fast-math optimization question

[Steve Ellcey]

On Thu, 2014-10-09 at 19:50 +0000, Joseph S. Myers wrote:
> On Thu, 9 Oct 2014, Steve Ellcey wrote:
> 
> > Do you know which pass does the simple
> > '(float)function((double)float_val)' demotion?  Maybe that would be a
> > good place to extend things.
> 
> convert.c does such transformations.  Maybe the transformations in there 
> could move to the match-and-simplify infrastructure - convert.c is not a 
> particularly good place for optimization, and having similar 
> transformations scattered around (fold-const, convert.c, front ends, SSA 
> optimizers) isn't helpful; hopefully match-and-simplify will allow some 
> unification of this sort of optimization.

I did a quick and dirty experiment with the match-and-simplify branch
just to get an idea of what it might be like.  The branch built for MIPS
right out of the box so that was great and I added a couple of rules
(see below) just to see if it would trigger the optimization I wanted
and it did.  I was impressed with the match-and-simplify infrastructure,
it seemed to work quite well.  Will this branch be included in GCC 5.0?

Steve Ellcey
sellcey@mips.com


Code added to match-builtin.pd:

 
(if (flag_unsafe_math_optimizations)
 /* Optimize "(float) expN(x)" [where x is type double] to
             "expNf((float) x)", i.e. call the 'f' single precision func */
 (simplify
  (convert (BUILT_IN_LOG @0))
  (if ((TYPE_MODE (type) == SFmode) && (TYPE_MODE (TREE_TYPE (@0)) == DFmode))
   (BUILT_IN_LOGF (convert @0))))
)

(if (flag_unsafe_math_optimizations)
 /* Optimize "(float) expN(x)" [where x is type double] to
             "expNf((float) x)", i.e. call the 'f' single precision func */
 (simplify
  (convert (BUILT_IN_SIN @0))
  (if ((TYPE_MODE (type) == SFmode) && (TYPE_MODE (TREE_TYPE (@0)) == DFmode))
   (BUILT_IN_SINF (convert @0))))
)