Re: GCC's -ffast-math behavior

[Tim Prince <n8tm@xxxxxxx>]

On 2/10/2012 1:39 AM, Miles Bader wrote:
> xunxun<xunxun1982@xxxxxxxxx>  writes:
> > > If you switch to using SSE floating-point, e.g. using "-mfpmath=sse",
> > > it will still call the library functions even when using -ffast-math
> > > (because the SSE unit doesn't have special instructions like "fsin" or
> > > "f2xm1").  I think SSE FP is typically faster than the 387 for many
> > > CPUs anyway.
> > Well, that's right.
> > But in my experience, -mfpmath=sse will slow my code very much.
>
> Hmm, I've always found SSE FP to be a speedup -- sometimes a _big_
> speedup -- over 387 FP, at least when one is using mostly primitive FP
> operations (mul, divide, sqrt, etc) ... I think it's worth testing, at
> least.

387 code may be faster when you mix float and double, e.g. by forcing 
double evaluation of float expressions by removing the f suffix from 
constants.
It has been a long time since I had 32-bit OS installed and could 
examine the broken mathinline.h headers which typically came with them. 
 You would need to fix those before trying -ffast-math.


--
Tim Prince