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[CodeGen] MachineVerifier to check early-clobber constraint #151421

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[CodeGen] MachineVerifier to check early-clobber constraint #151421

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[CodeGen] MachineVerifier to check early-clobber constraint
AbhayKanhere Jul 30, 2025
a0a0504
MAD_U64_U32 set early-clobber - Machine verifier
AbhayKanhere Aug 4, 2025
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7 changes: 7 additions & 0 deletions llvm/lib/CodeGen/MachineVerifier.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -2325,6 +2325,13 @@ void MachineVerifier::visitMachineInstrBefore(const MachineInstr *MI) {
report("Missing mayStore flag", MI);
}

// Verify earlyClobber def operand
if (MCID.getOperandConstraint(0, MCOI::EARLY_CLOBBER) != -1) {
if (!MI->getOperand(0).isReg())
report("Early clobber must be a register", MI);
if (!MI->getOperand(0).isEarlyClobber())
report("Missing earlyClobber flag", MI);
}
// Debug values must not have a slot index.
// Other instructions must have one, unless they are inside a bundle.
if (LiveInts) {
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1 change: 1 addition & 0 deletions llvm/lib/Target/AMDGPU/AMDGPUInstructionSelector.cpp
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Expand Up @@ -584,6 +584,7 @@ bool AMDGPUInstructionSelector::selectG_AMDGPU_MAD_64_32(
I.setDesc(TII.get(Opc));
I.addOperand(*MF, MachineOperand::CreateImm(0));
I.addImplicitDefUseOperands(*MF);
I.getOperand(0).setIsEarlyClobber(true);
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This should be a property of the instruction definition and not require manual setting

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@AbhayKanhere AbhayKanhere Aug 5, 2025 •
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earlyClobber and TiedTo are two of the OperandConstraint -- we check for tiedTo but not earlyClobber in machine verifier. on CPU targets register allocators strictly look at earlyClobber on operand. Recently we discovered that when MI.setDesc is used to modify the opcode, this constraint on Operand is not updated. However if new instruction is built and operand added using addOperand() , then earlyClobber is set correctly.. Looks like AMDGPU backend makes heavy use of setDesc (since the earlySelect expects in-place change to opcode) but does not update earlyClobber. I found many instances of this.

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@AbhayKanhere AbhayKanhere Aug 5, 2025 •
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Internally we discussed a variant for MI.setDesc(MCID, bool updateOpConstraint=false) that updates OperandConstraints if passed true, but setDesc is often part of a set of update operations on operands and not the last in the sequence so it is not always possible to update operand constraints there.

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The problem here is that MI.setDesc which only updates opcode, and MI.addOperand following MI.buildInstr/ that create new instruction behave differently in how Operand constraints are updated.

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ok, still needs tests

return constrainSelectedInstRegOperands(I, TII, TRI, RBI);
}

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