[v2] c++: Fix compile-time-hog in cp_fold_immediate_r [PR111660]
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Commit Message
On Thu, Oct 12, 2023 at 09:41:43PM -0400, Jason Merrill wrote:
> On 10/12/23 17:04, Marek Polacek wrote:
> > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> >
> > -- >8 --
> > My recent patch introducing cp_fold_immediate_r caused exponential
> > compile time with nested COND_EXPRs. The problem is that the COND_EXPR
> > case recursively walks the arms of a COND_EXPR, but after processing
> > both arms it doesn't end the walk; it proceeds to walk the
> > sub-expressions of the outermost COND_EXPR, triggering again walking
> > the arms of the nested COND_EXPR, and so on. This patch brings the
> > compile time down to about 0m0.033s.
> >
> > I've added some debug prints to make sure that the rest of cp_fold_r
> > is still performed as before.
> >
> > PR c++/111660
> >
> > gcc/cp/ChangeLog:
> >
> > * cp-gimplify.cc (cp_fold_immediate_r) <case COND_EXPR>: Return
> > integer_zero_node instead of break;.
> > (cp_fold_immediate): Return true if cp_fold_immediate_r returned
> > error_mark_node.
> >
> > gcc/testsuite/ChangeLog:
> >
> > * g++.dg/cpp0x/hog1.C: New test.
> > ---
> > gcc/cp/cp-gimplify.cc | 9 ++--
> > gcc/testsuite/g++.dg/cpp0x/hog1.C | 77 +++++++++++++++++++++++++++++++
> > 2 files changed, 82 insertions(+), 4 deletions(-)
> > create mode 100644 gcc/testsuite/g++.dg/cpp0x/hog1.C
> >
> > diff --git a/gcc/cp/cp-gimplify.cc b/gcc/cp/cp-gimplify.cc
> > index bdf6e5f98ff..ca622ca169a 100644
> > --- a/gcc/cp/cp-gimplify.cc
> > +++ b/gcc/cp/cp-gimplify.cc
> > @@ -1063,16 +1063,16 @@ cp_fold_immediate_r (tree *stmt_p, int *walk_subtrees, void *data_)
> > break;
> > if (TREE_OPERAND (stmt, 1)
> > && cp_walk_tree (&TREE_OPERAND (stmt, 1), cp_fold_immediate_r, data,
> > - nullptr))
> > + nullptr) == error_mark_node)
> > return error_mark_node;
> > if (TREE_OPERAND (stmt, 2)
> > && cp_walk_tree (&TREE_OPERAND (stmt, 2), cp_fold_immediate_r, data,
> > - nullptr))
> > + nullptr) == error_mark_node)
> > return error_mark_node;
> > /* We're done here. Don't clear *walk_subtrees here though: we're called
> > from cp_fold_r and we must let it recurse on the expression with
> > cp_fold. */
> > - break;
> > + return integer_zero_node;
>
> I'm concerned this will end up missing something like
>
> 1 ? 1 : ((1 ? 1 : 1), immediate())
>
> as the integer_zero_node from the inner ?: will prevent walk_tree from
> looking any farther.
You are right. The line above works as expected, but
1 ? 1 : ((1 ? 1 : id (42)), id (i));
shows the problem (when the expression isn't used as an initializer).
> Maybe we want to handle COND_EXPR in cp_fold_r instead of here?
I hope this version is better.
Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
-- >8 --
My recent patch introducing cp_fold_immediate_r caused exponential
compile time with nested COND_EXPRs. The problem is that the COND_EXPR
case recursively walks the arms of a COND_EXPR, but after processing
both arms it doesn't end the walk; it proceeds to walk the
sub-expressions of the outermost COND_EXPR, triggering again walking
the arms of the nested COND_EXPR, and so on. This patch brings the
compile time down to about 0m0.033s.
The ff_fold_immediate flag is unused after this patch but since I'm
using it in the P2564 patch, I'm not removing it now.
PR c++/111660
gcc/cp/ChangeLog:
* cp-gimplify.cc (cp_fold_immediate_r) <case COND_EXPR>: Don't
handle it here.
