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Re: [AArch64] Fix integer vabs intrinsics


On Fri, May 2, 2014 at 1:48 AM, James Greenhalgh
<james.greenhalgh@arm.com> wrote:
>
> Hi,
>
> Unlike the mid-end's concept of an ABS_EXPR, which treats overflow as
> undefined/impossible, the neon intrinsics vabs intrinsics should behave as
> the hardware. That is to say, the pseudo-code sequence:


Only for signed integer types.  You should be able to use an unsigned
integer type here instead.

>
>   a = vabs_s8 (vdup_n_s8 (-128));
>   assert (a >= 0);
>
> does not hold. As in hardware
>
>   abs (-128) == -128
>
> Folding vabs intrinsics to an ABS_EXPR is thus a mistake, and we should avoid
> it. In fact, we have to be even more careful than that, and keep the integer
> vabs intrinsics as an unspec in the back end.

No it is not.  The mistake is to use signed integer types here.  Just
add a conversion to an unsigned integer vector and it will work
correctly.
In fact the ABS rtl code is not undefined for the overflow.

Thanks,
Andrew Pinski

>
> We keep the standard pattern name around for the benefit of
> auto-vectorization.
>
> Tested on aarch64-none-elf with no issues.
>
> This will also be a bug on 4.9 (ugh), OK for trunk and gcc-4_9-branch?
>
> Thanks,
> James
>
> ---
> 2014-05-02  James Greenhalgh  <james.greenhalgh@arm.com>
>
>         * config/aarch64/aarch64-builtins.c (aarch64_fold_builtin): Don't
>         fold integer abs builtins.
>         * config/aarch64/aarch64-simd-builtins.def (abs): Split by integer
>         and floating point variants.
>         * config/aarch64/aarch64-simd.md (aarch64_abs<mode>): New.
>         * config/aarch64/iterators.md (unspec): Add UNSPEC_ABS.


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