[Bug c++/96594] New: Compiled code behaves differently with -O1 and -O0 on s390x

["mitya57 at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96594

            Bug ID: 96594
           Summary: Compiled code behaves differently with -O1 and -O0 on
                    s390x
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mitya57 at gmail dot com
  Target Milestone: ---

Created attachment 49050
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=49050&action=edit
Test case

Hi,

I was debugging Compiz test failures that started happening on Debian/Ubuntu
s390x in July. They are not directly related to GCC upgrade (old GCC 8 behaves
the same way), however I noticed a strange thing that I want to report here.

The attached file behaves differently when built with -O0 and -O1:

$ g++ test.cpp -g -O1 -lX11
$ xvfb-run ./a.out
setting value = 0x1020304
got value = 0xffffffff

$ g++ test.cpp -g -O0 -lX11
$ xvfb-run ./a.out
setting value = 0x1020304
got value = 0x0

And both these behaviors are wrong! Expected behavior is (as seen on x86_64):

$ xvfb-run ./a.out 
setting value = 0x1020304
got value = 0x1020304

However, this bug is about differing behaviors with -O1 and -O0.

The only dependency of this file is Xlib. So far I was not able to make an
example that does not depend on it.

I wanted to bisect a particular optimization that causes this behavior change,
but even with all options from
https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html#index-O1 it behaves
like -O0.

Some more things I noticed:

- Moving GetProperty and SetProperty functions away from the namespace makes it
behave like -O0.

- Adding __attribute__((noinline)) to SetProperty makes it behave like -O0. So
the bug may be related to inlining.

I am also attaching assembly dumps with -O1 and -O0.