The following change, which I bootstrapped on powerpc-unknown-linux-gnu 
(C and f95) without regressions, leads the Fortran front-end to generate 
expressions x**(-n) with n constant as if they were written (1/x)**n.

This will enable more opportunities for CSE in case multiple powers of 
the same base are needed in an expression.  E.g., the following:

function force(d)
force = d**(-2) + d**(-6) + d**(-8) + d**(-12)
end

will lead to the following floating point operations (when using 
-ffast-math, whereas the current method needs four divisions and uses no 
fused multiply-add.

         fdivs 1,1,13
         fmuls 0,1,1
         fmuls 1,1,0
         fmuls 13,0,0
         fmuls 1,1,1
         fadds 0,0,1
         fmadds 13,13,13,0
         fmadds 1,1,1,13

Paul, is this OK ?

-- 
Toon Moene - mailto:toon@moene.indiv.nluug.nl - phoneto: +31 346 214290
Saturnushof 14, 3738 XG  Maartensdijk, The Netherlands
Maintainer, GNU Fortran 77: http://gcc.gnu.org/onlinedocs/g77_news.html
GNU Fortran 95: http://gcc.gnu.org/fortran/ (under construction)