Should I look at creating a patch to fix
> > > the libstdc++ issues before resubmitting this patch for the C++
> > > frontend? Or should I submit a version of this patch without the
> > > `std::construct_at` changes and wait till libstdc++ gets fixed for
> that?
> > >
> >
> > I think we should fix libstdc++. There are probably only a few places
> that
> > need a fix, which cause all those failures.
> >
> > I can help with those fixes. I'll look into it after the weekend.
> >
>
> Thanks. I did end up getting a chance to look at it earlier today, and
> with the following patch I had no regressions when applying the frontend
> changes. Bootstrapped and regtested on x86_64-pc-linux-gnu.
>
Nice, thanks! This looks good at first glance, I'll review it properly ASAP.
> -- >8 --
>
> This patch ensures that the union members for std::string and
> std::variant are always properly set when a change occurs.
>
> libstdc++-v3/ChangeLog:
>
> * include/bits/basic_string.h: (basic_string(basic_string&&)):
> Activate _M_local_buf when needed.
> (basic_string(basic_string&&, const _Alloc&)): Likewise.
> * include/bits/basic_string.tcc: (basic_string::swap): Likewise.
> * include/std/variant: (__detail::__variant::__construct_n): New.
> (__detail::_variant::__emplace): Use __construct_n.
>
> Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
> ---
> libstdc++-v3/include/bits/basic_string.h | 7 +++--
> libstdc++-v3/include/bits/basic_string.tcc | 8 +++---
> libstdc++-v3/include/std/variant | 32 ++++++++++++++++++++--
> 3 files changed, 38 insertions(+), 9 deletions(-)
>
> diff --git a/libstdc++-v3/include/bits/basic_string.h
> b/libstdc++-v3/include/bits/basic_string.h
> index 09fd62afa66..7c342879827 100644
> --- a/libstdc++-v3/include/bits/basic_string.h
> +++ b/libstdc++-v3/include/bits/basic_string.h
> @@ -678,7 +678,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> {
> if (__str._M_is_local())
> {
> - traits_type::copy(_M_local_buf, __str._M_local_buf,
> + traits_type::copy(_M_use_local_data(), __str._M_local_buf,
> __str.length() + 1);
> }
> else
> @@ -691,7 +691,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> // basic_stringbuf relies on writing into unallocated capacity so
> // we mess up the contents if we put a '\0' in the string.
> _M_length(__str.length());
> - __str._M_data(__str._M_local_data());
> + __str._M_data(__str._M_use_local_data());
> __str._M_set_length(0);
> }
>
> @@ -717,6 +717,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> {
> if (__str._M_is_local())
> {
> + _M_use_local_data();
> traits_type::copy(_M_local_buf, __str._M_local_buf,
> __str.length() + 1);
> _M_length(__str.length());
> @@ -728,7 +729,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> _M_data(__str._M_data());
> _M_length(__str.length());
> _M_capacity(__str._M_allocated_capacity);
> - __str._M_data(__str._M_local_buf);
> + __str._M_data(__str._M_use_local_data());
> __str._M_set_length(0);
> }
> else
> diff --git a/libstdc++-v3/include/bits/basic_string.tcc
> b/libstdc++-v3/include/bits/basic_string.tcc
> index 104a517f794..ee6e57da555 100644
> --- a/libstdc++-v3/include/bits/basic_string.tcc
> +++ b/libstdc++-v3/include/bits/basic_string.tcc
> @@ -79,7 +79,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> }
> else if (__s.length())
> {
> - traits_type::copy(_M_local_buf, __s._M_local_buf,
> + traits_type::copy(_M_use_local_data(), __s._M_local_buf,
> __s.length() + 1);
> _M_length(__s.length());
> __s._M_set_length(0);
> @@ -87,7 +87,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> }
> else if (length())
> {
> - traits_type::copy(__s._M_local_buf, _M_local_buf,
> + traits_type::copy(__s._M_use_local_data(), _M_local_buf,
> length() + 1);
> __s._M_length(length());
> _M_set_length(0);
> @@ -97,7 +97,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> else
> {
> const size_type __tmp_capacity = __s._M_allocated_capacity;
> - traits_type::copy(__s._M_local_buf, _M_local_buf,
> + traits_type::copy(__s._M_use_local_data(), _M_local_buf,
> length() + 1);
> _M_data(__s._M_data());
> __s._M_data(__s._M_local_buf);
> @@ -108,7 +108,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> const size_type __tmp_capacity = _M_allocated_capacity;
> if (__s._M_is_local())
> {
> - traits_type::copy(_M_local_buf, __s._M_local_buf,
> + traits_type::copy(_M_use_local_data(), __s._M_local_buf,
> __s.length() + 1);
> __s._M_data(_M_data());
> _M_data(_M_local_buf);
> diff --git a/libstdc++-v3/include/std/variant
> b/libstdc++-v3/include/std/variant
> index c0e41740dcf..7f24e760bb1 100644
> --- a/libstdc++-v3/include/std/variant
> +++ b/libstdc++-v3/include/std/variant
> @@ -320,6 +320,33 @@ namespace __variant
> __get(_Variant&& __v) noexcept
> { return __variant::__get_n<_Np>(std::forward<_Variant>(__v)._M_u); }
>
> + // Gets the _Uninitialized to construct into for __u.
> + template<size_t _Np, typename _Union>
> + constexpr decltype(auto)
> + __construct_n(_Union& __u) noexcept
> + {
> + if constexpr (_Np == 0)
> + return &__u._M_first;
> + else if constexpr (_Np == 1)
> + {
> + std::_Construct(&__u._M_rest);
> + return &__u._M_rest._M_first;
> + }
> + else if constexpr (_Np == 2)
> + {
> + std::_Construct(&__u._M_rest);
> + std::_Construct(&__u._M_rest._M_rest);
> + return &__u._M_rest._M_rest._M_first;
> + }
> + else
> + {
> + std::_Construct(&__u._M_rest);
> + std::_Construct(&__u._M_rest._M_rest);
> + std::_Construct(&__u._M_rest._M_rest._M_rest);
> + return __variant::__construct_n<_Np -
> 3>(__u._M_rest._M_rest._M_rest);
> + }
> + }
> +
> template<typename... _Types>
> struct _Traits
> {
> @@ -536,8 +563,9 @@ namespace __variant
> __emplace(_Variant_storage<_Triv, _Types...>& __v, _Args&&... __args)
> {
> __v._M_reset();
> - auto* __addr = std::__addressof(__variant::__get_n<_Np>(__v._M_u));
> - std::_Construct(__addr, std::forward<_Args>(__args)...);
> + auto* __addr = __variant::__construct_n<_Np>(__v._M_u);
> + std::_Construct(__addr, in_place_index<0>,
> + std::forward<_Args>(__args)...);
> // Construction didn't throw, so can set the new index now:
> __v._M_index = _Np;
> }
> --
> 2.41.0
>
>
On Sat, 23 Sept 2023 at 08:30, Nathaniel Shead via Libstdc++
<libstdc++@gcc.gnu.org> wrote:
>
> On Sat, Sep 23, 2023 at 07:40:48AM +0100, Jonathan Wakely wrote:
> > On Sat, 23 Sept 2023, 01:39 Nathaniel Shead via Libstdc++, <
> > libstdc++@gcc.gnu.org> wrote:
> >
> > > Now that bootstrap has finished, I have gotten regressions in the
> > > following libstdc++ tests:
> > >
> > > Running libstdc++:libstdc++-dg/conformance.exp ...
> > > FAIL: 20_util/bitset/access/constexpr.cc -std=gnu++23 (test for excess
> > > errors)
> > > FAIL: 20_util/bitset/access/constexpr.cc -std=gnu++26 (test for excess
> > > errors)
> > > FAIL: 20_util/variant/constexpr.cc -std=gnu++20 (test for excess errors)
> > > FAIL: 20_util/variant/constexpr.cc -std=gnu++26 (test for excess errors)
> > > FAIL: 21_strings/basic_string/cons/char/constexpr.cc -std=gnu++20 (test
> > > for excess errors)
> > > FAIL: 21_strings/basic_string/cons/char/constexpr.cc -std=gnu++26 (test
> > > for excess errors)
> > > FAIL: 21_strings/basic_string/cons/wchar_t/constexpr.cc -std=gnu++20 (test
> > > for excess errors)
> > > FAIL: 21_strings/basic_string/cons/wchar_t/constexpr.cc -std=gnu++26 (test
> > > for excess errors)
> > > FAIL: 21_strings/basic_string/modifiers/swap/constexpr-wchar_t.cc
> > > -std=gnu++20 (test for excess errors)
> > > FAIL: 21_strings/basic_string/modifiers/swap/constexpr-wchar_t.cc
> > > -std=gnu++26 (test for excess errors)
> > > FAIL: 21_strings/basic_string/modifiers/swap/constexpr.cc -std=gnu++20
> > > (test for excess errors)
> > > FAIL: 21_strings/basic_string/modifiers/swap/constexpr.cc -std=gnu++26
> > > (test for excess errors)
> > > FAIL: std/ranges/adaptors/join_with/1.cc -std=gnu++23 (test for excess
> > > errors)
> > > UNRESOLVED: std/ranges/adaptors/join_with/1.cc -std=gnu++23 compilation
> > > failed to produce executable
> > > FAIL: std/ranges/adaptors/join_with/1.cc -std=gnu++26 (test for excess
> > > errors)
> > > UNRESOLVED: std/ranges/adaptors/join_with/1.cc -std=gnu++26 compilation
> > > failed to produce executable
> > >
> > > On investigation though it looks like the issue might be with libstdc++
> > > rather than the patch itself; running the failing tests using clang with
> > > libstdc++ also produces similar errors, and my reading of the code
> > > suggests that this is correct.
> > >
> > > What's the way forward here? Should I look at creating a patch to fix
> > > the libstdc++ issues before resubmitting this patch for the C++
> > > frontend? Or should I submit a version of this patch without the
> > > `std::construct_at` changes and wait till libstdc++ gets fixed for that?
> > >
> >
> > I think we should fix libstdc++. There are probably only a few places that
> > need a fix, which cause all those failures.
> >
> > I can help with those fixes. I'll look into it after the weekend.
> >
>
> Thanks. I did end up getting a chance to look at it earlier today, and
> with the following patch I had no regressions when applying the frontend
> changes. Bootstrapped and regtested on x86_64-pc-linux-gnu.
>
> -- >8 --
>
> This patch ensures that the union members for std::string and
> std::variant are always properly set when a change occurs.
>
> libstdc++-v3/ChangeLog:
>
> * include/bits/basic_string.h: (basic_string(basic_string&&)):
> Activate _M_local_buf when needed.
> (basic_string(basic_string&&, const _Alloc&)): Likewise.
> * include/bits/basic_string.tcc: (basic_string::swap): Likewise.
> * include/std/variant: (__detail::__variant::__construct_n): New.
> (__detail::_variant::__emplace): Use __construct_n.
>
> Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
> ---
> libstdc++-v3/include/bits/basic_string.h | 7 +++--
> libstdc++-v3/include/bits/basic_string.tcc | 8 +++---
> libstdc++-v3/include/std/variant | 32 ++++++++++++++++++++--
> 3 files changed, 38 insertions(+), 9 deletions(-)
>
> diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
> index 09fd62afa66..7c342879827 100644
> --- a/libstdc++-v3/include/bits/basic_string.h
> +++ b/libstdc++-v3/include/bits/basic_string.h
> @@ -678,7 +678,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> {
> if (__str._M_is_local())
> {
> - traits_type::copy(_M_local_buf, __str._M_local_buf,
> + traits_type::copy(_M_use_local_data(), __str._M_local_buf,
> __str.length() + 1);
> }
> else
> @@ -691,7 +691,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> // basic_stringbuf relies on writing into unallocated capacity so
> // we mess up the contents if we put a '\0' in the string.
