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Re: [PATCH] match.pd: reassociate multiplications with constants
On Mon, 17 Jul 2017, Marc Glisse wrote:
> > +/* Combine successive multiplications. Similar to above, but handling
> > + overflow is different. */
> > +(simplify
> > + (mult (mult @0 INTEGER_CST@1) INTEGER_CST@2)
> > + (with {
> > + bool overflow_p;
> > + wide_int mul = wi::mul (@1, @2, TYPE_SIGN (type), &overflow_p);
> > + }
> > + (if (!overflow_p || TYPE_OVERFLOW_WRAPS (type))
>
> I wonder if there are cases where this would cause trouble for saturating
> integers. The only case I can think of is when @2 is -1, but that's likely
> simplified to NEGATE_EXPR first.
Ah, yes, I think if @2 is -1 or 0 then we should not attempt this transform for
either saturating or sanitized types, just like in the first patch. I think
wrapping the 'with' with 'if (!integer_minus_onep (@2) && !integer_zerop (@2))'
works, since as you say it should become a negate/zero anyway?
Alexander