llvm dev - Dec 2009 - [LLVMdev] How to use property 'isCommutable' in target description file?

Hi everyone,

I  practice writing target description file with MSP430 reference.

I add a multiply-and-add instruction as below:

let isTwoAddress=1 in {

  def MULADD:Pseudo<(out GR16:$dst), (ins GR16:$src1, GR16:$src2,
GR16:$src3),

                                "muladd\t{$dst, $src2, $src3}",

                                [(set GR16:$dst, (add GR16:$src1, (mul
GR16:$src2, GR16:$src3)))]>

}

How can i tell the system X=A*B + C == X = B*A + C == X=C+A*B == X=C+B*A by
property 'isCommutable'? Is it necessary to do that?

***  Another question: Why set isCommutable = 1 in ADD16rr but NOT set in
ADD16ri?

let isCommutable = 1 in { // X = ADD Y, Z  == X = ADD Z, Y
def ADD16rr : Pseudo<(outs GR16:$dst), (ins GR16:$src1, GR16:$src2),
                     "add.w\t{$src2, $dst}",
                     [(set GR16:$dst, (add GR16:$src1, GR16:$src2)),
                      (implicit SRW)]>;
}
def ADD16ri : Pseudo<(outs GR16:$dst), (ins GR16:$src1, i16imm:$src2),
                     "add.w\t{$src2, $dst}",
                     [(set GR16:$dst, (add GR16:$src1, imm:$src2)),
                      (implicit SRW)]>;

Regards
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Hello
> How can i tell the system X=A*B + C == X = B*A + C == X=C+A*B == X=C+B*A by
> property 'isCommutable'? Is it necessary to do that?
Most probably you will need to write a special hook to commute this
instruction. However, everything depends on your target (e.g. if there
is an output register tied to one of the input).
> ***  Another question: Why set isCommutable = 1 in ADD16rr but NOT set in
> ADD16ri?
Because all msp430 instructions are two-address, this means that
output register is tied to first input. That's why you cannot swap
operands for reg-imm operations.

-- 
With best regards, Anton Korobeynikov
Faculty of Mathematics and Mechanics, Saint Petersburg State University

Thanks to Anton.

Frankly said i don't know the exact meaning and purpose of
'isCommutable'.
By my opinion "add.w r6, r7" != "add.w r7, r6", so we
shouldn't set
isCommutable = 1.

Who would like give an example to demonstrate what benifit it has if
'isCommuatble=1' in instruction selection, register allocation or other
process?

Regards



2009/12/7, Anton Korobeynikov <anton at
korobeynikov.info>:
>
> Hello
>
> > ***  Another question: Why set isCommutable = 1 in ADD16rr but NOT set
in
> > ADD16ri?
> Because all msp430 instructions are two-address, this means that
> output register is tied to first input. That's why you cannot swap
> operands for reg-imm operations.
>
> --
> With best regards, Anton Korobeynikov
> Faculty of Mathematics and Mechanics, Saint Petersburg State University
>
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