disabling the effects of -ffast-math at specific locations

[Sam Hocevar <sam at hocevar dot net>]

   I am using -ffast-math because 99% of the time it has a very positive
impact on my resulting binaries. However, from time to time I would like
to avoid some of the assumptions it makes.

   Consider the following code, doing some fast rounding magic:

     static double moo(double f, double g)
     {
         g *= 4503599627370496.0; // 2 ** 52
         f += g;
         f -= g;
         return f;
     }

   On amd64, it compiles to the following assembly code using -Os:

	.cfi_startproc
	mulsd	.LC0(%rip), %xmm1
	addsd	%xmm1, %xmm0
	subsd	%xmm1, %xmm0
	ret
	.cfi_endproc

   As documented, when using -ffast-math everything gets optimised away:

	.cfi_startproc
	rep
	ret
	.cfi_endproc

   But everything goes back to what I want with this asm call:

     static double moo(double f, double g)
     {
         g *= 4503599627370496.0; // 2 ** 52
         f += g;
         __asm__("" : "+x" (f));
         f -= g;
         return f;
     }

   So my question is: is this guaranteed to always work? Am I sure that
f will always be in an "x" register?

   And do I have a way to make this portable? On PowerPC it's just a
matter of replacing "+x" with "+f" (which is exactly what Apple does in
its libm). But on i386, I don't know in what kind of register f will be,
and I'm stuck with using "+m", which has an awful performance impact.

   Or maybe I'm trying to solve the wrong problem? Is there another
method that will more likely do what I want?

Cheers,
-- 
Sam.