[llvm-dev] New register class and patterns

Rail Shafigulin via llvm-dev llvm-dev at lists.llvm.org
Wed Feb 3 18:17:00 PST 2016

> def SDTX86CmpPTest : SDTypeProfile<1, 2, [SDTCisVT<0, i32>,
>                                           SDTCisVec<1>,
>                                           SDTCisSameAs<2, 1>]>;
> This is confusing to me. This tells me that there is 1 result but and 2
> operands. But then it says that operands 2 and 1 are of the same type,
> SDTCisSameAs<2, 1>. Given that operand numbering starts at 0, how can there
> be operands 2 and 1?
> The results are numbered starting from 0. In this case with 1 result, 0 is
> the output operand, and 1 and 2 are the inputs.
> Based on the previous answer my understanding is that LLVM is complaining
> because it doesn't know what register to use. What is unclear to me is why?
> I already had 2 register classes before and everything was working. All
> I've done is that I had added an extra class. After that LLVM started to
> complain. And this is what puzzles me.
> Did you add a register class for a special condition register? Did you set
> it as isAllocatable = 0?

I think I'm slowly getting it. To answer your question, no I did not set
isAllocaable = 0 for the new register class. But I'm still confused.
Original instruction does not have an output register. It sets a flag in
the special purpose register. So why creating a new register class would
cause a problem? In other words, since there is no output register, why
would LLVM start complaining. Below I'm repeating some code for reference.
Any help is appreciated.

def SDT_EscalaSetFlag      : SDTypeProfile<0, 3, [SDTCisSameAs<0, 1>]>;

def Esenciasetflag     : SDNode<"EsenciaISD::SET_FLAG", SDT_EsenciaSetFlag,

class SF_RI<bits<5> op2Val, string asmstr, PatLeaf Cond>
  : InstRI<0xf, (outs), (ins GPR:$rA, s16imm:$imm),
           !strconcat(asmstr, "i\t$rA, $imm"),
           [(Escalasetflag (i32 GPR:$rA), immSExt16:$imm, Cond)]> {
  bits<5> op2;
  bits<5> rA;
  bits<16> imm;

  let Inst{25-21} = op2;
  let Inst{20-16} = rA;
  let Inst{15-0} = imm;

  let format = AFrm;
  let op2 = op2Val;

Rail Shafigulin
Software Engineer
Esencia Technologies
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