[Bug tree-optimization/70291] muldc3 code generation could be smarter

[wilco at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70291

Wilco <wilco at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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                 CC|                            |wilco at gcc dot gnu.org

--- Comment #2 from Wilco <wilco at gcc dot gnu.org> ---
(In reply to Ramana Radhakrishnan from comment #0)
> __complex double x;
> __complex double y;
> __complex double z;
> double a, b, c, d;
> 
> int main (void)
> {
>   x = y * z;
>   return 0; 
> }
> 
> Could well be implemented as:
> 
> 
> int main (void)
> {
>   x = y * z;
>   if (isnan (__real x) && isnan (__imag__ x))
>     x = __muldc3 (y, z);
> 
>   return 0;
> }
> 
> essentially opencoding this as the standard suggests in G.5.1 => 6for C99.
> 
> spotted while looking at profiles that were reported on aarch64 with code
> compiled at O2 / O3.
> 
> I note that lowering this in this form in tree-complex.c will need a bit of
> book-keeping given that it's sort of bounded on the phi nodes and ssa_names
> before lowering begins but this could well be another math-optimization done
> later rather than munging it with the existing lowering.

Note that isnan (x) || isnan (y) is optimized to isunordered (x, y), so that
would be a faster check.

Also it may be beneficial to vectorize this despite the fallback.