Re: [PATCH 06/15] rs6000: Use rldiwi in constant construction

[Segher Boessenkool <segher at kernel dot crashing dot org>]

On Wed, Aug 12, 2015 at 08:55:51AM -0700, Richard Henderson wrote:
> On 08/12/2015 07:02 AM, Segher Boessenkool wrote:
> > On Tue, Aug 11, 2015 at 06:11:35PM -0700, Richard Henderson wrote:
> >> @@ -8173,6 +8173,13 @@ genimm_ppc::exam_search (HOST_WIDE_INT c, int budget)
> >>    if (exam_mask (-1, c, sub_budget))
> >>      return true;
> >>  
> >> +  /* If the two halves are equal, use an insert.  */
> >> +  if (c >> 32 == test && exam_sub (test, sub_budget))
> >> +    {
> >> +      opN (VEC_DUPLICATE, 0xffffffffu); /* RLDIMI */
> >> +      return true;
> >> +    }
> > 
> > Does this work for c with the high bit set?  I think you need
> > to cast it to unsigned HOST_WIDE_INT first?
> 
> Indeed, a sign-extension works better.  It means the base constant will use
> LIS+ORIS without trying to create an unsigned version.

Patch 8/15 changes this so that "test" is assigned the sign-extended low
32 bits right before this code; that should work just fine.

> If you're talking about ubsan sort of restrictions on shifting signed
> constants...  I choose to totally ignore that.

Good plan.  We rely on arithmetic shifts rounding towards negative
infinity, and so does the rest of the world.

> Certainly no where else in gcc
> has been audited for that, beginning with hwint.h itself.

Yes.  And there are much worse problems, like many things not working
right if your HOST_WIDE_INT would happen to be more than 64 bits; we
cannot really shake those out because there is no actual system to
test that on -- but it also doesn't actually matter, because there is
no system to run it on :-)


Segher