Re: Bug in builtin_floor optimization

[Roger Sayle <roger at eyesopen dot com>]

On Mon, 22 Aug 2005, Dale Johannesen wrote:
> There is some clever code in convert_to_real that converts
>
> double d;
> (float)floor(d)
>
> to
>
> floorf((float)d)
> ...
>
> Comments? Should I preserve the buggy behavior with -ffast-math?

Good catch.  This is indeed a -ffast-math (or more precisely a
flag_unsafe_math_optimizations) transformation.  I'd prefer to
keep these transformations with -ffast-math, as Jan described them
as significantly helping SPEC's mesa when they were added.

My one comment is that we should try to make sure that we continue
to optimize the common safe case (even without -ffast-math):

	float x, y;
	x = floor(y);

i.e. that (float)floor((double)y) is the same as floorf(y).
Hmm, it might be good to know the relative merits of the safe vs.
unsafe variants.  If the majority of the benefit is from the "safe"
form, I wouldn't be opposed to removing the "unsafe" form completely,
if people think its an optimization "too far".

Thanks for investigating this.

Roger
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