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Re: [AArch64] Fix integer vabs intrinsics


On Fri, May 02, 2014 at 10:00:15AM +0100, Andrew Pinski wrote:
> 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.

If anything, I think that puts us in a worse position. The issue that
inspires this patch is that GCC will happily fold:

  t1 = ABS_EXPR (x)
  t2 = GE_EXPR (t1, 0)

to

  t2 = TRUE

Surely an unsigned integer type is going to suffer the same fate? Certainly I
can imagine somewhere in the compiler there being a fold path for:

  (unsigned >= 0) == TRUE

> >
> >   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.
 
Here we are covering ourselves against a seperate issue. For auto-vectorized
code we want the SABD combine patterns to kick in whenever sensible. For
intrinsics code, in the case where vsub_s8 (x, y) would cause an underflow:

  vabs_s8 (vsub_s8 (x, y)) != vabd_s8 (x, y)

So in this case, the combine would be erroneous. Likewise SABA.

Thanks,
James


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