[Bug libgcc/60429] New: Miscompilation (aliasing) with -finline-functions

[linux at carewolf dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60429

            Bug ID: 60429
           Summary: Miscompilation (aliasing) with -finline-functions
           Product: gcc
           Version: 4.8.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libgcc
          Assignee: unassigned at gcc dot gnu.org
          Reporter: linux at carewolf dot com

After recently trying to build Qt with -O3, I found one of our tests failing.
After investigating I narrowed it down to qregion.cpp and the flag
-finline-functions (using -O2 -finline-functions).

Specificially the inlining of the function loadAET() in "QRegionPrivate
*PolygonRegion()" causes the problem. Adding __attribute__((noinline)) to
loadAET() solves the problem. Interestingly compiling without -finline-function
and just marking the loadAET as inline or always_inline, does not trigger the
issue.

The code is not Qt specific and is used in a other projects, though it is most
places compiled as C code.

Further testing shows the issue is also triggered in GCC 4.6, 4.7, and the
latest version of 4.9 I had. It is however NOT present in GCC 4.4, so this is a
regression.

Looking at the assembler and debug output, I believe the that the variable pAET
that should have been reloaded after loadAET() has been eliminated. This might
be what breaks the aliasing rules.