[LLVMdev] Tablegen: How to define an instruction that reads and writes the same register

[Peter Cooper]

Hi Tom

This is frequently done in the x86 target using a tablegen constraint.

For example, from X86InstrArithmetic.td we have

let Constraints = "$src1 = $dst" in {
def INC8r  : I<0xFE, MRM0r, (outs GR8 :$dst), (ins GR8 :$src1),
               "inc{b}\t$dst",
               [(set GR8:$dst, EFLAGS, (X86inc_flag GR8:$src1))]>;
}

Thanks,
Pete

On Jan 19, 2012, at 6:07 PM, Tom Stellard wrote:

> Hi,
> 
> Is it possible to define an Instruction with tablegen that reads and
> writes the same register?  For example, an increment instruction that
> reads a value from a register, adds one to it and then writes the result
> back to the same register.
> 
> Thanks,
> Tom
> 
> 
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