[LLVMdev] Swaps of FP registers

[Jonas Maebe]

On 04 Jul 2007, at 02:10, Scott Graham wrote:

> On 7/3/07, Fernando Magno Quintao Pereira <fernando at cs.ucla.edu>  
> wrote:
>>
>>      what is the best way to implement a swap of floating point  
>> registers
>> in X86? For the integer registers, I am using xchg. Is there a  
>> similar
>> instruction for floating point?
>
> FXCH swaps stN with st0, but you'd have to use memory for arbitrary  
> swaps I
> believe.

Well, you can use 3 FXCH's too (stX<->st0, stY<->st0, stX<->st0). I  
guess that would still be a lot faster than passing via memory (not  
to mention that you need an extra fpu stack location if you'd do it  
via memory).


Jonas