[AArch64], patch] PR71727 fix -mstrict-align

[Andrew Pinski]

On Tue, Jul 18, 2017 at 5:50 AM, Christophe Lyon
<christophe.lyon@linaro.org> wrote:
> Hello,
>
> I've received a complaint that GCC for AArch64 would generate
> vectorized code relying on unaligned memory accesses even when using
> -mstrict-align. This is a problem for code where such accesses lead to
> memory faults.
>
> A previous patch (r243333) introduced
> aarch64_builtin_support_vector_misalignment, which rejects such
> accesses when the element size is 64 bits, and accept them otherwise,
> which I think it shouldn't. The testcase added at that time only used
> 64 bits elements, and therefore didn't fully test the patch.
>
> The report I received is about vectorized accesses to an array of
> unsigned chars, whose start address is not aligned on a 128 bits
> boundary.
>
> The attached patch fixes the problem by making
> aarch64_builtin_support_vector_misalignment always return false when
> the misalignment is not known at compile time.
>
> I've also added a testcase, which tries to check if the array start
> address alignment is checked (using %16, and-ing with #15), so that
> loop peeling is performed *before* using vectorized accesses. Without
> the patch, vectorized accesses are used at the beginning of the array,
> and byte accesses are used for the remainder at the end, and there is
> not such 'and wX,wX,15'.
>
> BTW, I'm not sure about the same hook for arm... it seems to me it has
> a similar problem.
>
> OK?

I would keep part of the comment:
-  /* Misalignment factor is unknown at compile time but we know
-     it's word aligned.  */

Something like:
/* Misalignment factor is unknown at compile time. */

Otherwise it is not obvious what -1 means.

Other than I think this patch is good (I cannot approve though).

Thanks,
Andrew

>
> Thanks,
>
> Christophe