[LLVMdev] TableGen pattern for negated operand

[Ivan Llopard]

Hi Joe,

Le 11/05/2012 02:13, Joe Matarazzo a écrit :
> I've been unable to come up with the TableGen recipe to match a
> negated operand. My target asm syntax allows the following transform:
>
>    FNEG  r8, r5
>    MUL    r6, r8, r9
>
> to
>
>    MUL  r6, -r5, r9
>
> Is there a Pattern<>  syntax that would allow matching *any* opcode (or
> even some subset), not just MUL, with a FNEG'd operand?

You may custom lower all binops, no matter which one, and create custom 
nodes if it meets that condition. But I think a better solution would be 
to add pattern matching rules for all of them systematically. IMO it's 
cleaner and easier to maintain. You can use multiclasses :

multiclass Inst< node op, string opc>  {
     def _rr :  Instruction<(outs GPR32:$dst), (ins GPR32:$src1, 
GPR32:$src2),
         "!strcat(opc, " $dst, $src1, $src2")",
             [(set $dst, (op GPR32:$src1, GPR32:$src2))]>;

     def _fneg_rr :  Instruction<(outs GPR32:$dst), (ins GPR32:$src1, 
GPR32:$src2),
         "!strcat(opc, " $dst, -$src1, $src2")",
             [(set $dst, (op (fneg_su GPR32:$src1), GPR32:$src2)))]>;
}

>   I expect I can
> define a PatFrag:
>
> def fneg_su : PatFrag<(ops node:$val), (fneg node:$val), [{ return
> N->hasOneUse(); }]>;

AFAIK, you don't need to verify for hasOneUse() because the instruction 
selector will do it for you. Also, it's too restrictive if fneg_su is 
used alone in some other matching rule.

> and then use that in each target instruction patten in XXXInstrInfo.td, such as:
>
> def XXX_MUL : XXXInst<
>     (outs GPR32:$dst),
>     (ins GPR32:$src1, GPR32:$src2),
>     "mul $dst, -$src1, $src2",
>     [(set $dst, (mul (fneg_su GPR32:$src1), GPR32:$src2))]>;
>
> but I would like to believe there's a way to do this with a Pattern<>
> definition instead, with help from PatFrag and the SDNode_XFORM
> perhaps.
>
> I looked at the ARM target code with PatFrag for negated immediates
> but that approach doesn't seem possible with register operands, as I
> don't know what xform would operate on. With immediates you have a DAG
> node you can generate.
>
> Also, it does seem like this is a folding operation
> PerformDAGCombine() could do but that approach seems like it needs to
> modify the registerclass, or something similar, that would eventually
> get you to a PrintMethod that could insert the dash/negate in front of
> the operand. I didn't want to define a 'mirror' registerclass for my
> existing register set that would just have the Name as the negated
> version. That would have its own complications. Is there a superior
> way to do this with DAG combine?

I don't see how you can implement this in the combiner phase. May be 
someone else can help you.

Ivan

>
> Thanks,
> Joe
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