> _M_length(__str.length());
> - __str._M_data(__str._M_local_data());
> + __str._M_data(__str._M_use_local_data());
> __str._M_set_length(0);
> }
>
> @@ -717,6 +717,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> {
> if (__str._M_is_local())
> {
> + _M_use_local_data();
Lets add a cast to void to make it clear we're intentionally
discarding the return value here, and only calling it for its
constexpr "side effects":
(void) _M_use_local_data();
> traits_type::copy(_M_local_buf, __str._M_local_buf,
> __str.length() + 1);
> _M_length(__str.length());
> @@ -728,7 +729,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> _M_data(__str._M_data());
> _M_length(__str.length());
> _M_capacity(__str._M_allocated_capacity);
> - __str._M_data(__str._M_local_buf);
> + __str._M_data(__str._M_use_local_data());
> __str._M_set_length(0);
> }
> else
> diff --git a/libstdc++-v3/include/bits/basic_string.tcc b/libstdc++-v3/include/bits/basic_string.tcc
> index 104a517f794..ee6e57da555 100644
> --- a/libstdc++-v3/include/bits/basic_string.tcc
> +++ b/libstdc++-v3/include/bits/basic_string.tcc
> @@ -79,7 +79,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> }
> else if (__s.length())
> {
> - traits_type::copy(_M_local_buf, __s._M_local_buf,
> + traits_type::copy(_M_use_local_data(), __s._M_local_buf,
I think we should call _M_use_local_data() before calling
traits_type::copy, as you did for
basic_string(basic_string&& __str, const _Alloc& __a)
above.
The problem is that _M_use_local_data() returns the allocator's
pointer type, but traits_type::copy expects char_type*, and those
might not be the same type.
So:
@@ -79,6 +79,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
}
else if (__s.length())
{
+ (void) _M_use_local_data();
traits_type::copy(_M_local_buf, __s._M_local_buf,
__s.length() + 1);
_M_length(__s.length());
> __s.length() + 1);
> _M_length(__s.length());
> __s._M_set_length(0);
> @@ -87,7 +87,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> }
> else if (length())
> {
> - traits_type::copy(__s._M_local_buf, _M_local_buf,
> + traits_type::copy(__s._M_use_local_data(), _M_local_buf,
Same here, for __s._M_use_local_data()
> length() + 1);
> __s._M_length(length());
> _M_set_length(0);
> @@ -97,7 +97,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> else
> {
> const size_type __tmp_capacity = __s._M_allocated_capacity;
> - traits_type::copy(__s._M_local_buf, _M_local_buf,
> + traits_type::copy(__s._M_use_local_data(), _M_local_buf,
And again.
> length() + 1);
> _M_data(__s._M_data());
> __s._M_data(__s._M_local_buf);
> @@ -108,7 +108,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> const size_type __tmp_capacity = _M_allocated_capacity;
> if (__s._M_is_local())
> {
> - traits_type::copy(_M_local_buf, __s._M_local_buf,
> + traits_type::copy(_M_use_local_data(), __s._M_local_buf,
And again.
> __s.length() + 1);
> __s._M_data(_M_data());
> _M_data(_M_local_buf);
> diff --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant
> index c0e41740dcf..7f24e760bb1 100644
> --- a/libstdc++-v3/include/std/variant
> +++ b/libstdc++-v3/include/std/variant
> @@ -320,6 +320,33 @@ namespace __variant
> __get(_Variant&& __v) noexcept
> { return __variant::__get_n<_Np>(std::forward<_Variant>(__v)._M_u); }
>
> + // Gets the _Uninitialized to construct into for __u.
> + template<size_t _Np, typename _Union>
> + constexpr decltype(auto)
> + __construct_n(_Union& __u) noexcept
> + {
> + if constexpr (_Np == 0)
> + return &__u._M_first;
> + else if constexpr (_Np == 1)
> + {
> + std::_Construct(&__u._M_rest);
> + return &__u._M_rest._M_first;
> + }
> + else if constexpr (_Np == 2)
> + {
> + std::_Construct(&__u._M_rest);
> + std::_Construct(&__u._M_rest._M_rest);
> + return &__u._M_rest._M_rest._M_first;
> + }
> + else
> + {
> + std::_Construct(&__u._M_rest);
> + std::_Construct(&__u._M_rest._M_rest);
> + std::_Construct(&__u._M_rest._M_rest._M_rest);
> + return __variant::__construct_n<_Np - 3>(__u._M_rest._M_rest._M_rest);
> + }
> + }
This is nice, thanks for optimizing it to reduce the recursion depth.
> +
> template<typename... _Types>
> struct _Traits
> {
> @@ -536,8 +563,9 @@ namespace __variant
> __emplace(_Variant_storage<_Triv, _Types...>& __v, _Args&&... __args)
> {
> __v._M_reset();
> - auto* __addr = std::__addressof(__variant::__get_n<_Np>(__v._M_u));
> - std::_Construct(__addr, std::forward<_Args>(__args)...);
> + auto* __addr = __variant::__construct_n<_Np>(__v._M_u);
> + std::_Construct(__addr, in_place_index<0>,
> + std::forward<_Args>(__args)...);
> // Construction didn't throw, so can set the new index now:
> __v._M_index = _Np;
> }
> --
> 2.41.0
>
On Wed, Sep 27, 2023 at 03:13:35PM +0100, Jonathan Wakely wrote:
> On Sat, 23 Sept 2023 at 08:30, Nathaniel Shead via Libstdc++
> <libstdc++@gcc.gnu.org> wrote:
> >
> > On Sat, Sep 23, 2023 at 07:40:48AM +0100, Jonathan Wakely wrote:
> > > On Sat, 23 Sept 2023, 01:39 Nathaniel Shead via Libstdc++, <
> > > libstdc++@gcc.gnu.org> wrote:
> > >
> > > > Now that bootstrap has finished, I have gotten regressions in the
> > > > following libstdc++ tests:
> > > >
> > > > Running libstdc++:libstdc++-dg/conformance.exp ...
> > > > FAIL: 20_util/bitset/access/constexpr.cc -std=gnu++23 (test for excess
> > > > errors)
> > > > FAIL: 20_util/bitset/access/constexpr.cc -std=gnu++26 (test for excess
> > > > errors)
> > > > FAIL: 20_util/variant/constexpr.cc -std=gnu++20 (test for excess errors)
> > > > FAIL: 20_util/variant/constexpr.cc -std=gnu++26 (test for excess errors)
> > > > FAIL: 21_strings/basic_string/cons/char/constexpr.cc -std=gnu++20 (test
> > > > for excess errors)
> > > > FAIL: 21_strings/basic_string/cons/char/constexpr.cc -std=gnu++26 (test
> > > > for excess errors)
> > > > FAIL: 21_strings/basic_string/cons/wchar_t/constexpr.cc -std=gnu++20 (test
> > > > for excess errors)
> > > > FAIL: 21_strings/basic_string/cons/wchar_t/constexpr.cc -std=gnu++26 (test
> > > > for excess errors)
> > > > FAIL: 21_strings/basic_string/modifiers/swap/constexpr-wchar_t.cc
> > > > -std=gnu++20 (test for excess errors)
> > > > FAIL: 21_strings/basic_string/modifiers/swap/constexpr-wchar_t.cc
> > > > -std=gnu++26 (test for excess errors)
> > > > FAIL: 21_strings/basic_string/modifiers/swap/constexpr.cc -std=gnu++20
> > > > (test for excess errors)
> > > > FAIL: 21_strings/basic_string/modifiers/swap/constexpr.cc -std=gnu++26
> > > > (test for excess errors)
> > > > FAIL: std/ranges/adaptors/join_with/1.cc -std=gnu++23 (test for excess
> > > > errors)
> > > > UNRESOLVED: std/ranges/adaptors/join_with/1.cc -std=gnu++23 compilation
> > > > failed to produce executable
> > > > FAIL: std/ranges/adaptors/join_with/1.cc -std=gnu++26 (test for excess
> > > > errors)
> > > > UNRESOLVED: std/ranges/adaptors/join_with/1.cc -std=gnu++26 compilation
> > > > failed to produce executable
> > > >
> > > > On investigation though it looks like the issue might be with libstdc++
> > > > rather than the patch itself; running the failing tests using clang with
> > > > libstdc++ also produces similar errors, and my reading of the code
> > > > suggests that this is correct.
> > > >
> > > > What's the way forward here? Should I look at creating a patch to fix
> > > > the libstdc++ issues before resubmitting this patch for the C++
> > > > frontend? Or should I submit a version of this patch without the
> > > > `std::construct_at` changes and wait till libstdc++ gets fixed for that?
> > > >
> > >
> > > I think we should fix libstdc++. There are probably only a few places that
> > > need a fix, which cause all those failures.
> > >
> > > I can help with those fixes. I'll look into it after the weekend.
> > >
> >
> > Thanks. I did end up getting a chance to look at it earlier today, and
> > with the following patch I had no regressions when applying the frontend
> > changes. Bootstrapped and regtested on x86_64-pc-linux-gnu.
> >
> > -- >8 --
> >
> > This patch ensures that the union members for std::string and
> > std::variant are always properly set when a change occurs.
> >
> > libstdc++-v3/ChangeLog:
> >
> > * include/bits/basic_string.h: (basic_string(basic_string&&)):
> > Activate _M_local_buf when needed.
> > (basic_string(basic_string&&, const _Alloc&)): Likewise.
> > * include/bits/basic_string.tcc: (basic_string::swap): Likewise.
> > * include/std/variant: (__detail::__variant::__construct_n): New.
> > (__detail::_variant::__emplace): Use __construct_n.
> >
> > Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
> > ---
> > libstdc++-v3/include/bits/basic_string.h | 7 +++--
> > libstdc++-v3/include/bits/basic_string.tcc | 8 +++---
> > libstdc++-v3/include/std/variant | 32 ++++++++++++++++++++--
> > 3 files changed, 38 insertions(+), 9 deletions(-)
> >
> > diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
> > index 09fd62afa66..7c342879827 100644
> > --- a/libstdc++-v3/include/bits/basic_string.h
> > +++ b/libstdc++-v3/include/bits/basic_string.h
> > @@ -678,7 +678,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> > {
> > if (__str._M_is_local())
> > {
> > - traits_type::copy(_M_local_buf, __str._M_local_buf,
> > + traits_type::copy(_M_use_local_data(), __str._M_local_buf,
> > __str.length() + 1);
> > }
> > else
> > @@ -691,7 +691,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> > // basic_stringbuf relies on writing into unallocated capacity so
> > // we mess up the contents if we put a '\0' in the string.
> > _M_length(__str.length());
> > - __str._M_data(__str._M_local_data());
> > + __str._M_data(__str._M_use_local_data());
> > __str._M_set_length(0);
> > }
> >
> > @@ -717,6 +717,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> > {
> > if (__str._M_is_local())
> > {
> > + _M_use_local_data();
>
> Lets add a cast to void to make it clear we're intentionally
> discarding the return value here, and only calling it for its
> constexpr "side effects":
>
> (void) _M_use_local_data();
>
>
> > traits_type::copy(_M_local_buf, __str._M_local_buf,
> > __str.length() + 1);
> > _M_length(__str.length());
> > @@ -728,7 +729,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> > _M_data(__str._M_data());
> > _M_length(__str.length());
> > _M_capacity(__str._M_allocated_capacity);
> > - __str._M_data(__str._M_local_buf);
> > + __str._M_data(__str._M_use_local_data());
> > __str._M_set_length(0);
> > }
> > else
> > diff --git a/libstdc++-v3/include/bits/basic_string.tcc b/libstdc++-v3/include/bits/basic_string.tcc
> > index 104a517f794..ee6e57da555 100644
> > --- a/libstdc++-v3/include/bits/basic_string.tcc
> > +++ b/libstdc++-v3/include/bits/basic_string.tcc
> > @@ -79,7 +79,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > }
> > else if (__s.length())
> > {
> > - traits_type::copy(_M_local_buf, __s._M_local_buf,
> > + traits_type::copy(_M_use_local_data(), __s._M_local_buf,
>
> I think we should call _M_use_local_data() before calling
> traits_type::copy, as you did for
> basic_string(basic_string&& __str, const _Alloc& __a)
> above.
>
> The problem is that _M_use_local_data() returns the allocator's
> pointer type, but traits_type::copy expects char_type*, and those
> might not be the same type.
> So:
>
> @@ -79,6 +79,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> }
> else if (__s.length())
> {
> + (void) _M_use_local_data();
> traits_type::copy(_M_local_buf, __s._M_local_buf,
> __s.length() + 1);
> _M_length(__s.length());
>
>
>
> > __s.length() + 1);
> > _M_length(__s.length());
> > __s._M_set_length(0);
> > @@ -87,7 +87,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > }
> > else if (length())
> > {
> > - traits_type::copy(__s._M_local_buf, _M_local_buf,
> > + traits_type::copy(__s._M_use_local_data(), _M_local_buf,
>
> Same here, for __s._M_use_local_data()
>
> > length() + 1);
> > __s._M_length(length());
> > _M_set_length(0);
> > @@ -97,7 +97,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > else
> > {
> > const size_type __tmp_capacity = __s._M_allocated_capacity;
> > - traits_type::copy(__s._M_local_buf, _M_local_buf,
> > + traits_type::copy(__s._M_use_local_data(), _M_local_buf,
>
> And again.