(cp_fold_r): Handle COND_EXPR here.
gcc/testsuite/ChangeLog:
* g++.dg/cpp0x/hog1.C: New test.
* g++.dg/cpp2a/consteval36.C: New test.
---
gcc/cp/cp-gimplify.cc | 38 +++++-------
gcc/testsuite/g++.dg/cpp0x/hog1.C | 77 ++++++++++++++++++++++++
gcc/testsuite/g++.dg/cpp2a/consteval36.C | 18 ++++++
3 files changed, 111 insertions(+), 22 deletions(-)
create mode 100644 gcc/testsuite/g++.dg/cpp0x/hog1.C
create mode 100644 gcc/testsuite/g++.dg/cpp2a/consteval36.C
base-commit: 8be20f3b0bded7f9b690b27cbee58b283dbe827b
Comments
On 10/13/23 14:53, Marek Polacek wrote:
> On Thu, Oct 12, 2023 at 09:41:43PM -0400, Jason Merrill wrote:
>> On 10/12/23 17:04, Marek Polacek wrote:
>>> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
>>>
>>> -- >8 --
>>> My recent patch introducing cp_fold_immediate_r caused exponential
>>> compile time with nested COND_EXPRs. The problem is that the COND_EXPR
>>> case recursively walks the arms of a COND_EXPR, but after processing
>>> both arms it doesn't end the walk; it proceeds to walk the
>>> sub-expressions of the outermost COND_EXPR, triggering again walking
>>> the arms of the nested COND_EXPR, and so on. This patch brings the
>>> compile time down to about 0m0.033s.
>>>
>>> I've added some debug prints to make sure that the rest of cp_fold_r
>>> is still performed as before.
>>>
>>> PR c++/111660
>>>
>>> gcc/cp/ChangeLog:
>>>
>>> * cp-gimplify.cc (cp_fold_immediate_r) <case COND_EXPR>: Return
>>> integer_zero_node instead of break;.
>>> (cp_fold_immediate): Return true if cp_fold_immediate_r returned
>>> error_mark_node.
>>>
>>> gcc/testsuite/ChangeLog:
>>>
>>> * g++.dg/cpp0x/hog1.C: New test.
>>> ---
>>> gcc/cp/cp-gimplify.cc | 9 ++--
>>> gcc/testsuite/g++.dg/cpp0x/hog1.C | 77 +++++++++++++++++++++++++++++++
>>> 2 files changed, 82 insertions(+), 4 deletions(-)
>>> create mode 100644 gcc/testsuite/g++.dg/cpp0x/hog1.C
>>>
>>> diff --git a/gcc/cp/cp-gimplify.cc b/gcc/cp/cp-gimplify.cc
>>> index bdf6e5f98ff..ca622ca169a 100644
>>> --- a/gcc/cp/cp-gimplify.cc
>>> +++ b/gcc/cp/cp-gimplify.cc
>>> @@ -1063,16 +1063,16 @@ cp_fold_immediate_r (tree *stmt_p, int *walk_subtrees, void *data_)
>>> break;
>>> if (TREE_OPERAND (stmt, 1)
>>> && cp_walk_tree (&TREE_OPERAND (stmt, 1), cp_fold_immediate_r, data,
>>> - nullptr))
>>> + nullptr) == error_mark_node)
>>> return error_mark_node;
>>> if (TREE_OPERAND (stmt, 2)
>>> && cp_walk_tree (&TREE_OPERAND (stmt, 2), cp_fold_immediate_r, data,
>>> - nullptr))
>>> + nullptr) == error_mark_node)
>>> return error_mark_node;
>>> /* We're done here. Don't clear *walk_subtrees here though: we're called
>>> from cp_fold_r and we must let it recurse on the expression with
>>> cp_fold. */
>>> - break;
>>> + return integer_zero_node;
>>
>> I'm concerned this will end up missing something like
>>
>> 1 ? 1 : ((1 ? 1 : 1), immediate())
>>
>> as the integer_zero_node from the inner ?: will prevent walk_tree from
>> looking any farther.
