fast-math optimization question

[Steve Ellcey]

I have a -ffast-math (missing?) optimization question.  I noticed on MIPS
that if I compiled:

        #include <math.h>
        extern x;
        void foo() { x = sin(log(x)); }

GCC will extend 'x' to double precision, call the double precision log and sin
functions and then truncate the result to single precision.

If instead, I have:

        #include <math.h>
        extern x;
        void foo() { x = log(x); x = sin(x); }

Then GCC will call the single precision log and sin functions and not do
any extensions or truncations.  In addition to avoiding the extend/trunc
instructions the single precision log and sin functions are presumably
faster then the double precision ones making the entire code much faster.

Is there a reason why GCC couldn't (under -ffast-math) call the single
precision routines for the first case?

Steve Ellcey
sell...@mips.com