[v2] GCC-13/changes: Add note about iostream usage
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On 26/04/23 20:06 +0100, Jonathan Wakely wrote:
>On 26/04/23 09:53 -0700, Andrew Pinski wrote:
>>This adds a note about iostream usage so it does not catch others
>>in surpise like it has already.
>>
>>OK?
>
>Thanks, I agree we should add something, but have some comments below.
>
>>---
>>htdocs/gcc-13/changes.html | 5 +++++
>>1 file changed, 5 insertions(+)
>>
>>diff --git a/htdocs/gcc-13/changes.html b/htdocs/gcc-13/changes.html
>>index 70732ec0..7c83f7c4 100644
>>--- a/htdocs/gcc-13/changes.html
>>+++ b/htdocs/gcc-13/changes.html
>>@@ -25,6 +25,11 @@ You may also want to check out our
>><!-- .................................................................. -->
>><h2>Caveats</h2>
>><ul>
>>+ <li>libstdc++ uses constructors inside the library to initialize std::cout/std::cin, etc.
>>+ instead of having it done in each source which uses iostream header.
>
>We should use code font for std::cout, std::cin and iostream, and
>style it as <iostream> not just iostream.
>
>>+ This requires you to make sure the dynamic loader to load the new libstdc++v3 library
>>+ (examples of how to do this is to use -Wl,-rpath,... while linking or LD_LIBRARY_PATH
>>+ while running the program). </li>
>
>I think it would be better to link to https://gcc.gnu.org/onlinedocs/libstdc++/manual/using_dynamic_or_shared.html#manual.intro.using.linkage.dynamic
>
>How about:
>
> <li>For C++, construction of the global iostream objects
><code>std::cout</code>, <code>std::cin</code> etc. is now done
> inside the standard library, instead of in every source file that
> includes the <code><iostream></code> header. This change
> improves the start-up performance of C++ programs, but it means that
> code compiled with GCC 13.1 will crash if the correct version of
> <code>libstdc++.so</code> is not used at runtime. See the
> <a href="https://gcc.gnu.org/onlinedocs/libstdc++/manual/using_dynamic_or_shared.html#manual.intro.using.linkage.dynamic">documentation</a>
> about using the right <code>libstdc++.so</code> at runtime.
> </li>
Here's a proper patch proposal along those lines.
OK for wwwdocs?
commit cf408a8d7e9ee3c7efd5b4a3fa5697f4a85a036a
Author: Jonathan Wakely <jwakely@redhat.com>
Date: Fri Apr 28 13:47:12 2023 +0100
Add caveat about C++ iostream init changes (PR108969)
Co-authored-by: Andrew Pinski <apinski@marvell.com>
@@ -56,6 +56,18 @@ You may also want to check out our
<a href="https://gcc.gnu.org/onlinedocs/gcc-13.1.0/gcc/ARM-iWMMXt-Built-in-Functions.html">
iWMMXt built-in functions</a>.
</li>
+ <li>For C++, construction of the global iostream objects
+ <code>std::cout</code>, <code>std::cin</code> etc. is now done
+ inside the standard library, instead of in every source file that
+ includes the <code><iostream></code> header. This change
+ improves the start-up performance of C++ programs, but it means that
+ code compiled with GCC 13.1 will crash if the correct version of
+ <code>libstdc++.so</code> is not used at runtime. See the
+ <a href="https://gcc.gnu.org/onlinedocs/libstdc++/manual/using_dynamic_or_shared.html#manual.intro.using.linkage.dynamic">documentation</a>
+ about using the right <code>libstdc++.so</code> at runtime.
+ Future GCC releases will mitigate the problem so that the program
+ cannot be run at all with an older <code>libstdc++.so</code>.
+ </li>
</ul>