llvm dev - Jul 2007 - [LLVMdev] Swaps of FP registers

Dear guys,

     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?

My function to insert swaps is like:

void X86RegisterInfo::swapRegs(
     MachineBasicBlock & mbb,
     MachineBasicBlock::iterator mi,
     unsigned r1,
     unsigned r2,
     const TargetRegisterClass *rc
) const {
     unsigned Opc;
     if (rc == &X86::GR32RegClass) {
         Opc = X86::XCHG32rr;
     } else if (rc == &X86::GR16RegClass) {
         Opc = X86::XCHG16rr;
     } else if (rc == &X86::GR8RegClass) {
         Opc = X86::XCHG8rr;
     } else {
         assert(0 && "Unknown regclass in add swap");
         abort();
     }
     BuildMI(mbb, mi, Opc, 1, r1).addReg(r2);
}

thanks,

Fernando

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. I have no idea if it's the "best" though.
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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