On 10/27/2016 12:37 AM, Mathieu Malaterre wrote:
On Wed, Oct 26, 2016 at 4:01 PM, Martin Sebor <msebor@xxxxxxxxx> wrote:
I believe -Wcast-align should be made to warn on x86/64 as well, in
the light of the above, as now it is the most dangerous territory,
where you only get crashes if the compiler decides to vectorize code
AND you get unaligned memory (the crash I got was VERY hard to
reproduce).
The problem I see is a conflict between the C standard (accesses must
be aligned for int, etc.) and the x86/64 machine (accesses need not be
aligned for int, etc.), and GCC optimizations using the former, while
the warning flag (-Wcast-align) taking the latter.
I would like more comments, ideas on this in general as well.
-Wcast-align depends on the STRICT_ALIGNMENT GCC macro being
non-zero. The macro is set to zero for i386 which effectively
disables the warning.
I agree that it would be useful to enable -Wcast-align even for
targets that tolerate unaligned accesses, not just for the reason
you mention but also to help detect invalid alignment assumptions
that may happen to be safe on the current target but not on others.
-fsanitize=alignment ?
https://gcc.gnu.org/onlinedocs/gcc/Instrumentation-Options.html
The sanitizer is a viable option when running the instrumented
program and exercising the code paths of interest is feasible,
and when the pointers actually are misaligned. When that's not
possible (e.g., when cross-compiling, or typically in embedded
software compiled with -ffreestanding or similar), -Wcast-align
is the only alternative.
Martin
