Re: [committed] libstdc++: Disable cacheline alignment for DJGPP [PR109741]

[Martin Jambor <mjambor@suse.cz>]

Hello,

On Tue, May 16 2023, Jonathan Wakely via Gcc-patches wrote:
> Tested powerpc64le-linux. Builds OK on djgpp too.
>
> Pushed to trunk.
>
> -- >8 --
>
> DJGPP (and maybe other targets) uses MAX_OFILE_ALIGNMENT=16 which means
> that globals (and static objects) can't have alignment greater than 16.
> This causes an error for the locks defined in src/c++11/shared_ptr.cc
> because we try to align them to the cacheline size, to avoid false
> sharing.
>
> Add a configure check for the increased alignment, and live with false
> sharing where we can't increase the alignment.
>
> libstdc++-v3/ChangeLog:
>
> 	PR libstdc++/109741
> 	* acinclude.m4 (GLIBCXX_CHECK_ALIGNAS_CACHELINE): Define.
> 	* config.h.in: Regenerate.
> 	* configure: Regenerate.
> 	* configure.ac: Use GLIBCXX_CHECK_ALIGNAS_CACHELINE.
> 	* src/c++11/shared_ptr.cc (__gnu_internal::get_mutex): Do not
> 	align lock table if not supported. use __GCC_DESTRUCTIVE_SIZE
> 	instead of hardcoded 64.

The periodic tests that Martin Liška left behind for me (for now) to
look at are now complaining that the configure and configure.ac in
libstdc++ are not in sync, the difference being (drum roll):

diff --git a/libstdc++-v3/configure b/libstdc++-v3/configure
index 188be08d716..412c4bf0e85 100755
--- a/libstdc++-v3/configure
+++ b/libstdc++-v3/configure
@@ -71957,6 +71957,7 @@ _ACEOF
   fi
 
 
+# For src/c++11/shared_ptr.cc alignment.
 
 
   ac_ext=cpp


Can I commit this change or do I need to attempt to adjust the script to
ignore comments in configure or what would be the correct way to deal
with this? (Option requiring work on my side may take some time during
which the otherwise useful tests would not work, I am afraid).

Thanks,

Martin