>
> > length() + 1);
> > _M_data(__s._M_data());
> > __s._M_data(__s._M_local_buf);
> > @@ -108,7 +108,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > const size_type __tmp_capacity = _M_allocated_capacity;
> > if (__s._M_is_local())
> > {
> > - traits_type::copy(_M_local_buf, __s._M_local_buf,
> > + traits_type::copy(_M_use_local_data(), __s._M_local_buf,
>
> And again.
>
> > __s.length() + 1);
> > __s._M_data(_M_data());
> > _M_data(_M_local_buf);
> > diff --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant
> > index c0e41740dcf..7f24e760bb1 100644
> > --- a/libstdc++-v3/include/std/variant
> > +++ b/libstdc++-v3/include/std/variant
> > @@ -320,6 +320,33 @@ namespace __variant
> > __get(_Variant&& __v) noexcept
> > { return __variant::__get_n<_Np>(std::forward<_Variant>(__v)._M_u); }
> >
> > + // Gets the _Uninitialized to construct into for __u.
> > + template<size_t _Np, typename _Union>
> > + constexpr decltype(auto)
> > + __construct_n(_Union& __u) noexcept
> > + {
> > + if constexpr (_Np == 0)
> > + return &__u._M_first;
> > + else if constexpr (_Np == 1)
> > + {
> > + std::_Construct(&__u._M_rest);
> > + return &__u._M_rest._M_first;
> > + }
> > + else if constexpr (_Np == 2)
> > + {
> > + std::_Construct(&__u._M_rest);
> > + std::_Construct(&__u._M_rest._M_rest);
> > + return &__u._M_rest._M_rest._M_first;
> > + }
> > + else
> > + {
> > + std::_Construct(&__u._M_rest);
> > + std::_Construct(&__u._M_rest._M_rest);
> > + std::_Construct(&__u._M_rest._M_rest._M_rest);
> > + return __variant::__construct_n<_Np - 3>(__u._M_rest._M_rest._M_rest);
> > + }
> > + }
>
> This is nice, thanks for optimizing it to reduce the recursion depth.
>
> > +
> > template<typename... _Types>
> > struct _Traits
> > {
> > @@ -536,8 +563,9 @@ namespace __variant
> > __emplace(_Variant_storage<_Triv, _Types...>& __v, _Args&&... __args)
> > {
> > __v._M_reset();
> > - auto* __addr = std::__addressof(__variant::__get_n<_Np>(__v._M_u));
> > - std::_Construct(__addr, std::forward<_Args>(__args)...);
> > + auto* __addr = __variant::__construct_n<_Np>(__v._M_u);
> > + std::_Construct(__addr, in_place_index<0>,
> > + std::forward<_Args>(__args)...);
> > // Construction didn't throw, so can set the new index now:
> > __v._M_index = _Np;
> > }
> > --
> > 2.41.0
> >
>
Thanks for the comments, here's an updated version of the patch.
Bootstrapped and regtested on x86_64-pc-linux-gnu.
I'll note that there are some existing calls to `_M_use_local_data()`
already used only for their side effects without a cast to void, e.g.
/**
* @brief Default constructor creates an empty string.
*/
_GLIBCXX20_CONSTEXPR
basic_string()
_GLIBCXX_NOEXCEPT_IF(is_nothrow_default_constructible<_Alloc>::value)
: _M_dataplus(_M_local_data())
{
_M_use_local_data();
_M_set_length(0);
}
I haven't updated these, but should this be changed for consistency?
-- >8 --
This patch ensures that the union members for std::string and
std::variant are always properly set when a change occurs.
libstdc++-v3/ChangeLog:
* include/bits/basic_string.h: (basic_string(basic_string&&)):
Activate _M_local_buf when needed.
(basic_string(basic_string&&, const _Alloc&)): Likewise.
* include/bits/basic_string.tcc: (basic_string::swap): Likewise.
* include/std/variant: (__detail::__variant::__construct_n): New.
(__detail::__variant::__emplace): Use __construct_n.
Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
---
libstdc++-v3/include/bits/basic_string.h | 6 ++--
libstdc++-v3/include/bits/basic_string.tcc | 4 +++
libstdc++-v3/include/std/variant | 32 ++++++++++++++++++++--
3 files changed, 38 insertions(+), 4 deletions(-)
diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
index 09fd62afa66..4f94cd967cf 100644
--- a/libstdc++-v3/include/bits/basic_string.h
+++ b/libstdc++-v3/include/bits/basic_string.h
@@ -678,6 +678,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
{
if (__str._M_is_local())
{
+ (void)_M_use_local_data();
traits_type::copy(_M_local_buf, __str._M_local_buf,
__str.length() + 1);
}
@@ -691,7 +692,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
// basic_stringbuf relies on writing into unallocated capacity so
// we mess up the contents if we put a '\0' in the string.
_M_length(__str.length());
- __str._M_data(__str._M_local_data());
+ __str._M_data(__str._M_use_local_data());
__str._M_set_length(0);
}
@@ -717,6 +718,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
{
if (__str._M_is_local())
{
+ (void)_M_use_local_data();
traits_type::copy(_M_local_buf, __str._M_local_buf,
__str.length() + 1);
_M_length(__str.length());
@@ -728,7 +730,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
_M_data(__str._M_data());
_M_length(__str.length());
_M_capacity(__str._M_allocated_capacity);
- __str._M_data(__str._M_local_buf);
+ __str._M_data(__str._M_use_local_data());
__str._M_set_length(0);
}
else
diff --git a/libstdc++-v3/include/bits/basic_string.tcc b/libstdc++-v3/include/bits/basic_string.tcc
index 104a517f794..4bc98f2aea7 100644
--- a/libstdc++-v3/include/bits/basic_string.tcc
+++ b/libstdc++-v3/include/bits/basic_string.tcc
@@ -79,6 +79,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
}
else if (__s.length())
{
+ (void)_M_use_local_data();
traits_type::copy(_M_local_buf, __s._M_local_buf,
__s.length() + 1);
_M_length(__s.length());
@@ -87,6 +88,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
}
else if (length())
{
+ (void)__s._M_use_local_data();
traits_type::copy(__s._M_local_buf, _M_local_buf,
length() + 1);
__s._M_length(length());
@@ -97,6 +99,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
else
{
const size_type __tmp_capacity = __s._M_allocated_capacity;
+ (void)__s._M_use_local_data();
traits_type::copy(__s._M_local_buf, _M_local_buf,
length() + 1);
_M_data(__s._M_data());
@@ -108,6 +111,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
const size_type __tmp_capacity = _M_allocated_capacity;
if (__s._M_is_local())
{
+ (void)_M_use_local_data();
traits_type::copy(_M_local_buf, __s._M_local_buf,
__s.length() + 1);
__s._M_data(_M_data());
diff --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant
index c0e41740dcf..7f24e760bb1 100644
--- a/libstdc++-v3/include/std/variant
+++ b/libstdc++-v3/include/std/variant
@@ -320,6 +320,33 @@ namespace __variant
__get(_Variant&& __v) noexcept
{ return __variant::__get_n<_Np>(std::forward<_Variant>(__v)._M_u); }
+ // Gets the _Uninitialized to construct into for __u.
+ template<size_t _Np, typename _Union>
+ constexpr decltype(auto)
+ __construct_n(_Union& __u) noexcept
+ {
+ if constexpr (_Np == 0)
+ return &__u._M_first;
+ else if constexpr (_Np == 1)
+ {
+ std::_Construct(&__u._M_rest);
+ return &__u._M_rest._M_first;
+ }
+ else if constexpr (_Np == 2)
+ {
+ std::_Construct(&__u._M_rest);
+ std::_Construct(&__u._M_rest._M_rest);
+ return &__u._M_rest._M_rest._M_first;
+ }
+ else
+ {
+ std::_Construct(&__u._M_rest);
+ std::_Construct(&__u._M_rest._M_rest);
+ std::_Construct(&__u._M_rest._M_rest._M_rest);
+ return __variant::__construct_n<_Np - 3>(__u._M_rest._M_rest._M_rest);
+ }
+ }
+
template<typename... _Types>
struct _Traits
{
@@ -536,8 +563,9 @@ namespace __variant
__emplace(_Variant_storage<_Triv, _Types...>& __v, _Args&&... __args)
{
__v._M_reset();
- auto* __addr = std::__addressof(__variant::__get_n<_Np>(__v._M_u));
- std::_Construct(__addr, std::forward<_Args>(__args)...);
+ auto* __addr = __variant::__construct_n<_Np>(__v._M_u);
+ std::_Construct(__addr, in_place_index<0>,
+ std::forward<_Args>(__args)...);
// Construction didn't throw, so can set the new index now:
__v._M_index = _Np;
}
On Fri, 29 Sept 2023 at 00:25, Nathaniel Shead
<nathanieloshead@gmail.com> wrote:
>
> On Wed, Sep 27, 2023 at 03:13:35PM +0100, Jonathan Wakely wrote:
> > On Sat, 23 Sept 2023 at 08:30, Nathaniel Shead via Libstdc++
> > <libstdc++@gcc.gnu.org> wrote:
> > >
> > > On Sat, Sep 23, 2023 at 07:40:48AM +0100, Jonathan Wakely wrote:
> > > > On Sat, 23 Sept 2023, 01:39 Nathaniel Shead via Libstdc++, <
> > > > libstdc++@gcc.gnu.org> wrote:
> > > >
> > > > > Now that bootstrap has finished, I have gotten regressions in the
> > > > > following libstdc++ tests:
> > > > >
> > > > > Running libstdc++:libstdc++-dg/conformance.exp ...
> > > > > FAIL: 20_util/bitset/access/constexpr.cc -std=gnu++23 (test for excess
> > > > > errors)
> > > > > FAIL: 20_util/bitset/access/constexpr.cc -std=gnu++26 (test for excess
> > > > > errors)
> > > > > FAIL: 20_util/variant/constexpr.cc -std=gnu++20 (test for excess errors)
> > > > > FAIL: 20_util/variant/constexpr.cc -std=gnu++26 (test for excess errors)
> > > > > FAIL: 21_strings/basic_string/cons/char/constexpr.cc -std=gnu++20 (test
> > > > > for excess errors)
> > > > > FAIL: 21_strings/basic_string/cons/char/constexpr.cc -std=gnu++26 (test
> > > > > for excess errors)
> > > > > FAIL: 21_strings/basic_string/cons/wchar_t/constexpr.cc -std=gnu++20 (test
> > > > > for excess errors)
> > > > > FAIL: 21_strings/basic_string/cons/wchar_t/constexpr.cc -std=gnu++26 (test
> > > > > for excess errors)
> > > > > FAIL: 21_strings/basic_string/modifiers/swap/constexpr-wchar_t.cc
> > > > > -std=gnu++20 (test for excess errors)
> > > > > FAIL: 21_strings/basic_string/modifiers/swap/constexpr-wchar_t.cc
> > > > > -std=gnu++26 (test for excess errors)
> > > > > FAIL: 21_strings/basic_string/modifiers/swap/constexpr.cc -std=gnu++20
> > > > > (test for excess errors)
> > > > > FAIL: 21_strings/basic_string/modifiers/swap/constexpr.cc -std=gnu++26
> > > > > (test for excess errors)
> > > > > FAIL: std/ranges/adaptors/join_with/1.cc -std=gnu++23 (test for excess
> > > > > errors)
> > > > > UNRESOLVED: std/ranges/adaptors/join_with/1.cc -std=gnu++23 compilation
> > > > > failed to produce executable
> > > > > FAIL: std/ranges/adaptors/join_with/1.cc -std=gnu++26 (test for excess
> > > > > errors)
> > > > > UNRESOLVED: std/ranges/adaptors/join_with/1.cc -std=gnu++26 compilation
> > > > > failed to produce executable
> > > > >
> > > > > On investigation though it looks like the issue might be with libstdc++
> > > > > rather than the patch itself; running the failing tests using clang with
> > > > > libstdc++ also produces similar errors, and my reading of the code
> > > > > suggests that this is correct.
> > > > >
> > > > > What's the way forward here? Should I look at creating a patch to fix
> > > > > the libstdc++ issues before resubmitting this patch for the C++
> > > > > frontend? Or should I submit a version of this patch without the
> > > > > `std::construct_at` changes and wait till libstdc++ gets fixed for that?
> > > > >
> > > >
> > > > I think we should fix libstdc++. There are probably only a few places that
> > > > need a fix, which cause all those failures.
