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[ConstantFolding] Consolidate poison propagation for intrinsics #146878

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Merged
merged 4 commits into from
Jul 4, 2025
Merged

[ConstantFolding] Consolidate poison propagation for intrinsics #146878

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c1c32e2
Add test for ctpop with poison arg
nikic Jul 3, 2025
f867100
Consolidate poison constant folding
nikic Jul 3, 2025
6c1ed11
Adjust unit test
nikic Jul 3, 2025
95ab15c
Use IsaPred
nikic Jul 3, 2025
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3 changes: 3 additions & 0 deletions llvm/include/llvm/Analysis/ValueTracking.h
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Expand Up @@ -727,6 +727,9 @@ LLVM_ABI bool isGuaranteedToExecuteForEveryIteration(const Instruction *I,
/// getGuaranteedNonPoisonOp.
LLVM_ABI bool propagatesPoison(const Use &PoisonOp);

/// Return whether this intrinsic propagates poison for all operands.
LLVM_ABI bool intrinsicPropagatesPoison(Intrinsic::ID IID);

/// Return true if the given instruction must trigger undefined behavior
/// when I is executed with any operands which appear in KnownPoison holding
/// a poison value at the point of execution.
Expand Down
39 changes: 5 additions & 34 deletions llvm/lib/Analysis/ConstantFolding.cpp
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Expand Up @@ -2229,17 +2229,6 @@ static Constant *ConstantFoldScalarCall1(StringRef Name,
return nullptr;
}

if (isa<PoisonValue>(Operands[0])) {
// TODO: All of these operations should probably propagate poison.
switch (IntrinsicID) {
case Intrinsic::canonicalize:
case Intrinsic::sqrt:
return PoisonValue::get(Ty);
default:
break;
}
}

if (isa<UndefValue>(Operands[0])) {
// cosine(arg) is between -1 and 1. cosine(invalid arg) is NaN.
// ctpop() is between 0 and bitwidth, pick 0 for undef.
Expand Down Expand Up @@ -3228,11 +3217,6 @@ static Constant *ConstantFoldIntrinsicCall2(Intrinsic::ID IntrinsicID, Type *Ty,
case Intrinsic::smin:
case Intrinsic::umax:
case Intrinsic::umin:
// This is the same as for binary ops - poison propagates.
// TODO: Poison handling should be consolidated.
if (isa<PoisonValue>(Operands[0]) || isa<PoisonValue>(Operands[1]))
return PoisonValue::get(Ty);

if (!C0 && !C1)
return UndefValue::get(Ty);
if (!C0 || !C1)
Expand All @@ -3245,9 +3229,6 @@ static Constant *ConstantFoldIntrinsicCall2(Intrinsic::ID IntrinsicID, Type *Ty,

case Intrinsic::scmp:
case Intrinsic::ucmp:
if (isa<PoisonValue>(Operands[0]) || isa<PoisonValue>(Operands[1]))
return PoisonValue::get(Ty);

if (!C0 || !C1)
return ConstantInt::get(Ty, 0);

Expand Down Expand Up @@ -3314,11 +3295,6 @@ static Constant *ConstantFoldIntrinsicCall2(Intrinsic::ID IntrinsicID, Type *Ty,
}
case Intrinsic::uadd_sat:
case Intrinsic::sadd_sat:
// This is the same as for binary ops - poison propagates.
// TODO: Poison handling should be consolidated.
if (isa<PoisonValue>(Operands[0]) || isa<PoisonValue>(Operands[1]))
return PoisonValue::get(Ty);

if (!C0 && !C1)
return UndefValue::get(Ty);
if (!C0 || !C1)
Expand All @@ -3329,11 +3305,6 @@ static Constant *ConstantFoldIntrinsicCall2(Intrinsic::ID IntrinsicID, Type *Ty,
return ConstantInt::get(Ty, C0->sadd_sat(*C1));
case Intrinsic::usub_sat:
case Intrinsic::ssub_sat:
// This is the same as for binary ops - poison propagates.
// TODO: Poison handling should be consolidated.
if (isa<PoisonValue>(Operands[0]) || isa<PoisonValue>(Operands[1]))
return PoisonValue::get(Ty);

if (!C0 && !C1)
return UndefValue::get(Ty);
if (!C0 || !C1)
Expand Down Expand Up @@ -3592,11 +3563,6 @@ static Constant *ConstantFoldScalarCall3(StringRef Name,

if (IntrinsicID == Intrinsic::smul_fix ||
IntrinsicID == Intrinsic::smul_fix_sat) {
// poison * C -> poison
// C * poison -> poison
if (isa<PoisonValue>(Operands[0]) || isa<PoisonValue>(Operands[1]))
return PoisonValue::get(Ty);

const APInt *C0, *C1;
if (!getConstIntOrUndef(Operands[0], C0) ||
!getConstIntOrUndef(Operands[1], C1))
Expand Down Expand Up @@ -3670,6 +3636,11 @@ static Constant *ConstantFoldScalarCall(StringRef Name,
ArrayRef<Constant *> Operands,
const TargetLibraryInfo *TLI,
const CallBase *Call) {
if (IntrinsicID != Intrinsic::not_intrinsic &&
any_of(Operands, IsaPred<PoisonValue>) &&
intrinsicPropagatesPoison(IntrinsicID))
return PoisonValue::get(Ty);

if (Operands.size() == 1)
return ConstantFoldScalarCall1(Name, IntrinsicID, Ty, Operands, TLI, Call);

