Re: mmx register moves through memory

[Vahur Sinijärv <vahur dot sinijarv at cydonia dot ee>]

Richard Henderson wrote:

> > I do not see the point why you should discourage the register allocator 
> > from using mmx registers, move through memory is clearly inefficent and 
> > enlarges resulting code (if the function containing moves is inlined in 
> > several places, even more so).
> >    
>
> First, what you think is "clearly inefficient" is at least two cycles
> faster, at least for AMD (Intel hasn't published anything as useful as
> instruction latencies since early PentiumPro).  I'm not sure what sort
> of pipeline bypasses are or are not responsible, but *all* cross function
> unit moves are discouraged.
>
>  
I see, i did a speed test with GCC 4.1.0 and 3.4.4 on my athlon-xp and 
you are right. Direct moves between genregs and MMX are still useful 
when optimizing for size. GCC could do a bit more sophisticated guesses 
whether to use secondary memory for such moves or not, right now it just 
disables them all.

> Second, proper use of MMX requires proper placement of emms instructions.
> Allowing the register allocator to use MMX registers at will breaks that.
>
>
> r~
>  
That is true, but would register allocator choose MMX regs for anything 
else than MMX ops ?

Regards,
Vahur