>
> You are right. The line above works as expected, but
>
> 1 ? 1 : ((1 ? 1 : id (42)), id (i));
>
> shows the problem (when the expression isn't used as an initializer).
>
>> Maybe we want to handle COND_EXPR in cp_fold_r instead of here?
>
> I hope this version is better.
>
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
>
> -- >8 --
> My recent patch introducing cp_fold_immediate_r caused exponential
> compile time with nested COND_EXPRs. The problem is that the COND_EXPR
> case recursively walks the arms of a COND_EXPR, but after processing
> both arms it doesn't end the walk; it proceeds to walk the
> sub-expressions of the outermost COND_EXPR, triggering again walking
> the arms of the nested COND_EXPR, and so on. This patch brings the
> compile time down to about 0m0.033s.
Is this number still accurate for this version?
This change seems algorithmically better than the current code, but
still problematic: if we have nested COND_EXPR A/B/C/D/E, it looks like
we will end up cp_fold_immediate_r walking the arms of E five times,
once for each COND_EXPR.
What I was thinking by handling COND_EXPR in cp_fold_r was to cp_fold_r
walk its subtrees (or cp_fold_immediate_r if it's clear from op0 that
the branch isn't taken) so we can clear *walk_subtrees and we don't
fold_imm walk a node more than once.
> The ff_fold_immediate flag is unused after this patch but since I'm
> using it in the P2564 patch, I'm not removing it now.
>
> PR c++/111660
>
> gcc/cp/ChangeLog:
>
> * cp-gimplify.cc (cp_fold_immediate_r) <case COND_EXPR>: Don't
> handle it here.
> (cp_fold_r): Handle COND_EXPR here.
>
> gcc/testsuite/ChangeLog:
>
> * g++.dg/cpp0x/hog1.C: New test.
> * g++.dg/cpp2a/consteval36.C: New test.
> ---
> gcc/cp/cp-gimplify.cc | 38 +++++-------
> gcc/testsuite/g++.dg/cpp0x/hog1.C | 77 ++++++++++++++++++++++++
> gcc/testsuite/g++.dg/cpp2a/consteval36.C | 18 ++++++
> 3 files changed, 111 insertions(+), 22 deletions(-)
> create mode 100644 gcc/testsuite/g++.dg/cpp0x/hog1.C
> create mode 100644 gcc/testsuite/g++.dg/cpp2a/consteval36.C
>
> diff --git a/gcc/cp/cp-gimplify.cc b/gcc/cp/cp-gimplify.cc
> index bdf6e5f98ff..801cba141cb 100644
> --- a/gcc/cp/cp-gimplify.cc
> +++ b/gcc/cp/cp-gimplify.cc
> @@ -1052,27 +1052,6 @@ cp_fold_immediate_r (tree *stmt_p, int *walk_subtrees, void *data_)
>
> switch (TREE_CODE (stmt))
> {
> - /* Unfortunately we must handle code like
> - false ? bar () : 42
> - where we have to check bar too. The cp_fold call in cp_fold_r could
> - fold the ?: into a constant before we see it here. */
> - case COND_EXPR:
> - /* If we are called from cp_fold_immediate, we don't need to worry about
> - cp_fold folding away the COND_EXPR. */
> - if (data->flags & ff_fold_immediate)
> - break;
> - if (TREE_OPERAND (stmt, 1)