Expand Down
75 changes: 43 additions & 32 deletions llvm/lib/Analysis/ValueTracking.cpp
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Expand Up @@ -7879,6 +7879,47 @@ bool llvm::isGuaranteedToExecuteForEveryIteration(const Instruction *I,
llvm_unreachable("Instruction not contained in its own parent basic block.");
}

bool llvm::intrinsicPropagatesPoison(Intrinsic::ID IID) {
switch (IID) {
// TODO: Add more intrinsics.
case Intrinsic::sadd_with_overflow:
case Intrinsic::ssub_with_overflow:
case Intrinsic::smul_with_overflow:
case Intrinsic::uadd_with_overflow:
case Intrinsic::usub_with_overflow:
case Intrinsic::umul_with_overflow:
// If an input is a vector containing a poison element, the
// two output vectors (calculated results, overflow bits)'
// corresponding lanes are poison.
return true;
case Intrinsic::ctpop:
case Intrinsic::ctlz:
case Intrinsic::cttz:
case Intrinsic::abs:
case Intrinsic::smax:
case Intrinsic::smin:
case Intrinsic::umax:
case Intrinsic::umin:
case Intrinsic::scmp:
case Intrinsic::ucmp:
case Intrinsic::bitreverse:
case Intrinsic::bswap:
case Intrinsic::sadd_sat:
case Intrinsic::ssub_sat:
case Intrinsic::sshl_sat:
case Intrinsic::uadd_sat:
case Intrinsic::usub_sat:
case Intrinsic::ushl_sat:
case Intrinsic::smul_fix:
case Intrinsic::smul_fix_sat:
case Intrinsic::canonicalize:
case Intrinsic::sqrt:
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Suggested change
case Intrinsic::sqrt:
case Intrinsic::sqrt:
case Intrinsic::frexp:

See

llvm-project/llvm/lib/Analysis/ConstantFolding.cpp

Lines 3915 to 3916 in 438863a

if (isa<PoisonValue>(Op))
return {Op, PoisonValue::get(IntTy)};

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Consolidating the frexp case is more complicated due to the struct return. I think it needs some refactorings to ConstantFoldStructCall first.

return true;
default:
return false;
}
}

bool llvm::propagatesPoison(const Use &PoisonOp) {
const Operator *I = cast<Operator>(PoisonOp.getUser());
switch (I->getOpcode()) {
Expand All @@ -7889,38 +7930,8 @@ bool llvm::propagatesPoison(const Use &PoisonOp) {
case Instruction::Select:
return PoisonOp.getOperandNo() == 0;
case Instruction::Call:
if (auto *II = dyn_cast<IntrinsicInst>(I)) {
switch (II->getIntrinsicID()) {
// TODO: Add more intrinsics.
case Intrinsic::sadd_with_overflow:
case Intrinsic::ssub_with_overflow:
case Intrinsic::smul_with_overflow:
case Intrinsic::uadd_with_overflow:
case Intrinsic::usub_with_overflow:
case Intrinsic::umul_with_overflow:
// If an input is a vector containing a poison element, the
// two output vectors (calculated results, overflow bits)'
// corresponding lanes are poison.
return true;
case Intrinsic::ctpop:
case Intrinsic::ctlz:
case Intrinsic::cttz:
case Intrinsic::abs:
case Intrinsic::smax:
case Intrinsic::smin:
case Intrinsic::umax:
case Intrinsic::umin:
case Intrinsic::bitreverse:
case Intrinsic::bswap:
case Intrinsic::sadd_sat:
case Intrinsic::ssub_sat:
case Intrinsic::sshl_sat:
case Intrinsic::uadd_sat:
case Intrinsic::usub_sat:
case Intrinsic::ushl_sat:
return true;
}
}
if (auto *II = dyn_cast<IntrinsicInst>(I))
return intrinsicPropagatesPoison(II->getIntrinsicID());
return false;
case Instruction::ICmp:
case Instruction::FCmp:
Expand Down
8 changes: 8 additions & 0 deletions llvm/test/Transforms/InstSimplify/fold-intrinsics.ll
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Expand Up @@ -44,3 +44,11 @@ define void @powi_i16(float %V, ptr%P) {

ret void
}

define i32 @test_ctpop_poison(i32 %a) {
; CHECK-LABEL: @test_ctpop_poison(
; CHECK-NEXT: ret i32 poison
;
%res = tail call i32 @llvm.ctpop.i32(i32 poison)
ret i32 %res
}
2 changes: 1 addition & 1 deletion llvm/unittests/Analysis/ValueTrackingTest.cpp
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Expand Up @@ -910,7 +910,7 @@ TEST(ValueTracking, propagatesPoison) {
{false, "call i32 @llvm.fshr.i32(i32 %x, i32 %y, i32 %shamt)", 0},
{false, "call i32 @llvm.fshr.i32(i32 %x, i32 %y, i32 %shamt)", 1},
{false, "call i32 @llvm.fshr.i32(i32 %x, i32 %y, i32 %shamt)", 2},
{false, "call float @llvm.sqrt.f32(float %fx)", 0},
{true, "call float @llvm.sqrt.f32(float %fx)", 0},
{false, "call float @llvm.powi.f32.i32(float %fx, i32 %x)", 0},
{false, "call float @llvm.sin.f32(float %fx)", 0},
{false, "call float @llvm.cos.f32(float %fx)", 0},
Expand Down
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