Re: MMX regs and GCC

[Jeff Garzik <jgarzik@pobox.com>]

Ben Combee wrote:
> 
> > hmmm.  I have the Pentium Pro databooks, and the quoted text below seems
> > to imply direct moves between general registers are possible.  Since
> > reg<->MMX reg transfers are possible, that seems to imply that loading
> > and using data in MMX registers would be cheaper than loaded data from
> > memory.
> 
> Actually, we have considered adding code to do that to CodeWarrior -- the
> biggest problem is that using MMX registers inteferes with any usage of the
> FP registers, and on some MMX-capable chips, the time to handle a EMMS
> instruction can be quite substancial.  However, if you are already
> generating MMX or 3DNow! code, then spilling a 32-bit value to a MMX
> register is probably just fine, as long as by doing so you aren't causing
> additional spills of MMX registers (the CodeWarrior MMX-3DNow! calling
> convention that we use for 3DNow! optimized code specifies that MM4-MM7 are
> preserved across calls)

What do you think about the case of a program that does little or no
floating point at all?  It seems like that would be an optimal, and
common, case for using MMX registers, while minimized FP context
save/store.  Also I was wondering if it was possible to avoid EMMS by
using 3DNow for 'single' and 'float'.

Thanks,

	Jeff




-- 
Custom driver development	|    Never worry about theory as long
Open source programming		|    as the machinery does what it's
				|    supposed to do.  -- R. A. Heinlein