> - && cp_walk_tree (&TREE_OPERAND (stmt, 1), cp_fold_immediate_r, data,
> - nullptr))
> - return error_mark_node;
> - if (TREE_OPERAND (stmt, 2)
> - && cp_walk_tree (&TREE_OPERAND (stmt, 2), cp_fold_immediate_r, data,
> - nullptr))
> - return error_mark_node;
> - /* We're done here. Don't clear *walk_subtrees here though: we're called
> - from cp_fold_r and we must let it recurse on the expression with
> - cp_fold. */
> - break;
> case PTRMEM_CST:
> if (TREE_CODE (PTRMEM_CST_MEMBER (stmt)) == FUNCTION_DECL
> && DECL_IMMEDIATE_FUNCTION_P (PTRMEM_CST_MEMBER (stmt)))
> @@ -1163,7 +1142,22 @@ cp_fold_r (tree *stmt_p, int *walk_subtrees, void *data_)
> enum tree_code code = TREE_CODE (stmt);
>
> if (cxx_dialect > cxx17)
> - cp_fold_immediate_r (stmt_p, walk_subtrees, data);
> + {
> + /* Unfortunately we must handle code like
> + false ? bar () : 42
> + where we have to check bar too. The cp_fold call below could
> + fold the ?: into a constant before we've checked it. */
> + if (code == COND_EXPR)
> + {
> + if (TREE_OPERAND (stmt, 1))
> + cp_walk_tree (&TREE_OPERAND (stmt, 1), cp_fold_immediate_r, data,
> + nullptr);
> + if (TREE_OPERAND (stmt, 2))
> + cp_walk_tree (&TREE_OPERAND (stmt, 2), cp_fold_immediate_r, data,
> + nullptr);
> + }
> + cp_fold_immediate_r (stmt_p, walk_subtrees, data);
> + }
>
> *stmt_p = stmt = cp_fold (*stmt_p, data->flags);
>
> diff --git a/gcc/testsuite/g++.dg/cpp0x/hog1.C b/gcc/testsuite/g++.dg/cpp0x/hog1.C
> new file mode 100644
> index 00000000000..105a2e912c4
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp0x/hog1.C
> @@ -0,0 +1,77 @@
> +// PR c++/111660
> +// { dg-do compile { target c++11 } }
> +
> +enum Value {
> + LPAREN,
> + RPAREN,
> + LBRACE,
> + RBRACE,
> + LBRACK,
> + RBRACK,
> + CONDITIONAL,
> + COLON,
> + SEMICOLON,
> + COMMA,
> + PERIOD,
> + BIT_OR,
> + BIT_AND,
> + BIT_XOR,
> + BIT_NOT,
> + NOT,
> + LT,
> + GT,
> + MOD,
> + ASSIGN,
> + ADD,
> + SUB,
> + MUL,
> + DIV,
> + PRIVATE_NAME,
> + STRING,
> + TEMPLATE_SPAN,
> + IDENTIFIER,
> + WHITESPACE,
> + ILLEGAL,
> +};
> +
> +constexpr Value GetOneCharToken(char c) {
> + return
> + c == '(' ? LPAREN :
> + c == ')' ? RPAREN :
> + c == '{' ? LBRACE :
> + c == '}' ? RBRACE :
> + c == '[' ? LBRACK :
> + c == ']' ? RBRACK :
> + c == '?' ? CONDITIONAL :
> + c == ':' ? COLON :
> + c == ';' ? SEMICOLON :
> + c == ',' ? COMMA :
> + c == '.' ? PERIOD :
> + c == '|' ? BIT_OR :
> + c == '&' ? BIT_AND :
> + c == '^' ? BIT_XOR :
> + c == '~' ? BIT_NOT :
> + c == '!' ? NOT :
> + c == '<' ? LT :
> + c == '>' ? GT :
> + c == '%' ? MOD :
> + c == '=' ? ASSIGN :
> + c == '+' ? ADD :
> + c == '-' ? SUB :
> + c == '*' ? MUL :
> + c == '/' ? DIV :
> + c == '#' ? PRIVATE_NAME :
> + c == '"' ? STRING :
> + c == '\'' ? STRING :
> + c == '`' ? TEMPLATE_SPAN :