> > > >
> > > > I can help with those fixes. I'll look into it after the weekend.
> > > >
> > >
> > > Thanks. I did end up getting a chance to look at it earlier today, and
> > > with the following patch I had no regressions when applying the frontend
> > > changes. Bootstrapped and regtested on x86_64-pc-linux-gnu.
> > >
> > > -- >8 --
> > >
> > > This patch ensures that the union members for std::string and
> > > std::variant are always properly set when a change occurs.
> > >
> > > libstdc++-v3/ChangeLog:
> > >
> > > * include/bits/basic_string.h: (basic_string(basic_string&&)):
> > > Activate _M_local_buf when needed.
> > > (basic_string(basic_string&&, const _Alloc&)): Likewise.
> > > * include/bits/basic_string.tcc: (basic_string::swap): Likewise.
> > > * include/std/variant: (__detail::__variant::__construct_n): New.
> > > (__detail::_variant::__emplace): Use __construct_n.
> > >
> > > Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
> > > ---
> > > libstdc++-v3/include/bits/basic_string.h | 7 +++--
> > > libstdc++-v3/include/bits/basic_string.tcc | 8 +++---
> > > libstdc++-v3/include/std/variant | 32 ++++++++++++++++++++--
> > > 3 files changed, 38 insertions(+), 9 deletions(-)
> > >
> > > diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
> > > index 09fd62afa66..7c342879827 100644
> > > --- a/libstdc++-v3/include/bits/basic_string.h
> > > +++ b/libstdc++-v3/include/bits/basic_string.h
> > > @@ -678,7 +678,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> > > {
> > > if (__str._M_is_local())
> > > {
> > > - traits_type::copy(_M_local_buf, __str._M_local_buf,
> > > + traits_type::copy(_M_use_local_data(), __str._M_local_buf,
> > > __str.length() + 1);
> > > }
> > > else
> > > @@ -691,7 +691,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> > > // basic_stringbuf relies on writing into unallocated capacity so
> > > // we mess up the contents if we put a '\0' in the string.
> > > _M_length(__str.length());
> > > - __str._M_data(__str._M_local_data());
> > > + __str._M_data(__str._M_use_local_data());
> > > __str._M_set_length(0);
> > > }
> > >
> > > @@ -717,6 +717,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> > > {
> > > if (__str._M_is_local())
> > > {
> > > + _M_use_local_data();
> >
> > Lets add a cast to void to make it clear we're intentionally
> > discarding the return value here, and only calling it for its
> > constexpr "side effects":
> >
> > (void) _M_use_local_data();
> >
> >
> > > traits_type::copy(_M_local_buf, __str._M_local_buf,
> > > __str.length() + 1);
> > > _M_length(__str.length());
> > > @@ -728,7 +729,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> > > _M_data(__str._M_data());
> > > _M_length(__str.length());
> > > _M_capacity(__str._M_allocated_capacity);
> > > - __str._M_data(__str._M_local_buf);
> > > + __str._M_data(__str._M_use_local_data());
> > > __str._M_set_length(0);
> > > }
> > > else
> > > diff --git a/libstdc++-v3/include/bits/basic_string.tcc b/libstdc++-v3/include/bits/basic_string.tcc
> > > index 104a517f794..ee6e57da555 100644
> > > --- a/libstdc++-v3/include/bits/basic_string.tcc
> > > +++ b/libstdc++-v3/include/bits/basic_string.tcc
> > > @@ -79,7 +79,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > > }
> > > else if (__s.length())
> > > {
> > > - traits_type::copy(_M_local_buf, __s._M_local_buf,
> > > + traits_type::copy(_M_use_local_data(), __s._M_local_buf,
> >
> > I think we should call _M_use_local_data() before calling
> > traits_type::copy, as you did for
> > basic_string(basic_string&& __str, const _Alloc& __a)
> > above.
> >
> > The problem is that _M_use_local_data() returns the allocator's
> > pointer type, but traits_type::copy expects char_type*, and those
> > might not be the same type.
> > So:
> >
> > @@ -79,6 +79,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > }
> > else if (__s.length())
> > {
> > + (void) _M_use_local_data();
> > traits_type::copy(_M_local_buf, __s._M_local_buf,
> > __s.length() + 1);
> > _M_length(__s.length());
> >
> >
> >
> > > __s.length() + 1);
> > > _M_length(__s.length());
> > > __s._M_set_length(0);
> > > @@ -87,7 +87,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > > }
> > > else if (length())
> > > {
> > > - traits_type::copy(__s._M_local_buf, _M_local_buf,
> > > + traits_type::copy(__s._M_use_local_data(), _M_local_buf,
> >
> > Same here, for __s._M_use_local_data()
> >
> > > length() + 1);
> > > __s._M_length(length());
> > > _M_set_length(0);
> > > @@ -97,7 +97,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > > else
> > > {
> > > const size_type __tmp_capacity = __s._M_allocated_capacity;
> > > - traits_type::copy(__s._M_local_buf, _M_local_buf,
> > > + traits_type::copy(__s._M_use_local_data(), _M_local_buf,
> >
> > And again.
> >
> > > length() + 1);
> > > _M_data(__s._M_data());
> > > __s._M_data(__s._M_local_buf);
> > > @@ -108,7 +108,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > > const size_type __tmp_capacity = _M_allocated_capacity;
> > > if (__s._M_is_local())
> > > {
> > > - traits_type::copy(_M_local_buf, __s._M_local_buf,
> > > + traits_type::copy(_M_use_local_data(), __s._M_local_buf,
> >
> > And again.
> >
> > > __s.length() + 1);
> > > __s._M_data(_M_data());
> > > _M_data(_M_local_buf);
> > > diff --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant
> > > index c0e41740dcf..7f24e760bb1 100644
> > > --- a/libstdc++-v3/include/std/variant
> > > +++ b/libstdc++-v3/include/std/variant
> > > @@ -320,6 +320,33 @@ namespace __variant
> > > __get(_Variant&& __v) noexcept
> > > { return __variant::__get_n<_Np>(std::forward<_Variant>(__v)._M_u); }
> > >
> > > + // Gets the _Uninitialized to construct into for __u.
> > > + template<size_t _Np, typename _Union>
> > > + constexpr decltype(auto)
> > > + __construct_n(_Union& __u) noexcept
> > > + {
> > > + if constexpr (_Np == 0)
> > > + return &__u._M_first;
> > > + else if constexpr (_Np == 1)
> > > + {
> > > + std::_Construct(&__u._M_rest);
> > > + return &__u._M_rest._M_first;
> > > + }
> > > + else if constexpr (_Np == 2)
> > > + {
> > > + std::_Construct(&__u._M_rest);
> > > + std::_Construct(&__u._M_rest._M_rest);
> > > + return &__u._M_rest._M_rest._M_first;
> > > + }
> > > + else
> > > + {
> > > + std::_Construct(&__u._M_rest);
> > > + std::_Construct(&__u._M_rest._M_rest);
> > > + std::_Construct(&__u._M_rest._M_rest._M_rest);
> > > + return __variant::__construct_n<_Np - 3>(__u._M_rest._M_rest._M_rest);
> > > + }
> > > + }
> >
> > This is nice, thanks for optimizing it to reduce the recursion depth.
> >
> > > +
> > > template<typename... _Types>
> > > struct _Traits
> > > {
> > > @@ -536,8 +563,9 @@ namespace __variant
> > > __emplace(_Variant_storage<_Triv, _Types...>& __v, _Args&&... __args)
> > > {
> > > __v._M_reset();
> > > - auto* __addr = std::__addressof(__variant::__get_n<_Np>(__v._M_u));
> > > - std::_Construct(__addr, std::forward<_Args>(__args)...);
> > > + auto* __addr = __variant::__construct_n<_Np>(__v._M_u);
> > > + std::_Construct(__addr, in_place_index<0>,
> > > + std::forward<_Args>(__args)...);
> > > // Construction didn't throw, so can set the new index now:
> > > __v._M_index = _Np;
> > > }
> > > --
> > > 2.41.0
> > >
> >
>
> Thanks for the comments, here's an updated version of the patch.
> Bootstrapped and regtested on x86_64-pc-linux-gnu.
Great, I'll get this committed today - thanks!
>
> I'll note that there are some existing calls to `_M_use_local_data()`
> already used only for their side effects without a cast to void, e.g.
>
> /**
> * @brief Default constructor creates an empty string.
> */
> _GLIBCXX20_CONSTEXPR
> basic_string()
> _GLIBCXX_NOEXCEPT_IF(is_nothrow_default_constructible<_Alloc>::value)
> : _M_dataplus(_M_local_data())
> {
> _M_use_local_data();
> _M_set_length(0);
> }
>
> I haven't updated these, but should this be changed for consistency?
Yes, good idea. I can do that.
Thanks again for fixing these. I think this might fix some bug reports
about clang rejecting our std::string in constant expressions, so I'll
check those.
>
> -- >8 --
>
> This patch ensures that the union members for std::string and
> std::variant are always properly set when a change occurs.
>
> libstdc++-v3/ChangeLog:
>
> * include/bits/basic_string.h: (basic_string(basic_string&&)):
> Activate _M_local_buf when needed.
> (basic_string(basic_string&&, const _Alloc&)): Likewise.
> * include/bits/basic_string.tcc: (basic_string::swap): Likewise.
> * include/std/variant: (__detail::__variant::__construct_n): New.
> (__detail::__variant::__emplace): Use __construct_n.
>
> Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
> ---
> libstdc++-v3/include/bits/basic_string.h | 6 ++--
> libstdc++-v3/include/bits/basic_string.tcc | 4 +++
> libstdc++-v3/include/std/variant | 32 ++++++++++++++++++++--
> 3 files changed, 38 insertions(+), 4 deletions(-)
>
> diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
> index 09fd62afa66..4f94cd967cf 100644
> --- a/libstdc++-v3/include/bits/basic_string.h
> +++ b/libstdc++-v3/include/bits/basic_string.h
> @@ -678,6 +678,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> {
> if (__str._M_is_local())
> {
> + (void)_M_use_local_data();
> traits_type::copy(_M_local_buf, __str._M_local_buf,
> __str.length() + 1);
> }
> @@ -691,7 +692,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> // basic_stringbuf relies on writing into unallocated capacity so
> // we mess up the contents if we put a '\0' in the string.
> _M_length(__str.length());
> - __str._M_data(__str._M_local_data());
> + __str._M_data(__str._M_use_local_data());
> __str._M_set_length(0);
> }
>
> @@ -717,6 +718,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> {
> if (__str._M_is_local())
> {
> + (void)_M_use_local_data();
> traits_type::copy(_M_local_buf, __str._M_local_buf,
> __str.length() + 1);
> _M_length(__str.length());
> @@ -728,7 +730,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> _M_data(__str._M_data());
> _M_length(__str.length());
> _M_capacity(__str._M_allocated_capacity);
> - __str._M_data(__str._M_local_buf);
> + __str._M_data(__str._M_use_local_data());
> __str._M_set_length(0);
> }
> else
> diff --git a/libstdc++-v3/include/bits/basic_string.tcc b/libstdc++-v3/include/bits/basic_string.tcc
> index 104a517f794..4bc98f2aea7 100644
> --- a/libstdc++-v3/include/bits/basic_string.tcc
> +++ b/libstdc++-v3/include/bits/basic_string.tcc
> @@ -79,6 +79,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> }
> else if (__s.length())
> {
> + (void)_M_use_local_data();
> traits_type::copy(_M_local_buf, __s._M_local_buf,
> __s.length() + 1);
> _M_length(__s.length());
> @@ -87,6 +88,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> }
> else if (length())
> {
> + (void)__s._M_use_local_data();
> traits_type::copy(__s._M_local_buf, _M_local_buf,
> length() + 1);
> __s._M_length(length());
> @@ -97,6 +99,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> else
> {
> const size_type __tmp_capacity = __s._M_allocated_capacity;
> + (void)__s._M_use_local_data();
> traits_type::copy(__s._M_local_buf, _M_local_buf,
> length() + 1);
> _M_data(__s._M_data());
> @@ -108,6 +111,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> const size_type __tmp_capacity = _M_allocated_capacity;
> if (__s._M_is_local())
> {
> + (void)_M_use_local_data();
> traits_type::copy(_M_local_buf, __s._M_local_buf,
> __s.length() + 1);
> __s._M_data(_M_data());
> diff --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant
> index c0e41740dcf..7f24e760bb1 100644
> --- a/libstdc++-v3/include/std/variant
> +++ b/libstdc++-v3/include/std/variant
> @@ -320,6 +320,33 @@ namespace __variant
> __get(_Variant&& __v) noexcept
> { return __variant::__get_n<_Np>(std::forward<_Variant>(__v)._M_u); }
>
> + // Gets the _Uninitialized to construct into for __u.
