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Re: x86/64 alignment, optimization, -Wcast-align
- From: Mathieu Malaterre <mathieu dot malaterre at gmail dot com>
- To: Martin Sebor <msebor at gmail dot com>
- Cc: Balázs Oroszi <orobalage at gmail dot com>, gcc-help <gcc-help at gcc dot gnu dot org>
- Date: Thu, 27 Oct 2016 08:37:23 +0200
- Subject: Re: x86/64 alignment, optimization, -Wcast-align
- Authentication-results: sourceware.org; auth=none
- References: <CA+1z_s_3-urv_NQY7amyvdApFLTJ2Y5e8_ub=+LmN=oL7+y4YQ@mail.gmail.com> <b5f45bdb-aaff-fcd5-d1b6-e0d30e38f88b@gmail.com>
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
--
Mathieu