x86/64 alignment, optimization, -Wcast-align

[Martin Sebor]

On 10/27/2016 12:37 AM, Mathieu Malaterre wrote:
> On Wed, Oct 26, 2016 at 4:01 PM, Martin Sebor <msebor@gmail.com> 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