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[GlobalISel] Reduce KnownBits usage in matcher combines #92381

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May 16, 2024
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[GlobalISel] Reduce KnownBits usage in matcher combines #92381

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102 changes: 67 additions & 35 deletions llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -3220,8 +3220,15 @@ bool CombinerHelper::matchRedundantAnd(MachineInstr &MI,
Register AndDst = MI.getOperand(0).getReg();
Register LHS = MI.getOperand(1).getReg();
Register RHS = MI.getOperand(2).getReg();
KnownBits LHSBits = KB->getKnownBits(LHS);

// Check the RHS (maybe a constant) first, and if we have no KnownBits there,
// we can't do anything. If we do, then it depends on whether we have
// KnownBits on the LHS.
KnownBits RHSBits = KB->getKnownBits(RHS);
if (RHSBits.isUnknown())
return false;

KnownBits LHSBits = KB->getKnownBits(LHS);

// Check that x & Mask == x.
// x & 1 == x, always
Expand Down Expand Up @@ -3260,6 +3267,7 @@ bool CombinerHelper::matchRedundantOr(MachineInstr &MI, Register &Replacement) {
Register OrDst = MI.getOperand(0).getReg();
Register LHS = MI.getOperand(1).getReg();
Register RHS = MI.getOperand(2).getReg();

KnownBits LHSBits = KB->getKnownBits(LHS);
KnownBits RHSBits = KB->getKnownBits(RHS);

Expand Down Expand Up @@ -4253,43 +4261,67 @@ bool CombinerHelper::matchICmpToTrueFalseKnownBits(MachineInstr &MI,
int64_t &MatchInfo) {
assert(MI.getOpcode() == TargetOpcode::G_ICMP);
auto Pred = static_cast<CmpInst::Predicate>(MI.getOperand(1).getPredicate());
auto KnownLHS = KB->getKnownBits(MI.getOperand(2).getReg());

// We want to avoid calling KnownBits on the LHS if possible, as this combine
// has no filter and runs on every G_ICMP instruction. We can avoid calling
// KnownBits on the LHS in two cases:
//
// - The RHS is unknown: Constants are always on RHS. If the RHS is unknown
// we cannot do any transforms so we can safely bail out early.
// - The RHS is zero: we don't need to know the LHS to do unsigned <0 and
// >=0.
auto KnownRHS = KB->getKnownBits(MI.getOperand(3).getReg());
if (KnownRHS.isUnknown())
return false;

std::optional<bool> KnownVal;
switch (Pred) {
default:
llvm_unreachable("Unexpected G_ICMP predicate?");
case CmpInst::ICMP_EQ:
KnownVal = KnownBits::eq(KnownLHS, KnownRHS);
break;
case CmpInst::ICMP_NE:
KnownVal = KnownBits::ne(KnownLHS, KnownRHS);
break;
case CmpInst::ICMP_SGE:
KnownVal = KnownBits::sge(KnownLHS, KnownRHS);
break;
case CmpInst::ICMP_SGT:
KnownVal = KnownBits::sgt(KnownLHS, KnownRHS);
break;
case CmpInst::ICMP_SLE:
KnownVal = KnownBits::sle(KnownLHS, KnownRHS);
break;
case CmpInst::ICMP_SLT:
KnownVal = KnownBits::slt(KnownLHS, KnownRHS);
break;
case CmpInst::ICMP_UGE:
KnownVal = KnownBits::uge(KnownLHS, KnownRHS);
break;
case CmpInst::ICMP_UGT:
KnownVal = KnownBits::ugt(KnownLHS, KnownRHS);
break;
case CmpInst::ICMP_ULE:
KnownVal = KnownBits::ule(KnownLHS, KnownRHS);
break;
case CmpInst::ICMP_ULT:
KnownVal = KnownBits::ult(KnownLHS, KnownRHS);
break;
if (KnownRHS.isZero()) {
// ? uge 0 -> always true
// ? ult 0 -> always false
if (Pred == CmpInst::ICMP_UGE)
KnownVal = true;
else if (Pred == CmpInst::ICMP_ULT)
KnownVal = false;
}

if (!KnownVal) {
auto KnownLHS = KB->getKnownBits(MI.getOperand(2).getReg());
switch (Pred) {
default:
llvm_unreachable("Unexpected G_ICMP predicate?");
case CmpInst::ICMP_EQ:
KnownVal = KnownBits::eq(KnownLHS, KnownRHS);
break;
case CmpInst::ICMP_NE:
KnownVal = KnownBits::ne(KnownLHS, KnownRHS);
break;
case CmpInst::ICMP_SGE:
KnownVal = KnownBits::sge(KnownLHS, KnownRHS);
break;
case CmpInst::ICMP_SGT:
KnownVal = KnownBits::sgt(KnownLHS, KnownRHS);
break;
case CmpInst::ICMP_SLE:
KnownVal = KnownBits::sle(KnownLHS, KnownRHS);
break;
case CmpInst::ICMP_SLT:
KnownVal = KnownBits::slt(KnownLHS, KnownRHS);
break;
case CmpInst::ICMP_UGE:
KnownVal = KnownBits::uge(KnownLHS, KnownRHS);
break;
case CmpInst::ICMP_UGT:
KnownVal = KnownBits::ugt(KnownLHS, KnownRHS);
break;
case CmpInst::ICMP_ULE:
KnownVal = KnownBits::ule(KnownLHS, KnownRHS);
break;
case CmpInst::ICMP_ULT:
KnownVal = KnownBits::ult(KnownLHS, KnownRHS);
break;
}
}

if (!KnownVal)
return false;
MatchInfo =
Expand Down
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