[Bug middle-end/109849] suboptimal code for vector walking loop

["redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109849

--- Comment #28 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Martin Jambor from comment #27)
> Unfortunately I cannot reproduce this, the above (on pristine master
> commit 006e90e1344 on an x86_64-linux) results in:
> 
> Running target unix/-D_GLIBCXX_USE_CXX11_ABI=0
> Running
> /home/mjambor/gcc/small/src/libstdc++-v3/testsuite/libstdc++-dg/conformance.
> exp ...
> PASS: 21_strings/basic_string/operators/char/1.cc  -std=gnu++17 (test for
> excess errors)
> PASS: 21_strings/basic_string/operators/char/1.cc  -std=gnu++17 execution
> test

Oops, sorry, that particular FAIL needs either
--target_board=unix/-D_GLIBCXX_USE_CXX11_ABI=0/-D_GLIBCXX_DEBUG which then
makes it fail for all -std modes:


Schedule of variations:
    unix/-D_GLIBCXX_USE_CXX11_ABI=0/-D_GLIBCXX_DEBUG

Running target unix/-D_GLIBCXX_USE_CXX11_ABI=0/-D_GLIBCXX_DEBUG
Using /usr/share/dejagnu/baseboards/unix.exp as board description file for
target.
Using /usr/share/dejagnu/config/unix.exp as generic interface file for target.
Using /home/test/src/gcc/libstdc++-v3/testsuite/config/default.exp as
tool-and-target-specific interface file.
Running /home/test/src/gcc/libstdc++-v3/testsuite/libstdc++-dg/conformance.exp
...
FAIL: 21_strings/basic_string/operators/char/1.cc  -std=gnu++11 execution test
FAIL: 21_strings/basic_string/operators/char/1.cc  -std=gnu++14 execution test
FAIL: 21_strings/basic_string/operators/char/1.cc  -std=gnu++17 execution test
FAIL: 21_strings/basic_string/operators/char/1.cc  -std=gnu++20 execution test
FAIL: 21_strings/basic_string/operators/char/1.cc  -std=gnu++23 execution test
FAIL: 21_strings/basic_string/operators/char/1.cc  -std=gnu++26 execution test

Or just set GLIBCXX_TESTSUITE_STDS="17,20" in the env before running the test:

Schedule of variations:
    unix/-D_GLIBCXX_USE_CXX11_ABI=0

Running target unix/-D_GLIBCXX_USE_CXX11_ABI=0
Using /usr/share/dejagnu/baseboards/unix.exp as board description file for
target.
Using /usr/share/dejagnu/config/unix.exp as generic interface file for target.
Using /home/test/src/gcc/libstdc++-v3/testsuite/config/default.exp as
tool-and-target-specific interface file.
Running /home/test/src/gcc/libstdc++-v3/testsuite/libstdc++-dg/conformance.exp
...
FAIL: 21_strings/basic_string/operators/char/1.cc  -std=gnu++20 execution test

It seems I picked a bad example to give, which requires additional options to
FAIL.

Many of the other FAILs do not require _GLIBCXX_DEBUG or -std=gnu++20 to FAIL,
but the -D_GLIBCXX_USE_CXX11_ABI=0 option is necessary, at least for all the
ones I inspected. That option isn't used by default, but I run the full
testsuite with that several times a day, and with
GLIBCXX_TESTSUITE_STDS=98,11,14,17,20,23,26.


> Can you please try if
> https://gcc.gnu.org/pipermail/gcc-patches/2023-November/638318.html
> fixes this?

Testing now ...