> + c == '\\' ? IDENTIFIER :
> + c == ' ' ? WHITESPACE :
> + c == '\t' ? WHITESPACE :
> + c == '\v' ? WHITESPACE :
> + c == '\f' ? WHITESPACE :
> + c == '\r' ? WHITESPACE :
> + c == '\n' ? WHITESPACE :
> + ILLEGAL;
> +}
> +
> +int main() {}
> diff --git a/gcc/testsuite/g++.dg/cpp2a/consteval36.C b/gcc/testsuite/g++.dg/cpp2a/consteval36.C
> new file mode 100644
> index 00000000000..d6aea214d61
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/consteval36.C
> @@ -0,0 +1,18 @@
> +// PR c++/111660
> +// { dg-do compile { target c++20 } }
> +
> +consteval int id (int i) { return i; }
> +
> +void
> +g (int i)
> +{
> + 1 ? 1 : ((1 ? 1 : 1), id (i)); // { dg-error "'i' is not a constant expression" }
> + 1 ? 1 : ((1 ? 1 : 1), id (i), 1); // { dg-error "'i' is not a constant expression" }
> + 1 ? 1 : ((i ? 1 : 1), id (i), 1); // { dg-error "'i' is not a constant expression" }
> + 1 ? 1 : ((1 ? i : 1), id (i), 1); // { dg-error "'i' is not a constant expression" }
> + 1 ? 1 : ((1 ? 1 : i), id (i), 1); // { dg-error "'i' is not a constant expression" }
> + 1 ? 1 : ((i ? -i : i), id (i), 1); // { dg-error "'i' is not a constant expression" }
> + 1 ? 1 : ((1 ? 1 : id (i)), id (42), 1); // { dg-error "'i' is not a constant expression" }
> + 1 ? 1 : ((1 ? 1 : id (42)), id (i)); // { dg-error "'i' is not a constant expression" }
> + 1 ? 1 : ((1 ? 1 : id (42)), id (i), 1); // { dg-error "'i' is not a constant expression" }
> +}
>
> base-commit: 8be20f3b0bded7f9b690b27cbee58b283dbe827b
@@ -1052,27 +1052,6 @@ cp_fold_immediate_r (tree *stmt_p, int *walk_subtrees, void *data_)
switch (TREE_CODE (stmt))
{
- /* Unfortunately we must handle code like
- false ? bar () : 42
- where we have to check bar too. The cp_fold call in cp_fold_r could
- fold the ?: into a constant before we see it here. */
- case COND_EXPR:
- /* If we are called from cp_fold_immediate, we don't need to worry about
- cp_fold folding away the COND_EXPR. */
- if (data->flags & ff_fold_immediate)
- break;
- if (TREE_OPERAND (stmt, 1)
- && cp_walk_tree (&TREE_OPERAND (stmt, 1), cp_fold_immediate_r, data,
- nullptr))
- return error_mark_node;
- if (TREE_OPERAND (stmt, 2)
- && cp_walk_tree (&TREE_OPERAND (stmt, 2), cp_fold_immediate_r, data,
- nullptr))
- return error_mark_node;
- /* We're done here. Don't clear *walk_subtrees here though: we're called
- from cp_fold_r and we must let it recurse on the expression with
- cp_fold. */
- break;
case PTRMEM_CST:
if (TREE_CODE (PTRMEM_CST_MEMBER (stmt)) == FUNCTION_DECL
&& DECL_IMMEDIATE_FUNCTION_P (PTRMEM_CST_MEMBER (stmt)))
@@ -1163,7 +1142,22 @@ cp_fold_r (tree *stmt_p, int *walk_subtrees, void *data_)
enum tree_code code = TREE_CODE (stmt);
if (cxx_dialect > cxx17)
- cp_fold_immediate_r (stmt_p, walk_subtrees, data);
+ {
+ /* Unfortunately we must handle code like