> + template<size_t _Np, typename _Union>
> + constexpr decltype(auto)
> + __construct_n(_Union& __u) noexcept
> + {
> + if constexpr (_Np == 0)
> + return &__u._M_first;
> + else if constexpr (_Np == 1)
> + {
> + std::_Construct(&__u._M_rest);
> + return &__u._M_rest._M_first;
> + }
> + else if constexpr (_Np == 2)
> + {
> + std::_Construct(&__u._M_rest);
> + std::_Construct(&__u._M_rest._M_rest);
> + return &__u._M_rest._M_rest._M_first;
> + }
> + else
> + {
> + std::_Construct(&__u._M_rest);
> + std::_Construct(&__u._M_rest._M_rest);
> + std::_Construct(&__u._M_rest._M_rest._M_rest);
> + return __variant::__construct_n<_Np - 3>(__u._M_rest._M_rest._M_rest);
> + }
> + }
> +
> template<typename... _Types>
> struct _Traits
> {
> @@ -536,8 +563,9 @@ namespace __variant
> __emplace(_Variant_storage<_Triv, _Types...>& __v, _Args&&... __args)
> {
> __v._M_reset();
> - auto* __addr = std::__addressof(__variant::__get_n<_Np>(__v._M_u));
> - std::_Construct(__addr, std::forward<_Args>(__args)...);
> + auto* __addr = __variant::__construct_n<_Np>(__v._M_u);
> + std::_Construct(__addr, in_place_index<0>,
> + std::forward<_Args>(__args)...);
> // Construction didn't throw, so can set the new index now:
> __v._M_index = _Np;
> }
> --
> 2.41.0
>
On Fri, 29 Sept 2023 at 10:32, Jonathan Wakely <jwakely@redhat.com> wrote:
> > Thanks for the comments, here's an updated version of the patch.
> > Bootstrapped and regtested on x86_64-pc-linux-gnu.
>
> Great, I'll get this committed today - thanks!
That's done now.
> >
> > I'll note that there are some existing calls to `_M_use_local_data()`
> > already used only for their side effects without a cast to void, e.g.
> >
> > /**
> > * @brief Default constructor creates an empty string.
> > */
> > _GLIBCXX20_CONSTEXPR
> > basic_string()
> > _GLIBCXX_NOEXCEPT_IF(is_nothrow_default_constructible<_Alloc>::value)
> > : _M_dataplus(_M_local_data())
> > {
> > _M_use_local_data();
> > _M_set_length(0);
> > }
> >
> > I haven't updated these, but should this be changed for consistency?
>
> Yes, good idea. I can do that.
I started to do that, and decided it made more sense to split out the
constexpr loop from _M_use_local_data() into a separate function,
_M_init_local_buf(). Then we can use that for all the places where we
don't care about the return value. That avoids the overhead of using
pointer_traits::pointer_to when we don't need the return value (which
is admittedly only going to be an issue for -O0 code, but I think it's
cleaner this way anyway).
Please see the attached patch and let me know what you think.
> Thanks again for fixing these. I think this might fix some bug reports
> about clang rejecting our std::string in constant expressions, so I'll
> check those.
Your patch fixes https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110900
(so we should backport it to gcc-13 and gcc-12 too).
commit 2668979d3206ff6c039ac0165aae29377a15666c
Author: Jonathan Wakely <jwakely@redhat.com>
Date: Fri Sep 29 12:12:22 2023
libstdc++: Split std::basic_string::_M_use_local_data into two functions
This splits out the activate-the-union-member-for-constexpr logic from
_M_use_local_data, so that it can be used separately in cases that don't
need to use std::pointer_traits<pointer>::pointer_to to obtain the
return value.
This leaves only three uses of _M_use_local_data() which are all the
same form:
__s._M_data(_M_use_local_data());
__s._M_set_length(0);
We could remove _M_use_local_data() and change those three places to use
a new _M_reset() function that does:
_M_init_local_buf();
_M_data(_M_local_data());
_M_set_length(0);
This is left for a future change.
libstdc++-v3/ChangeLog:
* include/bits/basic_string.h (_M_init_local_buf()): New
function.
(_M_use_local_data()): Use _M_init_local_buf.
(basic_string(), basic_string(const Alloc&))
(basic_string(basic_string&&))
(basic_string(basic_string&&, const Alloc&)): Use
_M_init_local_buf instead of _M_use_local_data().
* include/bits/basic_string.tcc (swap(basic_string&))
(_M_construct(InIter, InIter, forward_iterator_tag))
(_M_construct(size_type, CharT), reserve()): Likewise.
(_M_construct(InIter, InIter, input_iterator_tag)): Remove call
to _M_use_local_data() and initialize the local buffer directly.
diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
index 4f94cd967cf..18a19b8dcbc 100644
--- a/libstdc++-v3/include/bits/basic_string.h
+++ b/libstdc++-v3/include/bits/basic_string.h
@@ -353,13 +353,23 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
// Ensure that _M_local_buf is the active member of the union.
__attribute__((__always_inline__))
_GLIBCXX14_CONSTEXPR
- pointer
- _M_use_local_data() _GLIBCXX_NOEXCEPT
+ void
+ _M_init_local_buf() _GLIBCXX_NOEXCEPT
{
#if __cpp_lib_is_constant_evaluated
if (std::is_constant_evaluated())
for (size_type __i = 0; __i <= _S_local_capacity; ++__i)
_M_local_buf[__i] = _CharT();
+#endif
+ }
+
+ __attribute__((__always_inline__))
+ _GLIBCXX14_CONSTEXPR
+ pointer
+ _M_use_local_data() _GLIBCXX_NOEXCEPT
+ {
+#if __glibcxx_is_constant_evaluated
+ _M_init_local_buf();
#endif
return _M_local_data();
}
@@ -522,7 +532,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
_GLIBCXX_NOEXCEPT_IF(is_nothrow_default_constructible<_Alloc>::value)
: _M_dataplus(_M_local_data())
{
- _M_use_local_data();
+ _M_init_local_buf();
_M_set_length(0);
}
@@ -534,7 +544,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
basic_string(const _Alloc& __a) _GLIBCXX_NOEXCEPT
: _M_dataplus(_M_local_data(), __a)
{
- _M_use_local_data();
+ _M_init_local_buf();
_M_set_length(0);
}
@@ -678,7 +688,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
{
if (__str._M_is_local())
{
- (void)_M_use_local_data();
+ _M_init_local_buf();
traits_type::copy(_M_local_buf, __str._M_local_buf,
__str.length() + 1);
}
@@ -718,7 +728,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
{
if (__str._M_is_local())
{
- (void)_M_use_local_data();
+ _M_init_local_buf();
traits_type::copy(_M_local_buf, __str._M_local_buf,
__str.length() + 1);
_M_length(__str.length());
diff --git a/libstdc++-v3/include/bits/basic_string.tcc b/libstdc++-v3/include/bits/basic_string.tcc
index 4bc98f2aea7..052eeb9e846 100644
--- a/libstdc++-v3/include/bits/basic_string.tcc
+++ b/libstdc++-v3/include/bits/basic_string.tcc
@@ -79,7 +79,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
}
else if (__s.length())
{
- (void)_M_use_local_data();
+ _M_init_local_buf();
traits_type::copy(_M_local_buf, __s._M_local_buf,
__s.length() + 1);
_M_length(__s.length());
@@ -88,7 +88,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
}
else if (length())
{
- (void)__s._M_use_local_data();
+ __s._M_init_local_buf();
traits_type::copy(__s._M_local_buf, _M_local_buf,
length() + 1);
__s._M_length(length());
@@ -99,7 +99,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
else
{
const size_type __tmp_capacity = __s._M_allocated_capacity;
- (void)__s._M_use_local_data();
+ __s._M_init_local_buf();
traits_type::copy(__s._M_local_buf, _M_local_buf,
length() + 1);
_M_data(__s._M_data());
@@ -111,7 +111,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
const size_type __tmp_capacity = _M_allocated_capacity;
if (__s._M_is_local())
{
- (void)_M_use_local_data();
+ _M_init_local_buf();
traits_type::copy(_M_local_buf, __s._M_local_buf,
__s.length() + 1);
__s._M_data(_M_data());
@@ -174,14 +174,18 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
size_type __len = 0;
size_type __capacity = size_type(_S_local_capacity);
- pointer __p = _M_use_local_data();
-
while (__beg != __end && __len < __capacity)
{
- __p[__len++] = *__beg;
+ _M_local_buf[__len++] = *__beg;
++__beg;
}
+#if __glibcxx_is_constant_evaluated
+ if (std::is_constant_evaluated())
+ for (size_type __i = __len; __i <= __capacity; ++__i)
+ _M_local_buf[__i] = _CharT();
+#endif
+
struct _Guard
{
_GLIBCXX20_CONSTEXPR
@@ -230,7 +234,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
_M_capacity(__dnew);
}
else
- _M_use_local_data();
+ _M_init_local_buf();
// Check for out_of_range and length_error exceptions.
struct _Guard
@@ -263,7 +267,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
_M_capacity(__n);
}
else
- _M_use_local_data();
+ _M_init_local_buf();
if (__n)
this->_S_assign(_M_data(), __n, __c);
@@ -372,7 +376,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
if (__length <= size_type(_S_local_capacity))
{
- this->_S_copy(_M_use_local_data(), _M_data(), __length + 1);
+ _M_init_local_buf();
+ this->_S_copy(_M_local_buf, _M_data(), __length + 1);
_M_destroy(__capacity);
_M_data(_M_local_data());
}
On Fri, Sep 29, 2023 at 04:06:33PM +0100, Jonathan Wakely wrote:
> On Fri, 29 Sept 2023 at 10:32, Jonathan Wakely <jwakely@redhat.com> wrote:
> > > Thanks for the comments, here's an updated version of the patch.
> > > Bootstrapped and regtested on x86_64-pc-linux-gnu.
> >
> > Great, I'll get this committed today - thanks!
>
> That's done now.
>
Thanks!
> > >
> > > I'll note that there are some existing calls to `_M_use_local_data()`
> > > already used only for their side effects without a cast to void, e.g.
> > >
> > > /**
> > > * @brief Default constructor creates an empty string.
> > > */
> > > _GLIBCXX20_CONSTEXPR
> > > basic_string()
> > > _GLIBCXX_NOEXCEPT_IF(is_nothrow_default_constructible<_Alloc>::value)
> > > : _M_dataplus(_M_local_data())
> > > {
> > > _M_use_local_data();
> > > _M_set_length(0);
> > > }
> > >
> > > I haven't updated these, but should this be changed for consistency?
> >
> > Yes, good idea. I can do that.
>
> I started to do that, and decided it made more sense to split out the
> constexpr loop from _M_use_local_data() into a separate function,
> _M_init_local_buf(). Then we can use that for all the places where we
> don't care about the return value. That avoids the overhead of using
> pointer_traits::pointer_to when we don't need the return value (which
> is admittedly only going to be an issue for -O0 code, but I think it's
> cleaner this way anyway).
>
> Please see the attached patch and let me know what you think.
I agree, and it also looks clearer to me what is happening.
>
> > Thanks again for fixing these. I think this might fix some bug reports
> > about clang rejecting our std::string in constant expressions, so I'll
> > check those.
>
> Your patch fixes https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110900
> (so we should backport it to gcc-13 and gcc-12 too).
> commit 2668979d3206ff6c039ac0165aae29377a15666c
> Author: Jonathan Wakely <jwakely@redhat.com>
> Date: Fri Sep 29 12:12:22 2023
>
> libstdc++: Split std::basic_string::_M_use_local_data into two functions
>
> This splits out the activate-the-union-member-for-constexpr logic from
> _M_use_local_data, so that it can be used separately in cases that don't
> need to use std::pointer_traits<pointer>::pointer_to to obtain the
> return value.
>
> This leaves only three uses of _M_use_local_data() which are all the
> same form:
>
> __s._M_data(_M_use_local_data());
> __s._M_set_length(0);
>
> We could remove _M_use_local_data() and change those three places to use
> a new _M_reset() function that does:
>
> _M_init_local_buf();
> _M_data(_M_local_data());
> _M_set_length(0);
>
> This is left for a future change.
>
> libstdc++-v3/ChangeLog:
>
> * include/bits/basic_string.h (_M_init_local_buf()): New
> function.
