Re: VEC_COND_EXPR optimizations

[Richard Biener <richard.guenther@gmail.com>]

On Fri, Jul 31, 2020 at 2:50 PM Richard Sandiford
<richard.sandiford@arm.com> wrote:
>
> Marc Glisse <marc.glisse@inria.fr> writes:
> > On Fri, 31 Jul 2020, Richard Sandiford wrote:
> >
> >> Marc Glisse <marc.glisse@inria.fr> writes:
> >>> +/* (c ? a : b) op (c ? d : e)  -->  c ? (a op d) : (b op e) */
> >>> + (simplify
> >>> +  (op (vec_cond:s @0 @1 @2) (vec_cond:s @0 @3 @4))
> >>> +  (with
> >>> +   {
> >>> +     tree rhs1, rhs2 = NULL;
> >>> +     rhs1 = fold_binary (op, type, @1, @3);
> >>> +     if (rhs1 && is_gimple_val (rhs1))
> >>> +       rhs2 = fold_binary (op, type, @2, @4);
> >>> +   }
> >>> +   (if (rhs2 && is_gimple_val (rhs2))
> >>> +    (vec_cond @0 { rhs1; } { rhs2; })))))
> >>> +#endif
> >>
> >> This one looks dangerous for potentially-trapping ops.
> >
> > I would expect the operation not to be folded if it can trap. Is that too
> > optimistic?
>
> Not sure TBH.  I was thinking of “trapping” in the sense of raising
> an IEEE exception, rather than in the could-throw/must-end-bb sense.
> I thought match.pd applied to things like FP addition as normal and
> it was up to individual patterns to check the appropriate properties.

I think it can be indeed defered to the simplification of (op @0 @2)
because that would be invalid if it removes a IEEE exception.
The VEC_COND_EXPR itself cannot trap.

Richard.

> Thanks,
> Richard