[LLVMdev] Selection Condition Codes

[Villmow, Micah]

Eli, Thanks for the tips.
 I've been able to get something working using a custom instruction
inserter, however, I'm still having the problem of linking together the
setcc and the select_cc commands. I want to turn the setcc into a
comparison and use the results in the select_cc register. However, the
comparison information is in the select_cc instruction and the result of
the comparison is in the setcc instruction. What I am trying to figure
out is using MachineInstruction/MachineBasicBlock, how I can access the
previous/next instruction without having to use an iterator and parse
over the whole block.

Is this possible? I've put comments in the code below to help understand
what I'm attempting to do.


case INST::SELECT_CC:
            // Here we want to write a custom instruction inserter for
the
            // select_cc operation. What we want to do is turn this into
            // a series of instructions that in the end become a compare
            // and a cmov_logical
            {
                MachineOperand Dst = MI->getOperand(0);
                MachineOperand TrueVal = MI->getOperand(1);
                MachineOperand FalseVal = MI->getOperand(2);
                MachineOperand CCFlag = MI->getOperand(3);
                CC = (INSTCC::CondCodes)MI->getOperand(3).getImm();
			// Here I want to get the destination register
of SET_CC instruction and place it as the first addReg
                TODO(Get setcc destination register);
                BuildMI(BB,
TII.get(INST::CMOVLOG_32),Dst.getReg()).addReg(CCFlag.getImm()).addReg(T
rueVal.getReg()).addReg(FalseVal.getReg());
            }
            break;
        case INST::SET_CC:
            {
                MachineOperand Dst = MI->getOperand(0);
                MachineOperand LHS = MI->getOperand(1);
                MachineOperand RHS = MI->getOperand(2);
			// I want to get the CCFlag from the select_CC
instruction
                  // and place it in the TII.get() field.
                BuildMI(BB, TII.get(INST::IL_COMPARE_INSTR),
Dst.getReg()).addReg(LHS.getReg()).addReg(RHS.getReg());
            }
        default:
};

Again,
 Thanks for any tips you might be able to share.

Micah

-----Original Message-----
From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu]
On Behalf Of Eli Friedman
Sent: Thursday, September 11, 2008 8:12 PM
To: LLVM Developers Mailing List
Subject: Re: [LLVMdev] Selection Condition Codes

On Thu, Sep 11, 2008 at 6:14 PM, Villmow, Micah <Micah.Villmow at amd.com>
wrote:
> I am attempting to lower the selectCC instruction to the instruction
set of
> the backend I'm working on and I cannot seem to find a way to
correctly
> implement this instruction. I know how this instruction should get
> implemented; I just have yet to find a way to do it. I want the
select_cc
> instruction to be lowered into a comparison followed by a conditional
move.

"setOperationAction(ISD::SELECT_CC        , MVT::Other, Expand);"
should do the trick; that will prevent SELECT_CC instructions from
being introduced.  This one is easy to miss.

> def CMOVLOG     : InstFormat<(outs GPR:$dst), (ins GPR:$LHS, GPR:$RHS,
> GPR:$TVAL, GPR:$FVAL, CCOp:$cond),

The condition isn't properly an input... this seems likely to confuse
ISel.  I'm not sure, though; I'm not really an expert on patterns, and
they can mess up in non-obvious ways.

> Maybe I'm approaching this from the wrong way, but I don't think this
> transformation should be that difficult.

If you need to transform a construct into multiple instructions, there
are a few different ways; one is a custom lowering, one is a selection
pattern, and another is a custom inserter.  Stuffing multiple actual
instructions into a single target instruction is generally a bad
approach.

-Eli
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