[Bug c/105449] New: suspicious optimization since GCC 10.1.0 from -

[Bug c/105449] New: suspicious optimization since GCC 10.1.0 from -

[gilles.gouaillardet at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105449

            Bug ID: 105449
           Summary: suspicious optimization since GCC 10.1.0 from -
           Product: gcc
           Version: 10.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gilles.gouaillardet at gmail dot com
  Target Milestone: ---

[Bug c/105449] suspicious optimization since GCC 10.1.0 from -

[gilles.gouaillardet at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105449

--- Comment #1 from Gilles Gouaillardet <gilles.gouaillardet at gmail dot com> ---
Created attachment 52916
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=52916&action=edit
a simple reproducer

[Bug c/105449] suspicious optimization since GCC 10.1.0 from -O2

[gilles.gouaillardet at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105449

Gilles Gouaillardet <gilles.gouaillardet at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |gilles.gouaillardet at gmail dot c
                   |                            |om
            Summary|suspicious optimization     |suspicious optimization
                   |since GCC 10.1.0 from -     |since GCC 10.1.0 from -O2

--- Comment #2 from Gilles Gouaillardet <gilles.gouaillardet at gmail dot com> ---
The attached reproducer works fine when using -O1 or less aggressive
optimization and on all GCC versions I tested.

However, starting from GCC 10.1.0 and when using -O2 or more aggressive
optimization, the local_pos() subroutine behaves differently compared to the
global_pos() and volatile_pos() subroutines. All these subroutines do virtually
the same things, but only the declaration of the bogus variable changes.

I suspect recent GCC versions perform some register inlining when the bogus
variable is declared as local and non volatile.


Can you please have a look at the reproducer and establish whether:
 - this reproducer has an undefined behavior, and the recent optimizations are
legit
 - this is a bug in GCC >= 10.1.0


For the record, here are my logs (on a x86_64 system, same behavior on aarch64.
Though I did not test, a different behavior is expected on a 64 bits big endian
processor).

$ gcc --version
gcc (GCC) 11.2.0
Copyright (C) 2021 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
$ gcc bug.c && ./a.out
global: 0
local: 0
volatile: 0

$ gcc -O2 bug.c && ./a.out
global: 0
local: 2   <--- UNEXPECTED VALUE
volatile: 0


FWIW, a consequence of this optimization is an issue that has been reported at
https://github.com/open-mpi/ompi/issues/10339

[Bug c/105449] suspicious optimization since GCC 10.1.0 from -O2

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105449

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |INVALID

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
You are violating C/C++ aliasing rules.

You have a store as "void*" but then access it as an "int*"
    void * bogus = (void*) 1;
    int *p = (int *) &bogus;
...
        if (p[pos] == 1) {

[Bug c/105449] suspicious optimization since GCC 10.1.0 from -O2

[gilles.gouaillardet at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105449

--- Comment #4 from Gilles Gouaillardet <gilles.gouaillardet at gmail dot com> ---
Thanks for the clarification!