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
