The conditions when convert from double to float is permitted?

[Marc Glisse]

On Thu, 10 Dec 2020, Xionghu Luo via Gcc wrote:

> I have a maybe silly question about whether there is any *standard*
> or *options* (like -ffast-math) for GCC that allow double to float
> demotion optimization?  For example,
>
> 1) from PR22326:
>
> #include <math.h>
>
> float foo(float f, float x, float y) {
> return (fabs(f)*x+y);
> }
>
> The fabs will return double result but it could be demoted to float
> actually since the function returns float finally.

With fp-contract, this is (float)fma((double)f,(double)x,(double)y). This 
could almost be transformed into fmaf(f,x,y), except that the double 
rounding may not be strictly equivalent. Still, that seems like it would 
be no problem with -funsafe-math-optimizations, just like turning 
(float)((double)x*(double)y) into x*y, as long as it is a single operation 
with casts on all inputs and output. Whether there are cases that can be 
optimized without -funsafe-math-optimizations is harder to tell.

-- 
Marc Glisse