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From: "ebotcazou at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/67701] Unnecessary/bad instructions for u32-casted access to external symbol (assumes misaligned, superfluous load)
Date: Thu, 24 Sep 2015 15:14:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67701

--- Comment #7 from Eric Botcazou <ebotcazou at gcc dot gnu.org> ---
> Yes, AFAIK this was some obscure situation with SSE on x86.  IIRC
> code doing unaligned scalar accesses (which is ok on x86) but then
> vectorized using peeling for alignment (which cannot succeed if the
> element is not naturally aligned) and segfaulting for the emitted
> aligned move instructions.

I see, thanks for the insight.

> Maybe these days the legacy has been cleaned up enough so we can
> remove that conservative handling again...  I think it also causes
> us to handle
> 
> char c[4];
> 
> int main()
> {
>   if (!((unsigned long)c & 3))
>     return *(int *)c;
>   return c[0];
> }
> 
> too conservatively as we expand
> 
>   _5 = MEM[(int *)&c];
> 
> and thus lost the flow-sensitive info.

The problem is that, in order to fix a legitimate issue on x86, the change
pessimizes the code for strict-alignment platforms, where the said issue
doesn't exist since there are unaligned accesses in the source code.  And of
course only for them, since x86 has unaligned load/stores.  So, in the end,
this is a net loss for strict-alignment platforms.