> (_M_use_local_data()): Use _M_init_local_buf.
> (basic_string(), basic_string(const Alloc&))
> (basic_string(basic_string&&))
> (basic_string(basic_string&&, const Alloc&)): Use
> _M_init_local_buf instead of _M_use_local_data().
> * include/bits/basic_string.tcc (swap(basic_string&))
> (_M_construct(InIter, InIter, forward_iterator_tag))
> (_M_construct(size_type, CharT), reserve()): Likewise.
> (_M_construct(InIter, InIter, input_iterator_tag)): Remove call
> to _M_use_local_data() and initialize the local buffer directly.
>
> diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
> index 4f94cd967cf..18a19b8dcbc 100644
> --- a/libstdc++-v3/include/bits/basic_string.h
> +++ b/libstdc++-v3/include/bits/basic_string.h
> @@ -353,13 +353,23 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> // Ensure that _M_local_buf is the active member of the union.
> __attribute__((__always_inline__))
> _GLIBCXX14_CONSTEXPR
> - pointer
> - _M_use_local_data() _GLIBCXX_NOEXCEPT
> + void
> + _M_init_local_buf() _GLIBCXX_NOEXCEPT
> {
> #if __cpp_lib_is_constant_evaluated
> if (std::is_constant_evaluated())
> for (size_type __i = 0; __i <= _S_local_capacity; ++__i)
> _M_local_buf[__i] = _CharT();
> +#endif
> + }
> +
> + __attribute__((__always_inline__))
> + _GLIBCXX14_CONSTEXPR
> + pointer
> + _M_use_local_data() _GLIBCXX_NOEXCEPT
> + {
> +#if __glibcxx_is_constant_evaluated
> + _M_init_local_buf();
> #endif
> return _M_local_data();
> }
What's the difference between __cpp_lib_is_constant_evaluated and
__glibcxx_is_constant_evaluated? Should these lines be using the same
macro here?
> @@ -522,7 +532,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> _GLIBCXX_NOEXCEPT_IF(is_nothrow_default_constructible<_Alloc>::value)
> : _M_dataplus(_M_local_data())
> {
> - _M_use_local_data();
> + _M_init_local_buf();
> _M_set_length(0);
> }
>
> @@ -534,7 +544,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> basic_string(const _Alloc& __a) _GLIBCXX_NOEXCEPT
> : _M_dataplus(_M_local_data(), __a)
> {
> - _M_use_local_data();
> + _M_init_local_buf();
> _M_set_length(0);
> }
>
> @@ -678,7 +688,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> {
> if (__str._M_is_local())
> {
> - (void)_M_use_local_data();
> + _M_init_local_buf();
> traits_type::copy(_M_local_buf, __str._M_local_buf,
> __str.length() + 1);
> }
> @@ -718,7 +728,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> {
> if (__str._M_is_local())
> {
> - (void)_M_use_local_data();
> + _M_init_local_buf();
> traits_type::copy(_M_local_buf, __str._M_local_buf,
> __str.length() + 1);
> _M_length(__str.length());
> diff --git a/libstdc++-v3/include/bits/basic_string.tcc b/libstdc++-v3/include/bits/basic_string.tcc
> index 4bc98f2aea7..052eeb9e846 100644
> --- a/libstdc++-v3/include/bits/basic_string.tcc
> +++ b/libstdc++-v3/include/bits/basic_string.tcc
> @@ -79,7 +79,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> }
> else if (__s.length())
> {
> - (void)_M_use_local_data();
> + _M_init_local_buf();
> traits_type::copy(_M_local_buf, __s._M_local_buf,
> __s.length() + 1);
> _M_length(__s.length());
> @@ -88,7 +88,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> }
> else if (length())
> {
> - (void)__s._M_use_local_data();
> + __s._M_init_local_buf();
> traits_type::copy(__s._M_local_buf, _M_local_buf,
> length() + 1);
> __s._M_length(length());
> @@ -99,7 +99,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> else
> {
> const size_type __tmp_capacity = __s._M_allocated_capacity;
> - (void)__s._M_use_local_data();
> + __s._M_init_local_buf();
> traits_type::copy(__s._M_local_buf, _M_local_buf,
> length() + 1);
> _M_data(__s._M_data());
> @@ -111,7 +111,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> const size_type __tmp_capacity = _M_allocated_capacity;
> if (__s._M_is_local())
> {
> - (void)_M_use_local_data();
> + _M_init_local_buf();
> traits_type::copy(_M_local_buf, __s._M_local_buf,
> __s.length() + 1);
> __s._M_data(_M_data());
> @@ -174,14 +174,18 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> size_type __len = 0;
> size_type __capacity = size_type(_S_local_capacity);
>
> - pointer __p = _M_use_local_data();
> -
> while (__beg != __end && __len < __capacity)
> {
> - __p[__len++] = *__beg;
> + _M_local_buf[__len++] = *__beg;
> ++__beg;
> }
>
> +#if __glibcxx_is_constant_evaluated
> + if (std::is_constant_evaluated())
> + for (size_type __i = __len; __i <= __capacity; ++__i)
> + _M_local_buf[__i] = _CharT();
> +#endif
> +
I wonder if maybe this should still be a call to `_M_init_local_buf()`
above, where the `_M_use_local_data()` used to be? That way the logic
stays in one place, and I don't imagine the compile time savings of not
immediately overwriting the first __len characters would be significant.
> struct _Guard
> {
> _GLIBCXX20_CONSTEXPR
> @@ -230,7 +234,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> _M_capacity(__dnew);
> }
> else
> - _M_use_local_data();
> + _M_init_local_buf();
>
> // Check for out_of_range and length_error exceptions.
> struct _Guard
> @@ -263,7 +267,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> _M_capacity(__n);
> }
> else
> - _M_use_local_data();
> + _M_init_local_buf();
>
> if (__n)
> this->_S_assign(_M_data(), __n, __c);
> @@ -372,7 +376,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>
> if (__length <= size_type(_S_local_capacity))
> {
> - this->_S_copy(_M_use_local_data(), _M_data(), __length + 1);
> + _M_init_local_buf();
> + this->_S_copy(_M_local_buf, _M_data(), __length + 1);
> _M_destroy(__capacity);
> _M_data(_M_local_data());
> }
But looks good to me; I tried applying this on top of my frontend patch
and it looks like everything still works correctly.
On Fri, 29 Sept 2023 at 17:29, Nathaniel Shead
<nathanieloshead@gmail.com> wrote:
>
> On Fri, Sep 29, 2023 at 04:06:33PM +0100, Jonathan Wakely wrote:
> > On Fri, 29 Sept 2023 at 10:32, Jonathan Wakely <jwakely@redhat.com> wrote:
> > > > Thanks for the comments, here's an updated version of the patch.
> > > > Bootstrapped and regtested on x86_64-pc-linux-gnu.
> > >
> > > Great, I'll get this committed today - thanks!
> >
> > That's done now.
> >
>
> Thanks!
>
> > > >
> > > > I'll note that there are some existing calls to `_M_use_local_data()`
> > > > already used only for their side effects without a cast to void, e.g.
> > > >
> > > > /**
> > > > * @brief Default constructor creates an empty string.
> > > > */
> > > > _GLIBCXX20_CONSTEXPR
> > > > basic_string()
> > > > _GLIBCXX_NOEXCEPT_IF(is_nothrow_default_constructible<_Alloc>::value)
> > > > : _M_dataplus(_M_local_data())
> > > > {
> > > > _M_use_local_data();
> > > > _M_set_length(0);
> > > > }
> > > >
> > > > I haven't updated these, but should this be changed for consistency?
> > >
> > > Yes, good idea. I can do that.
> >
> > I started to do that, and decided it made more sense to split out the
> > constexpr loop from _M_use_local_data() into a separate function,
> > _M_init_local_buf(). Then we can use that for all the places where we
> > don't care about the return value. That avoids the overhead of using
> > pointer_traits::pointer_to when we don't need the return value (which
> > is admittedly only going to be an issue for -O0 code, but I think it's
> > cleaner this way anyway).
> >
> > Please see the attached patch and let me know what you think.
>
> I agree, and it also looks clearer to me what is happening.
Good, I'll make this change next week then.
>
> >
> > > Thanks again for fixing these. I think this might fix some bug reports
> > > about clang rejecting our std::string in constant expressions, so I'll
> > > check those.
> >
> > Your patch fixes https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110900
> > (so we should backport it to gcc-13 and gcc-12 too).
>
> > commit 2668979d3206ff6c039ac0165aae29377a15666c
> > Author: Jonathan Wakely <jwakely@redhat.com>
> > Date: Fri Sep 29 12:12:22 2023
> >
> > libstdc++: Split std::basic_string::_M_use_local_data into two functions
> >
> > This splits out the activate-the-union-member-for-constexpr logic from
> > _M_use_local_data, so that it can be used separately in cases that don't
> > need to use std::pointer_traits<pointer>::pointer_to to obtain the
> > return value.
> >
> > This leaves only three uses of _M_use_local_data() which are all the
> > same form:
> >
> > __s._M_data(_M_use_local_data());
> > __s._M_set_length(0);
> >
> > We could remove _M_use_local_data() and change those three places to use
> > a new _M_reset() function that does:
> >
> > _M_init_local_buf();
> > _M_data(_M_local_data());
> > _M_set_length(0);
> >
> > This is left for a future change.
> >
> > libstdc++-v3/ChangeLog:
> >
> > * include/bits/basic_string.h (_M_init_local_buf()): New
> > function.
> > (_M_use_local_data()): Use _M_init_local_buf.
> > (basic_string(), basic_string(const Alloc&))
> > (basic_string(basic_string&&))
> > (basic_string(basic_string&&, const Alloc&)): Use
> > _M_init_local_buf instead of _M_use_local_data().
> > * include/bits/basic_string.tcc (swap(basic_string&))
> > (_M_construct(InIter, InIter, forward_iterator_tag))
> > (_M_construct(size_type, CharT), reserve()): Likewise.
> > (_M_construct(InIter, InIter, input_iterator_tag)): Remove call
> > to _M_use_local_data() and initialize the local buffer directly.
> >
> > diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
> > index 4f94cd967cf..18a19b8dcbc 100644
> > --- a/libstdc++-v3/include/bits/basic_string.h
> > +++ b/libstdc++-v3/include/bits/basic_string.h
> > @@ -353,13 +353,23 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> > // Ensure that _M_local_buf is the active member of the union.
> > __attribute__((__always_inline__))
> > _GLIBCXX14_CONSTEXPR
> > - pointer
> > - _M_use_local_data() _GLIBCXX_NOEXCEPT
> > + void
> > + _M_init_local_buf() _GLIBCXX_NOEXCEPT
> > {
> > #if __cpp_lib_is_constant_evaluated
> > if (std::is_constant_evaluated())
> > for (size_type __i = 0; __i <= _S_local_capacity; ++__i)
> > _M_local_buf[__i] = _CharT();
> > +#endif
> > + }
> > +
> > + __attribute__((__always_inline__))
> > + _GLIBCXX14_CONSTEXPR
> > + pointer
> > + _M_use_local_data() _GLIBCXX_NOEXCEPT
> > + {
> > +#if __glibcxx_is_constant_evaluated
> > + _M_init_local_buf();
> > #endif
> > return _M_local_data();
> > }
>
> What's the difference between __cpp_lib_is_constant_evaluated and
> __glibcxx_is_constant_evaluated? Should these lines be using the same
> macro here?
Ah, yeah, they could be the same.
The difference is that (for most feature test macros) the
__glibcxx_ftm form is defined after including <bits/version.h>, but
the __cpp_lib_ftm form is only defined only by headers that do:
#define __glibcxx_want_ftm
#include <bits/version.h>
This means we can ensure that the __cpp_lib_ftm form is only defined
by headers where we actually want to define it, e.g. in the headers
that [version.syn] in the standard says should define the macro. So
for our own internal uses, we should generally rely on the
__glibcxx_ftm one. Users should rely on __cpp_lib_ftm after including
the correct header. For example, __glibcxx_atomic_wait is defined
after including <bits/atomic_wait.h> and so is available for our own
uses, but __cpp_lib_atomic_wait is only defined after including
<atomic>, not just <bits/atomic_wait.h>. Or at least, that's the plan
- I have a pending patch to make everything I just said true :-)
Currently we "leak" the __cpp_lib_ftm forms into lots of <bits/xxx.h>
internal headers. That will change next week.
In this specific case, it doesn't matter, because
__cpp_lib_is_constant_evaluated is defined by <type_traits>, and every
header includes that. So it doesn't really matter whether our internal
uses are __cpp_lib_is_constant_evaluated or
__glibcxx_is_constant_evaluated. But it would be good to be
consistent.
