llvm dev - Jul 2008 - [LLVMdev] What happened to XCHG_rr?

Hi, guys,

     What is the opcode of the instruction to swap two registers in X86?

     After updating my LLVM branch, I realized that there is no longer an 
opcode for xchg with two register operands in X86GenInstrNames.inc. I 
found only instructions to swap memory and registers: XCHG16rm, XCHG32rm, 
XCHG64rm and XCHG8rm.

I am updating from LLVM 2.1 to current trunk. The names that I was using 
in LLVM 2.1 were: XCHG8rr, XCHG16rr, XCHG32rr and XCHG64rr.

Ps.: Evan, thank you for answering the question about IMPLICIT_DEF's.

All the best,

Fernando

On Jul 20, 2008, at 11:50 AM, Fernando Magno Quintao Pereira
wrote:
> I am updating from LLVM 2.1 to current trunk. The names that I was  
> using
> in LLVM 2.1 were: XCHG8rr, XCHG16rr, XCHG32rr and XCHG64rr.
>
> Ps.: Evan, thank you for answering the question about IMPLICIT_DEF's.
I think they were removed just because noone was using them.  Also,  
the JIT encoding may have not been correct, I don't recall.

-Chris

I was using them to do SSA-elimination after register allocation. I can 
implement swaps using three XOR's, but then the code becomes a little 
bigger and slower. I think even for the sake of completeness, the X86 
back-end should offer the possibility of swapping two registers with one 
single instruction. Do you guys think there is any possibility that those 
instructions could come back to the main trunk of LLVM?

Just as a side note, swaps are very important for SSA-based register 
allocation. If you cannot swap two live ranges atomically, then the 
problem of finding the minimum coloring becomes NP-complete, whereas it is 
linear on the size of the interference graph otherwise.

best,

Fernando
> On Jul 20, 2008, at 11:50 AM, Fernando Magno Quintao Pereira wrote:
>> I am updating from LLVM 2.1 to current trunk. The names that I was
>> using
>> in LLVM 2.1 were: XCHG8rr, XCHG16rr, XCHG32rr and XCHG64rr.
>>
>> Ps.: Evan, thank you for answering the question about
IMPLICIT_DEF's.
>
> I think they were removed just because noone was using them.  Also,
> the JIT encoding may have not been correct, I don't recall.
>
> -Chris
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