[Bug c++/111714] New: Strange behavior when casting std::size_t to bool, UB or compiler bug?

[Bug c++/111714] New: Strange behavior when casting std::size_t to bool, UB or compiler bug?

[carlosgalvezp at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111714

            Bug ID: 111714
           Summary: Strange behavior when casting std::size_t to bool, UB
                    or compiler bug?
           Product: gcc
           Version: 9.4.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: carlosgalvezp at gmail dot com
  Target Milestone: ---

Created attachment 56067
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=56067&action=edit
Test case

Hi!

We are bumping from GCC 7.5 to 9.4 and one of our tests fail. The test works on
GCC 10 and above, as well as Clang trunk. Sanitizers (GCC and Clang) and
Valgrind don't report anything.

I have managed to create a minimal example here:
https://godbolt.org/z/3q7c4qYzE (attached to the bug as a file as well).

There's also comments about changes to the code that can make the test pass.

This smells a lot like UB, but I can't figure out what is wrong with the code.
Otherwise it could well be a compiler bug. Is there anything obvious you can
see that could explain the behavior?

Thanks!

[Bug c++/111714] Strange behavior when casting std::size_t to bool, UB or compiler bug?

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111714

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
looks like a bug in GCC 9.x, note that's EOL and thus will receive no fixes. 
You can try to bisect where it was fixed since GCC 10.1 seems to work.  There
might be a duplicate fixed bugreport for this.

[Bug c++/111714] Strange behavior when casting std::size_t to bool, UB or compiler bug?

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111714

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
From IV-OPTs dup:
inv_expr 3:     (-(<unnamed-signed:1>) _13 - (<unnamed-signed:1>) &input) - -1
inv_expr 4:     -(<unnamed-signed:1>) _13 - (<unnamed-signed:1>) &input

That is totally bogus (that was even in GCC 8)

[Bug tree-optimization/111714] Strange behavior when casting std::size_t to bool, UB or compiler bug?

[carlosgalvezp at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111714

--- Comment #3 from Carlos Galvez <carlosgalvezp at gmail dot com> ---
Thanks for the quick response! Unfortunately we are stuck on GCC 9 for reasons
so I'll try to shuffle the code around a bit to make it work :)

[Bug tree-optimization/111714] Strange behavior when casting std::size_t to bool, UB or compiler bug?

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111714

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |wrong-code

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Carlos Galvez from comment #3)
> Thanks for the quick response! Unfortunately we are stuck on GCC 9 for
> reasons so I'll try to shuffle the code around a bit to make it work :)

Using -fno-ivopts is another options since that is where the bad code is
introduced.