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[RISCV] Explicitly check for supported opcodes in optimizeCondBranch. NFC #145622

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Merged
merged 3 commits into from
Jun 26, 2025
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[RISCV] Explicitly check for supported opcodes in optimizeCondBranch. NFC #145622

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cfb5abc
[RISCV] Explicitly check for supported opcodes in optimizeCondBranch.
topperc Jun 25, 2025
817d497
fixup! Use addReg(RISCV::X0)
topperc Jun 26, 2025
5f834d4
Merge remote-tracking branch 'origin/main' into pr/condbranch
topperc Jun 26, 2025
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78 changes: 44 additions & 34 deletions llvm/lib/Target/RISCV/RISCVInstrInfo.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1379,40 +1379,49 @@ bool RISCVInstrInfo::isFromLoadImm(const MachineRegisterInfo &MRI,
}

bool RISCVInstrInfo::optimizeCondBranch(MachineInstr &MI) const {
bool IsSigned = false;
bool IsEquality = false;
switch (MI.getOpcode()) {
default:
return false;
case RISCV::BEQ:
case RISCV::BNE:
IsEquality = true;
break;
case RISCV::BGE:
case RISCV::BLT:
IsSigned = true;
break;
case RISCV::BGEU:
case RISCV::BLTU:
break;
}

MachineBasicBlock *MBB = MI.getParent();
MachineRegisterInfo &MRI = MBB->getParent()->getRegInfo();

MachineBasicBlock *TBB, *FBB;
SmallVector<MachineOperand, 3> Cond;
if (analyzeBranch(*MBB, TBB, FBB, Cond, /*AllowModify=*/false))
return false;
const MachineOperand &LHS = MI.getOperand(0);
const MachineOperand &RHS = MI.getOperand(1);
MachineBasicBlock *TBB = MI.getOperand(2).getMBB();

RISCVCC::CondCode CC = static_cast<RISCVCC::CondCode>(Cond[0].getImm());
RISCVCC::CondCode CC = getCondFromBranchOpc(MI.getOpcode());
assert(CC != RISCVCC::COND_INVALID);

auto modifyBranch = [&]() {
// Build the new branch and remove the old one.
BuildMI(*MBB, MI, MI.getDebugLoc(),
getBrCond(static_cast<RISCVCC::CondCode>(Cond[0].getImm())))
.add(Cond[1])
.add(Cond[2])
.addMBB(TBB);
MI.eraseFromParent();
};

// Canonicalize conditional branches which can be constant folded into
// beqz or bnez. We can't modify the CFG here.
int64_t C0, C1;
if (isFromLoadImm(MRI, Cond[1], C0) && isFromLoadImm(MRI, Cond[2], C1)) {
unsigned NewCC =
evaluateCondBranch(CC, C0, C1) ? RISCVCC::COND_EQ : RISCVCC::COND_NE;
Cond[0] = MachineOperand::CreateImm(NewCC);
Cond[1] = Cond[2] = MachineOperand::CreateReg(RISCV::X0, /*isDef=*/false);
modifyBranch();
if (isFromLoadImm(MRI, LHS, C0) && isFromLoadImm(MRI, RHS, C1)) {
unsigned NewOpc = evaluateCondBranch(CC, C0, C1) ? RISCV::BEQ : RISCV::BNE;
// Build the new branch and remove the old one.
BuildMI(*MBB, MI, MI.getDebugLoc(), get(NewOpc))
.addReg(RISCV::X0)
.addReg(RISCV::X0)
.addMBB(TBB);
MI.eraseFromParent();
return true;
}

if (CC == RISCVCC::COND_EQ || CC == RISCVCC::COND_NE)
if (IsEquality)
return false;

// For two constants C0 and C1 from
Expand All @@ -1432,8 +1441,6 @@ bool RISCVInstrInfo::optimizeCondBranch(MachineInstr &MI) const {
//
// To make sure this optimization is really beneficial, we only
// optimize for cases where Y had only one use (i.e. only used by the branch).
MachineOperand &LHS = MI.getOperand(0);
MachineOperand &RHS = MI.getOperand(1);
// Try to find the register for constant Z; return
// invalid register otherwise.
auto searchConst = [&](int64_t C1) -> Register {
Expand All @@ -1449,23 +1456,25 @@ bool RISCVInstrInfo::optimizeCondBranch(MachineInstr &MI) const {
return Register();
};

unsigned NewOpc = RISCVCC::getBrCond(getOppositeBranchCondition(CC));

// Might be case 1.
// Don't change 0 to 1 since we can use x0.
// For unsigned cases changing -1U to 0 would be incorrect.
// The incorrect case for signed would be INT_MAX, but isFromLoadImm can't
// return that.
if (isFromLoadImm(MRI, LHS, C0) && C0 != 0 && LHS.getReg().isVirtual() &&
MRI.hasOneUse(LHS.getReg()) &&
(CC == RISCVCC::COND_GE || CC == RISCVCC::COND_LT || C0 != -1)) {
MRI.hasOneUse(LHS.getReg()) && (IsSigned || C0 != -1)) {
assert(isInt<12>(C0) && "Unexpected immediate");
if (Register RegZ = searchConst(C0 + 1)) {
reverseBranchCondition(Cond);
Cond[1] = MachineOperand::CreateReg(RHS.getReg(), /*isDef=*/false);
Cond[2] = MachineOperand::CreateReg(RegZ, /*isDef=*/false);
BuildMI(*MBB, MI, MI.getDebugLoc(), get(NewOpc))
.add(RHS)
.addReg(RegZ)
.addMBB(TBB);
// We might extend the live range of Z, clear its kill flag to
// account for this.
MRI.clearKillFlags(RegZ);
modifyBranch();
MI.eraseFromParent();
return true;
}
}
Expand All @@ -1479,13 +1488,14 @@ bool RISCVInstrInfo::optimizeCondBranch(MachineInstr &MI) const {
MRI.hasOneUse(RHS.getReg())) {
assert(isInt<12>(C0) && "Unexpected immediate");
if (Register RegZ = searchConst(C0 - 1)) {
reverseBranchCondition(Cond);
Cond[1] = MachineOperand::CreateReg(RegZ, /*isDef=*/false);
Cond[2] = MachineOperand::CreateReg(LHS.getReg(), /*isDef=*/false);
BuildMI(*MBB, MI, MI.getDebugLoc(), get(NewOpc))
.addReg(RegZ)
.add(LHS)
.addMBB(TBB);
// We might extend the live range of Z, clear its kill flag to
// account for this.
MRI.clearKillFlags(RegZ);
modifyBranch();
MI.eraseFromParent();
return true;
}
}
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