[LLVMdev] Operand order in dag pattern matching in td files

[Craig Topper]

You've unfortunately chosen a complex example.

Your second question is needs be answered first. null_frag causes the
pattern to be dropped.

Now having covered that the reason the operands are in the order they are
is because the only instruction that doesn't use null_frag is this one

defm r213 : fma3s_rm<opc213, !strconcat(OpStr, !strconcat("213", PackTy)),
                     x86memop, RC, OpVT, mem_frag, OpNode>

Which specifies the operand order as 213 thus why they are 2, 1, 3 in the
pattern.

~Craig

On Fri, Nov 16, 2012 at 12:19 AM, Anitha B Gollamudi <
anitha.boyapati at gmail.com> wrote:

> On 16 November 2012 13:41, Anitha B Gollamudi <anitha.boyapati at gmail.com>
> wrote:
> > Hi,
> >
> > I have a simple question w.r.t the order of operands used in dag
> > pattern matching in target files. Some of them seem intuitive. But I
> > want to get it clarified anyway. I am using a pattern from
> > X86InstrFMA.td in the below example. Consider FMA3 pattern
> > (simplified).
> >
> > let Constraints = "$src1 = $dst" in {
> > multiclass fma3s_rm<bits<8> opc, string OpcodeStr, X86MemOperand
> x86memop,
> >                     RegisterClass RC, ValueType OpVT, PatFrag mem_frag,
> >                     SDPatternOperator OpNode = null_frag> {
> >
> >   def r     : FMA3<opc, MRMSrcReg, (outs RC:$dst),
> >                    (ins RC:$src1, RC:$src2, RC:$src3),
> >                    !strconcat(OpcodeStr,
> >                               "\t{$src3, $src2, $dst|$dst, $src2,
> $src3}"),
> >                    [(set RC:$dst,
> >                      (OpVT (OpNode RC:$src2, RC:$src1, RC:$src3)))]>;
> >
> >
> > Given that it is an FMA pattern, the OpNode can be one of X86Fmadd,
> > X86Fmsub (among other opnodes). The respective operations are :
> > (a*b+c) and (a*b-c). My questions are:
> >
> > 1. Why does $src2 and not $src1 come first in (OpNode RC:$src2,
> > RC:$src1, RC:$src3) ?  In other words what is the difference between
> > (a) and (b):
> >    (a)    [(set RC:$dst, (OpVT (OpNode RC:$src2, RC:$src1, RC:$src3)))]
> >    (b)    [(set RC:$dst,(OpVT (OpNode RC:$src1, RC:$src2, RC:$src3)))]
> >
>
> Typo.
>
> > 2. Why happens when OpNode is null_frag?
>
> 2. What happens when OpNode is null_frag?
>
>
>
>
>
> --
>  Anitha
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>



-- 
~Craig
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