Richard Sandiford <richard.sandif...@arm.com> writes:
>> And yes, it is icky. But it is sound, as far as I can see.
>
> I really disagree that it's sound, but no point me saying why again :-)
>
> (It could certainly be made to work with sufficient hacks of course,
> like pretty much anything could, but I don't think that's the same thing.)
For an example, we have:
/* Maybe simplify x + 0 to x. The two expressions are equivalent
when x is NaN, infinite, or finite and nonzero. They aren't
when x is -0 and the rounding mode is not towards -infinity,
since (-0) + 0 is then 0. */
if (!HONOR_SIGNED_ZEROS (mode) && trueop1 == CONST0_RTX (mode))
return op0;
I think it's plausible that people will care about accurate rounding
but not signed zeroes. In that mode we could have:
(set (reg:DF r3) (plus:DF (reg:DF r1) (reg:DF r2)))
(set (reg:DF r4) (const_double:DF 0.0))
(set (reg:SF r5) (float_narrow:SF (plus:DF (reg:DF r3) (reg:DF r4))))
Then combine through normal structural simplification could (with the
rule above) fold all this down to:
(set (reg:SF r5) (float_narrow:SF (plus:DF (reg:DF r1) (reg:DF r2))))
where the truncation is now fused with r1+r2 instead of r3+r4. We would
have to have to add specific checks to avoid this happening, it wouldn't
fall out naturally from structural PoV.
Thanks,
Richard
