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. I've attempted to use a custom rule pasted below, but
it keeps giving me an error about the first argument.
What I attempted is as follows:
class InstFormat<dag outs, dag ins, string asmstr, list<dag>
pattern>
: Instruction {
let Namespace = "Inst";
dag OutOperandList = outs;
dag InOperandList = ins;
let Pattern = pattern;
let AsmString = asmstr;
}
def SDTGenTernaryOp : SDTypeProfile<1, 3, [ SDTCisSameAs<0,
1>,SDTCisSameAs<0, 2>, SDTCisSameAs<0, 3> ]>;
def cmov_logical : SDNode<"INSTISD::CMOVLOG", SDTGenTernaryOp>;
let PrintMethod = "printCCOperand" in
def CCOp : Operand<i32>;
def CMOVLOG : InstFormat<(outs GPR:$dst), (ins GPR:$LHS, GPR:$RHS,
GPR:$TVAL, GPR:$FVAL, CCOp:$cond),
"$cond $LHS, $LHS, $RHS\ncmov_logical
$dst, $LHS, $TVAL, $FVAL",
[(set GPR:$dst, (selectcc GPR:$LHS,
GPR:$RHS, GPR:$TVAL, GPR:$FVAL, CCOp:$cond))]>;
Maybe I'm approaching this from the wrong way, but I don't think this
transformation should be that difficult.
Basically I want dst = select $LHS, $RHS, $TVAL, $FVAL, $condOp to be
transformed into
$condOp tmp, $LHS, $RHS
cmov_logical dst, tmp, $TVAL, $FVAL
My attempt at doing so with a custom OperationAction also failed. So,
what would I need to do to figure out how to implement this correctly?
Is there any place you can point me to which shows how to go from a
single instruction to multiple consecutive instructions? A function that
I can study would be nice since my problem is even though I can look at
the other backends, I don't completely understand everything that is
going on so I probably overlooked this already.
Thanks,
Micah Villmow
Systems Engineer
Advanced Technology & Performance
Advanced Micro Devices Inc.
4555 Great America Pkwy,
Santa Clara, CA. 95054
P: 408-572-6219
F: 408-572-6596
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On Thu, Sep 11, 2008 at 6:14 PM, Villmow, Micah <Micah.Villmow at amd.com> wrote:> I am attempting to lower the selectCC instruction to the instruction set of > the backend I'm working on and I cannot seem to find a way to correctly > implement this instruction. I know how this instruction should get > implemented; I just have yet to find a way to do it. I want the select_cc > instruction to be lowered into a comparison followed by a conditional move."setOperationAction(ISD::SELECT_CC , MVT::Other, Expand);" should do the trick; that will prevent SELECT_CC instructions from being introduced. This one is easy to miss.> def CMOVLOG : InstFormat<(outs GPR:$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
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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