[ValueTracking] ComputeNumSignBitsImpl - add basic handling of BITCAST nodes #127218
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vortex73
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llvmbot
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llvm:instcombine
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@llvm/pr-subscribers-llvm-transforms Author: Narayan (vortex73) ChangesWhen a wider scalar/vector type containing all sign bits is bitcast to a narrower vector type, we can deduce that the resulting narrow elements will also be all sign bits. This matches existing behavior in SelectionDAG and helps optimize cases involving SSE intrinsics where sign-extended values are bitcast between different vector types. The current implementation fails to recognize that an arithmetic right shift is redundant when applied to elements that are already known to be all sign bits. This PR improves ComputeNumSignBitsImpl to track this information through bitcasts, enabling the optimization of such cases. Closes #87624 Full diff: https://github.com/llvm/llvm-project/pull/127218.diff 2 Files Affected:
diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index 8a9ad55366ee7..b0b963c14f8a5 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -3922,6 +3922,35 @@ static unsigned ComputeNumSignBitsImpl(const Value *V,
if (auto *U = dyn_cast<Operator>(V)) {
switch (Operator::getOpcode(V)) {
default: break;
+ case Instruction::BitCast: {
+ Value *Src = U->getOperand(0);
+ Type *SrcTy = Src->getType();
+ Type *SrcScalarTy = SrcTy->getScalarType();
+
+ if (!SrcScalarTy->isIntegerTy() && !SrcScalarTy->isFloatingPointTy())
+ break;
+
+ unsigned SrcBits = SrcTy->getScalarSizeInBits();
+
+ if ((SrcBits % TyBits) != 0)
+ break;
+
+ if (auto *DstVTy = dyn_cast<FixedVectorType>(Ty)) {
+ unsigned Scale = SrcBits / TyBits;
+
+ APInt SrcDemandedElts =
+ APInt::getSplat(DstVTy->getNumElements() / Scale, APInt(1, 1));
+
+ Tmp = ComputeNumSignBits(Src, SrcDemandedElts, Depth + 1, Q);
+ if (Tmp == SrcBits)
+ return TyBits;
+ } else {
+ Tmp = ComputeNumSignBits(Src, APInt(1, 1), Depth + 1, Q);
+ if (Tmp == SrcBits)
+ return TyBits;
+ }
+ break;
+ }
case Instruction::SExt:
Tmp = TyBits - U->getOperand(0)->getType()->getScalarSizeInBits();
return ComputeNumSignBits(U->getOperand(0), DemandedElts, Depth + 1, Q) +
diff --git a/llvm/test/Transforms/InstCombine/compute-sign-bits-bitcast.ll b/llvm/test/Transforms/InstCombine/compute-sign-bits-bitcast.ll
new file mode 100644
index 0000000000000..35ba53687c646
--- /dev/null
+++ b/llvm/test/Transforms/InstCombine/compute-sign-bits-bitcast.ll
@@ -0,0 +1,34 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+; RUN: opt -passes=instcombine -S < %s | FileCheck %s
+
+define i32 @test_compute_sign_bits() {
+; CHECK-LABEL: define i32 @test_compute_sign_bits() {
+; CHECK-NEXT: [[ENTRY:.*:]]
+; CHECK-NEXT: ret i32 -1
+;
+entry:
+ %a = add i8 -1, 0
+ %b = bitcast i8 %a to <4 x i2>
+ %c = ashr <4 x i2> %b, <i2 1, i2 1, i2 1, i2 1>
+ %d = bitcast <4 x i2> %c to i8
+ %e = sext i8 %d to i32
+ ret i32 %e
+}
+
+; Test with sign extension to ensure proper sign bit tracking
+define <4 x i2> @test_sext_bitcast(<1 x i8> %a0, <1 x i8> %a1) {
+; CHECK-LABEL: define <4 x i2> @test_sext_bitcast(
+; CHECK-SAME: <1 x i8> [[A0:%.*]], <1 x i8> [[A1:%.*]]) {
+; CHECK-NEXT: [[ENTRY:.*:]]
+; CHECK-NEXT: [[CMP:%.*]] = icmp sgt <1 x i8> [[A0]], [[A1]]
+; CHECK-NEXT: [[EXT:%.*]] = sext <1 x i1> [[CMP]] to <1 x i8>
+; CHECK-NEXT: [[SUB:%.*]] = bitcast <1 x i8> [[EXT]] to <4 x i2>
+; CHECK-NEXT: ret <4 x i2> [[SUB]]
+;
+entry:
+ %cmp = icmp sgt <1 x i8> %a0, %a1
+ %ext = sext <1 x i1> %cmp to <1 x i8>
+ %sub = bitcast <1 x i8> %ext to <4 x i2>
+ %result = ashr <4 x i2> %sub, <i2 1, i2 1, i2 1, i2 1>
+ ret <4 x i2> %result
+}
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Member
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@llvm/pr-subscribers-llvm-analysis Author: Narayan (vortex73) ChangesWhen a wider scalar/vector type containing all sign bits is bitcast to a narrower vector type, we can deduce that the resulting narrow elements will also be all sign bits. This matches existing behavior in SelectionDAG and helps optimize cases involving SSE intrinsics where sign-extended values are bitcast between different vector types. The current implementation fails to recognize that an arithmetic right shift is redundant when applied to elements that are already known to be all sign bits. This PR improves ComputeNumSignBitsImpl to track this information through bitcasts, enabling the optimization of such cases. Closes #87624 Full diff: https://github.com/llvm/llvm-project/pull/127218.diff 2 Files Affected:
diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index 8a9ad55366ee7..b0b963c14f8a5 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -3922,6 +3922,35 @@ static unsigned ComputeNumSignBitsImpl(const Value *V,
if (auto *U = dyn_cast<Operator>(V)) {
switch (Operator::getOpcode(V)) {
default: break;
+ case Instruction::BitCast: {
+ Value *Src = U->getOperand(0);
+ Type *SrcTy = Src->getType();
+ Type *SrcScalarTy = SrcTy->getScalarType();
+
+ if (!SrcScalarTy->isIntegerTy() && !SrcScalarTy->isFloatingPointTy())
+ break;
+
+ unsigned SrcBits = SrcTy->getScalarSizeInBits();
+
+ if ((SrcBits % TyBits) != 0)
+ break;
+
+ if (auto *DstVTy = dyn_cast<FixedVectorType>(Ty)) {
+ unsigned Scale = SrcBits / TyBits;
+
+ APInt SrcDemandedElts =
+ APInt::getSplat(DstVTy->getNumElements() / Scale, APInt(1, 1));
+
+ Tmp = ComputeNumSignBits(Src, SrcDemandedElts, Depth + 1, Q);
+ if (Tmp == SrcBits)
+ return TyBits;
+ } else {
+ Tmp = ComputeNumSignBits(Src, APInt(1, 1), Depth + 1, Q);
+ if (Tmp == SrcBits)
+ return TyBits;
+ }
+ break;
+ }
case Instruction::SExt:
Tmp = TyBits - U->getOperand(0)->getType()->getScalarSizeInBits();
return ComputeNumSignBits(U->getOperand(0), DemandedElts, Depth + 1, Q) +
diff --git a/llvm/test/Transforms/InstCombine/compute-sign-bits-bitcast.ll b/llvm/test/Transforms/InstCombine/compute-sign-bits-bitcast.ll
new file mode 100644
index 0000000000000..35ba53687c646
--- /dev/null
+++ b/llvm/test/Transforms/InstCombine/compute-sign-bits-bitcast.ll
@@ -0,0 +1,34 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+; RUN: opt -passes=instcombine -S < %s | FileCheck %s
+
+define i32 @test_compute_sign_bits() {
+; CHECK-LABEL: define i32 @test_compute_sign_bits() {
+; CHECK-NEXT: [[ENTRY:.*:]]
+; CHECK-NEXT: ret i32 -1
+;
+entry:
+ %a = add i8 -1, 0
+ %b = bitcast i8 %a to <4 x i2>
+ %c = ashr <4 x i2> %b, <i2 1, i2 1, i2 1, i2 1>
+ %d = bitcast <4 x i2> %c to i8
+ %e = sext i8 %d to i32
+ ret i32 %e
+}
+
+; Test with sign extension to ensure proper sign bit tracking
+define <4 x i2> @test_sext_bitcast(<1 x i8> %a0, <1 x i8> %a1) {
+; CHECK-LABEL: define <4 x i2> @test_sext_bitcast(
+; CHECK-SAME: <1 x i8> [[A0:%.*]], <1 x i8> [[A1:%.*]]) {
+; CHECK-NEXT: [[ENTRY:.*:]]
+; CHECK-NEXT: [[CMP:%.*]] = icmp sgt <1 x i8> [[A0]], [[A1]]
+; CHECK-NEXT: [[EXT:%.*]] = sext <1 x i1> [[CMP]] to <1 x i8>
+; CHECK-NEXT: [[SUB:%.*]] = bitcast <1 x i8> [[EXT]] to <4 x i2>
+; CHECK-NEXT: ret <4 x i2> [[SUB]]
+;
+entry:
+ %cmp = icmp sgt <1 x i8> %a0, %a1
+ %ext = sext <1 x i1> %cmp to <1 x i8>
+ %sub = bitcast <1 x i8> %ext to <4 x i2>
+ %result = ashr <4 x i2> %sub, <i2 1, i2 1, i2 1, i2 1>
+ ret <4 x i2> %result
+}
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Contributor
Author
dtcxzyw
reviewed
Feb 24, 2025
RKSimon
reviewed
Feb 24, 2025
Contributor
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@RKSimon Please have a look now. Since the original issue was only concerned with * → vector casts, I've not pushed my implementation for casts to scalars. If is a valid addition I could spend some time on that. |
RKSimon
reviewed
Mar 4, 2025
RKSimon
reviewed
Mar 5, 2025
RKSimon
reviewed
Mar 5, 2025
RKSimon
reviewed
Mar 5, 2025
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When a wider scalar/vector type containing all sign bits is bitcast to a narrower vector type, we can deduce that the resulting narrow elements will also be all sign bits. This matches existing behavior in SelectionDAG and helps optimize cases involving SSE intrinsics where sign-extended values are bitcast between different vector types.
The current implementation fails to recognize that an arithmetic right shift is redundant when applied to elements that are already known to be all sign bits. This PR improves ComputeNumSignBitsImpl to track this information through bitcasts, enabling the optimization of such cases.
Closes #87624