On Tue, 10 Nov 2015, Richard Biener wrote:
> Looks ok but I wonder if this is dead code with
>
> (for pows (POW)
> sqrts (SQRT)
> cbrts (CBRT)
> (simplify
> (pows @0 REAL_CST@1)
> (with {
> const REAL_VALUE_TYPE *value = TREE_REAL_CST_PTR (@1);
> REAL_VALUE_TYPE tmp;
> }
> (switch
> ...
> /* pow(x,0.5) -> sqrt(x). */
> (if (flag_unsafe_math_optimizations
> && canonicalize_math_p ()
> && real_equal (value, &dconsthalf))
> (sqrts @0))
>
> also wondering here about canonicalize_math_p (), I'd expected the
> reverse transform as canonicalization. Also wondering about
> flag_unsafe_math_optimizations (missing from the vectorizer pattern).
pow(x,0.5) -> sqrt(x) is unsafe because: pow (-0, 0.5) is specified in
Annex F to be +0 but sqrt (-0) is -0; pow (-Inf, 0.5) is specified in
Annex F to be +Inf, but sqrt (-Inf) is NaN with "invalid" exception
raised. I think it's safe in other cases (the reverse of course is not
safe, sqrt is a fully-specified correctly-rounded IEEE operation and pow
isn't).
--
Joseph S. Myers
[email protected]
