Re: GCC and Floating-Point (A proposal)

[Richard Guenther <richard.guenther@gmail.com>]

On 5/26/05, Scott Robert Ladd <scott.ladd@coyotegulch.com> wrote:
> Allan Sandfeld Jensen wrote:
> >>Yes. I still don't understand why gcc doesn't do -ffast-math by
> >>default like all other compilers.
> 
> Vincent Lefevre wrote:
> > No! And I really don't think that other compilers do that.
> > It would be very bad, would not conform to the C standard[*]
> > and would make lots of codes fail.
> 
> Perhaps what needs to be changed is the definition of -ffast-math
> itself. Some people (myself included) view it from the standpoint of
> using the full capabilities of our processors' hardware intrinsics;
> however, -ffast-math *also* implies the rearrangement of code that
> violates Standard behavior. Thus it does two things that perhaps should
> not be combined.
> 
> To be more pointed, it is -funsafe-math-optimizations (implied by
> -ffast-math) that is in need of adjustment.
> 
> May I be so bold as to suggest that -funsafe-math-optimizations be
> reduced in scope to perform exactly what it's name implies:
> transformations that may slightly alter the meanding of code. Then move
> the use of hardware intrinsics to a new -fhardware-math switch.

I think the other options implied by -ffast-math apart from
-funsafe-math-optimizations should (and do?) enable the use of
hardware intrinsics already.  It's only that some of the optimzations
guarded by -funsafe-math-optimizations could be applied in general.
A good start may be to enumerate the transformations done on a
Wiki page and list the flags it is guarded with.

Richard.