+ false ? bar () : 42
+ where we have to check bar too. The cp_fold call below could
+ fold the ?: into a constant before we've checked it. */
+ if (code == COND_EXPR)
+ {
+ if (TREE_OPERAND (stmt, 1))
+ cp_walk_tree (&TREE_OPERAND (stmt, 1), cp_fold_immediate_r, data,
+ nullptr);
+ if (TREE_OPERAND (stmt, 2))
+ cp_walk_tree (&TREE_OPERAND (stmt, 2), cp_fold_immediate_r, data,
+ nullptr);
+ }
+ cp_fold_immediate_r (stmt_p, walk_subtrees, data);
+ }
*stmt_p = stmt = cp_fold (*stmt_p, data->flags);
new file mode 100644
@@ -0,0 +1,77 @@
+// PR c++/111660
+// { dg-do compile { target c++11 } }
+
+enum Value {
+ LPAREN,
+ RPAREN,
+ LBRACE,
+ RBRACE,
+ LBRACK,
+ RBRACK,
+ CONDITIONAL,
+ COLON,
+ SEMICOLON,
+ COMMA,
+ PERIOD,
+ BIT_OR,
+ BIT_AND,
+ BIT_XOR,
+ BIT_NOT,
+ NOT,
+ LT,
+ GT,
+ MOD,
+ ASSIGN,
+ ADD,
+ SUB,
+ MUL,
+ DIV,
+ PRIVATE_NAME,
+ STRING,
+ TEMPLATE_SPAN,
+ IDENTIFIER,
+ WHITESPACE,
+ ILLEGAL,
+};
+
+constexpr Value GetOneCharToken(char c) {
+ return
+ c == '(' ? LPAREN :
+ c == ')' ? RPAREN :
+ c == '{' ? LBRACE :
+ c == '}' ? RBRACE :
+ c == '[' ? LBRACK :
+ c == ']' ? RBRACK :
+ c == '?' ? CONDITIONAL :
+ c == ':' ? COLON :
+ c == ';' ? SEMICOLON :
+ c == ',' ? COMMA :
+ c == '.' ? PERIOD :
+ c == '|' ? BIT_OR :
+ c == '&' ? BIT_AND :
+ c == '^' ? BIT_XOR :
+ c == '~' ? BIT_NOT :
+ c == '!' ? NOT :
+ c == '<' ? LT :
+ c == '>' ? GT :
+ c == '%' ? MOD :
+ c == '=' ? ASSIGN :
+ c == '+' ? ADD :
+ c == '-' ? SUB :
+ c == '*' ? MUL :
+ c == '/' ? DIV :
+ c == '#' ? PRIVATE_NAME :
+ c == '"' ? STRING :
+ c == '\'' ? STRING :
+ c == '`' ? TEMPLATE_SPAN :
+ c == '\\' ? IDENTIFIER :
+ c == ' ' ? WHITESPACE :
+ c == '\t' ? WHITESPACE :
+ c == '\v' ? WHITESPACE :
+ c == '\f' ? WHITESPACE :
+ c == '\r' ? WHITESPACE :
+ c == '\n' ? WHITESPACE :
+ ILLEGAL;
+}
+
+int main() {}
new file mode 100644
@@ -0,0 +1,18 @@
+// PR c++/111660
+// { dg-do compile { target c++20 } }
+
+consteval int id (int i) { return i; }
+
+void
+g (int i)
+{
+ 1 ? 1 : ((1 ? 1 : 1), id (i)); // { dg-error "'i' is not a constant expression" }
+ 1 ? 1 : ((1 ? 1 : 1), id (i), 1); // { dg-error "'i' is not a constant expression" }
+ 1 ? 1 : ((i ? 1 : 1), id (i), 1); // { dg-error "'i' is not a constant expression" }
+ 1 ? 1 : ((1 ? i : 1), id (i), 1); // { dg-error "'i' is not a constant expression" }
+ 1 ? 1 : ((1 ? 1 : i), id (i), 1); // { dg-error "'i' is not a constant expression" }
+ 1 ? 1 : ((i ? -i : i), id (i), 1); // { dg-error "'i' is not a constant expression" }
+ 1 ? 1 : ((1 ? 1 : id (i)), id (42), 1); // { dg-error "'i' is not a constant expression" }
+ 1 ? 1 : ((1 ? 1 : id (42)), id (i)); // { dg-error "'i' is not a constant expression" }
+ 1 ? 1 : ((1 ? 1 : id (42)), id (i), 1); // { dg-error "'i' is not a constant expression" }
+}