>
> > @@ -522,7 +532,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> > _GLIBCXX_NOEXCEPT_IF(is_nothrow_default_constructible<_Alloc>::value)
> > : _M_dataplus(_M_local_data())
> > {
> > - _M_use_local_data();
> > + _M_init_local_buf();
> > _M_set_length(0);
> > }
> >
> > @@ -534,7 +544,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> > basic_string(const _Alloc& __a) _GLIBCXX_NOEXCEPT
> > : _M_dataplus(_M_local_data(), __a)
> > {
> > - _M_use_local_data();
> > + _M_init_local_buf();
> > _M_set_length(0);
> > }
> >
> > @@ -678,7 +688,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> > {
> > if (__str._M_is_local())
> > {
> > - (void)_M_use_local_data();
> > + _M_init_local_buf();
> > traits_type::copy(_M_local_buf, __str._M_local_buf,
> > __str.length() + 1);
> > }
> > @@ -718,7 +728,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> > {
> > if (__str._M_is_local())
> > {
> > - (void)_M_use_local_data();
> > + _M_init_local_buf();
> > traits_type::copy(_M_local_buf, __str._M_local_buf,
> > __str.length() + 1);
> > _M_length(__str.length());
> > diff --git a/libstdc++-v3/include/bits/basic_string.tcc b/libstdc++-v3/include/bits/basic_string.tcc
> > index 4bc98f2aea7..052eeb9e846 100644
> > --- a/libstdc++-v3/include/bits/basic_string.tcc
> > +++ b/libstdc++-v3/include/bits/basic_string.tcc
> > @@ -79,7 +79,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > }
> > else if (__s.length())
> > {
> > - (void)_M_use_local_data();
> > + _M_init_local_buf();
> > traits_type::copy(_M_local_buf, __s._M_local_buf,
> > __s.length() + 1);
> > _M_length(__s.length());
> > @@ -88,7 +88,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > }
> > else if (length())
> > {
> > - (void)__s._M_use_local_data();
> > + __s._M_init_local_buf();
> > traits_type::copy(__s._M_local_buf, _M_local_buf,
> > length() + 1);
> > __s._M_length(length());
> > @@ -99,7 +99,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > else
> > {
> > const size_type __tmp_capacity = __s._M_allocated_capacity;
> > - (void)__s._M_use_local_data();
> > + __s._M_init_local_buf();
> > traits_type::copy(__s._M_local_buf, _M_local_buf,
> > length() + 1);
> > _M_data(__s._M_data());
> > @@ -111,7 +111,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > const size_type __tmp_capacity = _M_allocated_capacity;
> > if (__s._M_is_local())
> > {
> > - (void)_M_use_local_data();
> > + _M_init_local_buf();
> > traits_type::copy(_M_local_buf, __s._M_local_buf,
> > __s.length() + 1);
> > __s._M_data(_M_data());
> > @@ -174,14 +174,18 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > size_type __len = 0;
> > size_type __capacity = size_type(_S_local_capacity);
> >
> > - pointer __p = _M_use_local_data();
> > -
> > while (__beg != __end && __len < __capacity)
> > {
> > - __p[__len++] = *__beg;
> > + _M_local_buf[__len++] = *__beg;
> > ++__beg;
> > }
> >
> > +#if __glibcxx_is_constant_evaluated
> > + if (std::is_constant_evaluated())
> > + for (size_type __i = __len; __i <= __capacity; ++__i)
> > + _M_local_buf[__i] = _CharT();
> > +#endif
> > +
>
> I wonder if maybe this should still be a call to `_M_init_local_buf()`
> above, where the `_M_use_local_data()` used to be? That way the logic
> stays in one place, and I don't imagine the compile time savings of not
> immediately overwriting the first __len characters would be significant.
Yeah, I went back and forth on that, but you're probably right. I'll
do as you suggested.
On Fri, 29 Sept 2023 at 17:46, Jonathan Wakely <jwakely.gcc@gmail.com> wrote:
>
> On Fri, 29 Sept 2023 at 17:29, Nathaniel Shead
> <nathanieloshead@gmail.com> wrote:
> >
> > On Fri, Sep 29, 2023 at 04:06:33PM +0100, Jonathan Wakely wrote:
> > > On Fri, 29 Sept 2023 at 10:32, Jonathan Wakely <jwakely@redhat.com> wrote:
> > > > > Thanks for the comments, here's an updated version of the patch.
> > > > > Bootstrapped and regtested on x86_64-pc-linux-gnu.
> > > >
> > > > Great, I'll get this committed today - thanks!
> > >
> > > That's done now.
> > >
> >
> > Thanks!
> >
> > > > >
> > > > > I'll note that there are some existing calls to `_M_use_local_data()`
> > > > > already used only for their side effects without a cast to void, e.g.
> > > > >
> > > > > /**
> > > > > * @brief Default constructor creates an empty string.
> > > > > */
> > > > > _GLIBCXX20_CONSTEXPR
> > > > > basic_string()
> > > > > _GLIBCXX_NOEXCEPT_IF(is_nothrow_default_constructible<_Alloc>::value)
> > > > > : _M_dataplus(_M_local_data())
> > > > > {
> > > > > _M_use_local_data();
> > > > > _M_set_length(0);
> > > > > }
> > > > >
> > > > > I haven't updated these, but should this be changed for consistency?
> > > >
> > > > Yes, good idea. I can do that.
> > >
> > > I started to do that, and decided it made more sense to split out the
> > > constexpr loop from _M_use_local_data() into a separate function,
> > > _M_init_local_buf(). Then we can use that for all the places where we
> > > don't care about the return value. That avoids the overhead of using
> > > pointer_traits::pointer_to when we don't need the return value (which
> > > is admittedly only going to be an issue for -O0 code, but I think it's
> > > cleaner this way anyway).
> > >
> > > Please see the attached patch and let me know what you think.
> >
> > I agree, and it also looks clearer to me what is happening.
>
> Good, I'll make this change next week then.
>
>
> >
> > >
> > > > Thanks again for fixing these. I think this might fix some bug reports
> > > > about clang rejecting our std::string in constant expressions, so I'll
> > > > check those.
> > >
> > > Your patch fixes https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110900
> > > (so we should backport it to gcc-13 and gcc-12 too).
> >
> > > commit 2668979d3206ff6c039ac0165aae29377a15666c
> > > Author: Jonathan Wakely <jwakely@redhat.com>
> > > Date: Fri Sep 29 12:12:22 2023
> > >
> > > libstdc++: Split std::basic_string::_M_use_local_data into two functions
> > >
> > > This splits out the activate-the-union-member-for-constexpr logic from
> > > _M_use_local_data, so that it can be used separately in cases that don't
> > > need to use std::pointer_traits<pointer>::pointer_to to obtain the
> > > return value.
> > >
> > > This leaves only three uses of _M_use_local_data() which are all the
> > > same form:
> > >
> > > __s._M_data(_M_use_local_data());
> > > __s._M_set_length(0);
> > >
> > > We could remove _M_use_local_data() and change those three places to use
> > > a new _M_reset() function that does:
> > >
> > > _M_init_local_buf();
> > > _M_data(_M_local_data());
> > > _M_set_length(0);
> > >
> > > This is left for a future change.
> > >
> > > libstdc++-v3/ChangeLog:
> > >
> > > * include/bits/basic_string.h (_M_init_local_buf()): New
> > > function.
> > > (_M_use_local_data()): Use _M_init_local_buf.
> > > (basic_string(), basic_string(const Alloc&))
> > > (basic_string(basic_string&&))
> > > (basic_string(basic_string&&, const Alloc&)): Use
> > > _M_init_local_buf instead of _M_use_local_data().
> > > * include/bits/basic_string.tcc (swap(basic_string&))
> > > (_M_construct(InIter, InIter, forward_iterator_tag))
> > > (_M_construct(size_type, CharT), reserve()): Likewise.
> > > (_M_construct(InIter, InIter, input_iterator_tag)): Remove call
> > > to _M_use_local_data() and initialize the local buffer directly.
> > >
> > > diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
> > > index 4f94cd967cf..18a19b8dcbc 100644
> > > --- a/libstdc++-v3/include/bits/basic_string.h
> > > +++ b/libstdc++-v3/include/bits/basic_string.h
> > > @@ -353,13 +353,23 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> > > // Ensure that _M_local_buf is the active member of the union.
> > > __attribute__((__always_inline__))
> > > _GLIBCXX14_CONSTEXPR
> > > - pointer
> > > - _M_use_local_data() _GLIBCXX_NOEXCEPT
> > > + void
> > > + _M_init_local_buf() _GLIBCXX_NOEXCEPT
> > > {
> > > #if __cpp_lib_is_constant_evaluated
> > > if (std::is_constant_evaluated())
> > > for (size_type __i = 0; __i <= _S_local_capacity; ++__i)
> > > _M_local_buf[__i] = _CharT();
> > > +#endif
> > > + }
> > > +
> > > + __attribute__((__always_inline__))
> > > + _GLIBCXX14_CONSTEXPR
> > > + pointer
> > > + _M_use_local_data() _GLIBCXX_NOEXCEPT
> > > + {
> > > +#if __glibcxx_is_constant_evaluated
> > > + _M_init_local_buf();
> > > #endif
> > > return _M_local_data();
> > > }
> >
> > What's the difference between __cpp_lib_is_constant_evaluated and
> > __glibcxx_is_constant_evaluated? Should these lines be using the same
> > macro here?
>
> Ah, yeah, they could be the same.
>
> The difference is that (for most feature test macros) the
> __glibcxx_ftm form is defined after including <bits/version.h>, but
> the __cpp_lib_ftm form is only defined only by headers that do:
>
> #define __glibcxx_want_ftm
> #include <bits/version.h>
>
> This means we can ensure that the __cpp_lib_ftm form is only defined
> by headers where we actually want to define it, e.g. in the headers
> that [version.syn] in the standard says should define the macro. So
> for our own internal uses, we should generally rely on the
> __glibcxx_ftm one. Users should rely on __cpp_lib_ftm after including
> the correct header. For example, __glibcxx_atomic_wait is defined
> after including <bits/atomic_wait.h> and so is available for our own
> uses, but __cpp_lib_atomic_wait is only defined after including
> <atomic>, not just <bits/atomic_wait.h>. Or at least, that's the plan
> - I have a pending patch to make everything I just said true :-)
> Currently we "leak" the __cpp_lib_ftm forms into lots of <bits/xxx.h>
> internal headers. That will change next week.
>
> In this specific case, it doesn't matter, because
> __cpp_lib_is_constant_evaluated is defined by <type_traits>, and every
> header includes that. So it doesn't really matter whether our internal
> uses are __cpp_lib_is_constant_evaluated or
> __glibcxx_is_constant_evaluated. But it would be good to be
> consistent.
