llvm dev - Feb 2014 - [LLVMdev] selecting ISD node - help

Hi Tim,


Tim Northover-2 wrote
> The code used for DIV is around X86ISelDAGToDAG.cpp:2415, but from a
> glance the key points seem to be:
> 1. use the second result of getCopyToReg (i.e. SDValue(setIdNode, 1))
> in the RDMSR node.
> 2. Give your RDMSR node type MVT::Glue instead of MVT::Other
I tried doing what you said, and the DAG looks like how I think it supposed
to look like (attached the picture below). 

	  if(Subtarget->is64Bit())
	  {
		  idReg = X86::RCX;
		  resultReg = MF.addLiveIn(X86::RAX, &X86::GR64RegClass);
	  }
	  else
	  {
		  idReg = X86::ECX;
		  resultReg = MF.addLiveIn(X86::EAX, &X86::GR32RegClass);
	  }

	  idRegValue = CurDAG->getRegister(idReg, resultType);

	  SDValue setIdNode = CurDAG->getCopyToReg(CurDAG->getEntryNode(), dl,
idRegValue, id, SDValue());
	  SDValue rdmsrNode = SDValue(CurDAG->getMachineNode(X86::RDMSR, dl,
MVT::Glue, setIdNode.getValue(1)), 0);
	  SDValue resultNode = CurDAG->getCopyFromReg(CurDAG->getEntryNode(), dl,
X86::EAX, MVT::i32, rdmsrNode);
	  
	  return resultNode.getNode();

but I've a couple of problems:

getCopyToReg doesn't show on the final assembly
	  __Z5test2v proc near
	  push    ebp
	  mov     ebp, esp
	  rdmsr
	  pop     ebp
	  retn
	  __Z5test2v endp

When I add rdmsr + rdmsr, first the rdmsr calls are made and only then he
adds eax, eax.
	  __Z5test3v proc near
	  push    ebp
	  mov     ebp, esp
	  rdmsr
	  rdmsr
	  add     eax, eax
	  pop     ebp
	  retn
	  __Z5test3v endp
Maybe it's because I use MF.addLiveIn but when I tried using X86::EAX it
failed with the message: 
	  # Machine code for function _Z5test2v: Post SSA

	  BB#0: derived from LLVM BB %entry
	          %vreg2<def> = COPY %EAX; GR32:%vreg2
 	         RDMSR
 	         %EAX<def> = COPY %vreg2; GR32:%vreg2
  	        RET %EAX<kill>

	  # End machine code for function _Z5test2v.

	  *** Bad machine code: Using an undefined physical register ***
	  - function:    _Z5test2v
	  - basic block: BB#0 entry (0x2ff8228)
	  - instruction: %vreg2<def> = COPY %EAX; GR32:%vreg2
	  - operand 1:   %EAX
	  LLVM ERROR: Found 1 machine code errors.
	  Stack dump:
	  0.      Program arguments:
C:\Users\DaAn\Documents\Projects\llvm-3.4\build\bin\Debug\llc.exe
-filetype=obj -view-sched-dags
C:\Users\DaAn\Documents\Projects\llvm-3.4\build\bin\Debug\test.bc
	  1.      Running pass 'Function Pass Manager' on module
'C:\Users\DaAn\Documents\Projects\llvm-3.4\build\bin\Debug\test.bc'.
	  2.      Running pass 'Greedy Register Allocator' on function
'@_Z5test2v'

<http://llvm.1065342.n5.nabble.com/file/n65821/dag._Z5test2v-4ac530.png> 
<http://llvm.1065342.n5.nabble.com/file/n65821/dag._Z5test3v-e16193.png> 



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Hi DaAn,
> I tried doing what you said, and the DAG looks like how I think it supposed
> to look like (attached the picture below).
I'm a little concerned that the CopyFromRegs are referencing
"%vreg0"
rather than %EAX, but that might be an LLVM-internal thing (perhaps
because of your addLiveIns). Other than that they look right to me
too, so I wouldn't worry about it for now (your code looks OK).
> getCopyToReg doesn't show on the final assembly
Ah, I think I may have just spotted the reason for that. The RDMSR
instruction hasn't been marked as using EDX:EAX or defining ECX
(incidentally, the Intel manual doesn't seem to make any mention of
RCX).

That's probably causing LLVM to drop the COPY instructions you're
generating (thinking they're dead code). I'd suggest changing the
X86InstrSystem.td:443 to be something like:

    let Defs = [ECX], Uses = [EAX, EDX] in
    def RDMSR : I<0x32, RawFrm, (outs), (ins), "rdmsr", [],
IIC_RDMSR>, TB;
> Maybe it's because I use MF.addLiveIn but when I tried using X86::EAX
it
> failed with the message:
>           # Machine code for function _Z5test2v: Post SSA
>
>           BB#0: derived from LLVM BB %entry
>                   %vreg2<def> = COPY %EAX; GR32:%vreg2
>                  RDMSR
>                  %EAX<def> = COPY %vreg2; GR32:%vreg2
addLiveIn is definitely wrong (%eax isn't live upon entry to the
function). That snippet looks like the CopyFromReg and CopyToReg are
backwards to me. And the reuse of %vreg2 isn't inspiring confidence,
though it might be correct.

I've not looked in detail, but there's code in InstrEmitter.cpp
hinting that the ordering-constraints we want from the Glue may be
ignored if there isn't any register dependence, so I'd suggest fixing
the RDMSR definition and removing the addLiveIn together. With luck
everything will start working.

Cheers.

Tim.

>     let Defs = [ECX], Uses = [EAX, EDX] in
>     def RDMSR : I<0x32, RawFrm, (outs), (ins), "rdmsr", [],
IIC_RDMSR>, TB;
Sorry, that's backwards. I think it should be:

     let Defs = [EAX, EDX], Uses = [ECX] in
     def RDMSR : I<0x32, RawFrm, (outs), (ins), "rdmsr", [],
IIC_RDMSR>, TB;

Cheers.

Tim.