>
> >
> > > @@ -522,7 +532,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> > > _GLIBCXX_NOEXCEPT_IF(is_nothrow_default_constructible<_Alloc>::value)
> > > : _M_dataplus(_M_local_data())
> > > {
> > > - _M_use_local_data();
> > > + _M_init_local_buf();
> > > _M_set_length(0);
> > > }
> > >
> > > @@ -534,7 +544,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> > > basic_string(const _Alloc& __a) _GLIBCXX_NOEXCEPT
> > > : _M_dataplus(_M_local_data(), __a)
> > > {
> > > - _M_use_local_data();
> > > + _M_init_local_buf();
> > > _M_set_length(0);
> > > }
> > >
> > > @@ -678,7 +688,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> > > {
> > > if (__str._M_is_local())
> > > {
> > > - (void)_M_use_local_data();
> > > + _M_init_local_buf();
> > > traits_type::copy(_M_local_buf, __str._M_local_buf,
> > > __str.length() + 1);
> > > }
> > > @@ -718,7 +728,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> > > {
> > > if (__str._M_is_local())
> > > {
> > > - (void)_M_use_local_data();
> > > + _M_init_local_buf();
> > > traits_type::copy(_M_local_buf, __str._M_local_buf,
> > > __str.length() + 1);
> > > _M_length(__str.length());
> > > diff --git a/libstdc++-v3/include/bits/basic_string.tcc b/libstdc++-v3/include/bits/basic_string.tcc
> > > index 4bc98f2aea7..052eeb9e846 100644
> > > --- a/libstdc++-v3/include/bits/basic_string.tcc
> > > +++ b/libstdc++-v3/include/bits/basic_string.tcc
> > > @@ -79,7 +79,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > > }
> > > else if (__s.length())
> > > {
> > > - (void)_M_use_local_data();
> > > + _M_init_local_buf();
> > > traits_type::copy(_M_local_buf, __s._M_local_buf,
> > > __s.length() + 1);
> > > _M_length(__s.length());
> > > @@ -88,7 +88,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > > }
> > > else if (length())
> > > {
> > > - (void)__s._M_use_local_data();
> > > + __s._M_init_local_buf();
> > > traits_type::copy(__s._M_local_buf, _M_local_buf,
> > > length() + 1);
> > > __s._M_length(length());
> > > @@ -99,7 +99,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > > else
> > > {
> > > const size_type __tmp_capacity = __s._M_allocated_capacity;
> > > - (void)__s._M_use_local_data();
> > > + __s._M_init_local_buf();
> > > traits_type::copy(__s._M_local_buf, _M_local_buf,
> > > length() + 1);
> > > _M_data(__s._M_data());
> > > @@ -111,7 +111,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > > const size_type __tmp_capacity = _M_allocated_capacity;
> > > if (__s._M_is_local())
> > > {
> > > - (void)_M_use_local_data();
> > > + _M_init_local_buf();
> > > traits_type::copy(_M_local_buf, __s._M_local_buf,
> > > __s.length() + 1);
> > > __s._M_data(_M_data());
> > > @@ -174,14 +174,18 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> > > size_type __len = 0;
> > > size_type __capacity = size_type(_S_local_capacity);
> > >
> > > - pointer __p = _M_use_local_data();
> > > -
> > > while (__beg != __end && __len < __capacity)
> > > {
> > > - __p[__len++] = *__beg;
> > > + _M_local_buf[__len++] = *__beg;
> > > ++__beg;
> > > }
> > >
> > > +#if __glibcxx_is_constant_evaluated
> > > + if (std::is_constant_evaluated())
> > > + for (size_type __i = __len; __i <= __capacity; ++__i)
> > > + _M_local_buf[__i] = _CharT();
> > > +#endif
> > > +
> >
> > I wonder if maybe this should still be a call to `_M_init_local_buf()`
> > above, where the `_M_use_local_data()` used to be? That way the logic
> > stays in one place, and I don't imagine the compile time savings of not
> > immediately overwriting the first __len characters would be significant.
>
>
> Yeah, I went back and forth on that, but you're probably right. I'll
> do as you suggested.
Here's what I've pushed to trunk after testing on x86_64-linux.
Thanks again for the fixes.
commit 405a4140fc30bce86b1ec885a98bb17704e0c8c6
Author: Jonathan Wakely <jwakely@redhat.com>
Date: Fri Sep 29 12:12:22 2023
libstdc++: Split std::basic_string::_M_use_local_data into two functions
This splits out the activate-the-union-member-for-constexpr logic from
_M_use_local_data, so that it can be used separately in cases that don't
need to use std::pointer_traits<pointer>::pointer_to to obtain the
return value.
This leaves only three uses of _M_use_local_data() which are all of the
same form:
__s._M_data(_M_use_local_data());
__s._M_set_length(0);
We could remove _M_use_local_data() and change those three places to use
a new _M_reset() function that does:
_M_init_local_buf();
_M_data(_M_local_data());
_M_set_length(0);
This is left for a future change.
libstdc++-v3/ChangeLog:
* include/bits/basic_string.h (_M_init_local_buf()): New
function.
(_M_use_local_data()): Use _M_init_local_buf.
(basic_string(), basic_string(const Alloc&))
(basic_string(basic_string&&))
(basic_string(basic_string&&, const Alloc&)): Use
_M_init_local_buf instead of _M_use_local_data().
* include/bits/basic_string.tcc (swap(basic_string&))
(_M_construct(InIter, InIter, input_iterator_tag))
(_M_construct(InIter, InIter, forward_iterator_tag))
(_M_construct(size_type, CharT), reserve()): Likewise.
diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
index 4f94cd967cf..0fa32afeb84 100644
--- a/libstdc++-v3/include/bits/basic_string.h
+++ b/libstdc++-v3/include/bits/basic_string.h
@@ -353,13 +353,23 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
// Ensure that _M_local_buf is the active member of the union.
__attribute__((__always_inline__))
_GLIBCXX14_CONSTEXPR
- pointer
- _M_use_local_data() _GLIBCXX_NOEXCEPT
+ void
+ _M_init_local_buf() _GLIBCXX_NOEXCEPT
{
#if __cpp_lib_is_constant_evaluated
if (std::is_constant_evaluated())
for (size_type __i = 0; __i <= _S_local_capacity; ++__i)
_M_local_buf[__i] = _CharT();
+#endif
+ }
+
+ __attribute__((__always_inline__))
+ _GLIBCXX14_CONSTEXPR
+ pointer
+ _M_use_local_data() _GLIBCXX_NOEXCEPT
+ {
+#if __cpp_lib_is_constant_evaluated
+ _M_init_local_buf();
#endif
return _M_local_data();
}
@@ -522,7 +532,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
_GLIBCXX_NOEXCEPT_IF(is_nothrow_default_constructible<_Alloc>::value)
: _M_dataplus(_M_local_data())
{
- _M_use_local_data();
+ _M_init_local_buf();
_M_set_length(0);
}
@@ -534,7 +544,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
basic_string(const _Alloc& __a) _GLIBCXX_NOEXCEPT
: _M_dataplus(_M_local_data(), __a)
{
- _M_use_local_data();
+ _M_init_local_buf();
_M_set_length(0);
}
@@ -678,7 +688,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
{
if (__str._M_is_local())
{
- (void)_M_use_local_data();
+ _M_init_local_buf();
traits_type::copy(_M_local_buf, __str._M_local_buf,
__str.length() + 1);
}
@@ -718,7 +728,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
{
if (__str._M_is_local())
{
- (void)_M_use_local_data();
+ _M_init_local_buf();
traits_type::copy(_M_local_buf, __str._M_local_buf,
__str.length() + 1);
_M_length(__str.length());
diff --git a/libstdc++-v3/include/bits/basic_string.tcc b/libstdc++-v3/include/bits/basic_string.tcc
index 4bc98f2aea7..f0a44e5e881 100644
--- a/libstdc++-v3/include/bits/basic_string.tcc
+++ b/libstdc++-v3/include/bits/basic_string.tcc
@@ -79,7 +79,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
}
else if (__s.length())
{
- (void)_M_use_local_data();
+ _M_init_local_buf();
traits_type::copy(_M_local_buf, __s._M_local_buf,
__s.length() + 1);
_M_length(__s.length());
@@ -88,7 +88,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
}
else if (length())
{
- (void)__s._M_use_local_data();
+ __s._M_init_local_buf();
traits_type::copy(__s._M_local_buf, _M_local_buf,
length() + 1);
__s._M_length(length());
@@ -99,7 +99,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
else
{
const size_type __tmp_capacity = __s._M_allocated_capacity;
- (void)__s._M_use_local_data();
+ __s._M_init_local_buf();
traits_type::copy(__s._M_local_buf, _M_local_buf,
length() + 1);
_M_data(__s._M_data());
@@ -111,7 +111,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
const size_type __tmp_capacity = _M_allocated_capacity;
if (__s._M_is_local())
{
- (void)_M_use_local_data();
+ _M_init_local_buf();
traits_type::copy(_M_local_buf, __s._M_local_buf,
__s.length() + 1);
__s._M_data(_M_data());
@@ -174,11 +174,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
size_type __len = 0;
size_type __capacity = size_type(_S_local_capacity);
- pointer __p = _M_use_local_data();
+ _M_init_local_buf();
while (__beg != __end && __len < __capacity)
{
- __p[__len++] = *__beg;
+ _M_local_buf[__len++] = *__beg;
++__beg;
}
@@ -230,7 +230,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
_M_capacity(__dnew);
}
else
- _M_use_local_data();
+ _M_init_local_buf();
// Check for out_of_range and length_error exceptions.
struct _Guard
@@ -263,7 +263,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
_M_capacity(__n);
}
else
- _M_use_local_data();
+ _M_init_local_buf();
if (__n)
this->_S_assign(_M_data(), __n, __c);
@@ -372,7 +372,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
if (__length <= size_type(_S_local_capacity))
{
- this->_S_copy(_M_use_local_data(), _M_data(), __length + 1);
+ _M_init_local_buf();
+ this->_S_copy(_M_local_buf, _M_data(), __length + 1);
_M_destroy(__capacity);
_M_data(_M_local_data());
}
@@ -678,7 +678,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
{
if (__str._M_is_local())
{
- traits_type::copy(_M_local_buf, __str._M_local_buf,
+ traits_type::copy(_M_use_local_data(), __str._M_local_buf,
__str.length() + 1);
}
else
@@ -691,7 +691,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
// basic_stringbuf relies on writing into unallocated capacity so
// we mess up the contents if we put a '\0' in the string.
_M_length(__str.length());
- __str._M_data(__str._M_local_data());
+ __str._M_data(__str._M_use_local_data());
__str._M_set_length(0);
}
@@ -717,6 +717,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
{
if (__str._M_is_local())
{
+ _M_use_local_data();
traits_type::copy(_M_local_buf, __str._M_local_buf,
__str.length() + 1);
_M_length(__str.length());
@@ -728,7 +729,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
_M_data(__str._M_data());
_M_length(__str.length());
_M_capacity(__str._M_allocated_capacity);
- __str._M_data(__str._M_local_buf);
+ __str._M_data(__str._M_use_local_data());
__str._M_set_length(0);
}
else
@@ -79,7 +79,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
}
else if (__s.length())
{
- traits_type::copy(_M_local_buf, __s._M_local_buf,
+ traits_type::copy(_M_use_local_data(), __s._M_local_buf,
__s.length() + 1);
_M_length(__s.length());
__s._M_set_length(0);
@@ -87,7 +87,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
}
else if (length())
{
- traits_type::copy(__s._M_local_buf, _M_local_buf,
+ traits_type::copy(__s._M_use_local_data(), _M_local_buf,
length() + 1);
__s._M_length(length());
_M_set_length(0);
@@ -97,7 +97,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
else
{
const size_type __tmp_capacity = __s._M_allocated_capacity;
- traits_type::copy(__s._M_local_buf, _M_local_buf,
+ traits_type::copy(__s._M_use_local_data(), _M_local_buf,
length() + 1);
_M_data(__s._M_data());
__s._M_data(__s._M_local_buf);
@@ -108,7 +108,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
const size_type __tmp_capacity = _M_allocated_capacity;
if (__s._M_is_local())
{
- traits_type::copy(_M_local_buf, __s._M_local_buf,
+ traits_type::copy(_M_use_local_data(), __s._M_local_buf,
__s.length() + 1);
__s._M_data(_M_data());
_M_data(_M_local_buf);
@@ -320,6 +320,33 @@ namespace __variant
__get(_Variant&& __v) noexcept
{ return __variant::__get_n<_Np>(std::forward<_Variant>(__v)._M_u); }
+ // Gets the _Uninitialized to construct into for __u.
+ template<size_t _Np, typename _Union>
+ constexpr decltype(auto)
+ __construct_n(_Union& __u) noexcept
+ {
+ if constexpr (_Np == 0)
+ return &__u._M_first;
+ else if constexpr (_Np == 1)
+ {
+ std::_Construct(&__u._M_rest);
+ return &__u._M_rest._M_first;
+ }
+ else if constexpr (_Np == 2)
+ {
+ std::_Construct(&__u._M_rest);
+ std::_Construct(&__u._M_rest._M_rest);
+ return &__u._M_rest._M_rest._M_first;
+ }
+ else
+ {
+ std::_Construct(&__u._M_rest);
+ std::_Construct(&__u._M_rest._M_rest);
+ std::_Construct(&__u._M_rest._M_rest._M_rest);
+ return __variant::__construct_n<_Np - 3>(__u._M_rest._M_rest._M_rest);
+ }
+ }
+
template<typename... _Types>
struct _Traits
{
@@ -536,8 +563,9 @@ namespace __variant
__emplace(_Variant_storage<_Triv, _Types...>& __v, _Args&&... __args)
{
__v._M_reset();
- auto* __addr = std::__addressof(__variant::__get_n<_Np>(__v._M_u));
- std::_Construct(__addr, std::forward<_Args>(__args)...);
+ auto* __addr = __variant::__construct_n<_Np>(__v._M_u);
+ std::_Construct(__addr, in_place_index<0>,
+ std::forward<_Args>(__args)...);
// Construction didn't throw, so can set the new index now:
__v._M_index = _Np;
}