[llvm] [VectorCombine] Fold reduce(trunc(x)) -> trunc(reduce(x)) iff cost effective (PR #81852)
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Thu Feb 15 04:54:13 PST 2024
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-backend-x86
Author: Simon Pilgrim (RKSimon)
<details>
<summary>Changes</summary>
Vector truncations can be pretty expensive, especially on X86, whilst scalar truncations are often free.
If the cost of performing the add/mul/and/or/xor reduction is cheap enough on the pre-truncated type, then avoid the vector truncation entirely.
Fixes https://github.com/llvm/llvm-project/issues/81469
---
Full diff: https://github.com/llvm/llvm-project/pull/81852.diff
2 Files Affected:
- (modified) llvm/lib/Transforms/Vectorize/VectorCombine.cpp (+63)
- (added) llvm/test/Transforms/VectorCombine/X86/reduction-of-truncations.ll (+91)
``````````diff
diff --git a/llvm/lib/Transforms/Vectorize/VectorCombine.cpp b/llvm/lib/Transforms/Vectorize/VectorCombine.cpp
index f18711ba30b708..20fb9de7c75aa9 100644
--- a/llvm/lib/Transforms/Vectorize/VectorCombine.cpp
+++ b/llvm/lib/Transforms/Vectorize/VectorCombine.cpp
@@ -111,6 +111,7 @@ class VectorCombine {
bool scalarizeLoadExtract(Instruction &I);
bool foldShuffleOfBinops(Instruction &I);
bool foldShuffleFromReductions(Instruction &I);
+ bool foldTruncFromReductions(Instruction &I);
bool foldSelectShuffle(Instruction &I, bool FromReduction = false);
void replaceValue(Value &Old, Value &New) {
@@ -1526,6 +1527,67 @@ bool VectorCombine::foldShuffleFromReductions(Instruction &I) {
return foldSelectShuffle(*Shuffle, true);
}
+/// Determine if its more efficient to fold:
+/// reduce(trunc(x)) -> trunc(reduce(x)).
+bool VectorCombine::foldTruncFromReductions(Instruction &I) {
+ auto *II = dyn_cast<IntrinsicInst>(&I);
+ if (!II)
+ return false;
+
+ unsigned ReductionOpc = 0;
+ switch (II->getIntrinsicID()) {
+ case Intrinsic::vector_reduce_add:
+ ReductionOpc = Instruction::Add;
+ break;
+ case Intrinsic::vector_reduce_mul:
+ ReductionOpc = Instruction::Mul;
+ break;
+ case Intrinsic::vector_reduce_and:
+ ReductionOpc = Instruction::And;
+ break;
+ case Intrinsic::vector_reduce_or:
+ ReductionOpc = Instruction::Or;
+ break;
+ case Intrinsic::vector_reduce_xor:
+ ReductionOpc = Instruction::Xor;
+ break;
+ default:
+ return false;
+ }
+ Value *ReductionSrc = I.getOperand(0);
+
+ Value *TruncSrc;
+ if (!match(ReductionSrc, m_Trunc(m_OneUse(m_Value(TruncSrc)))))
+ return false;
+
+ auto *Trunc = cast<CastInst>(ReductionSrc);
+ auto *TruncTy = cast<VectorType>(TruncSrc->getType());
+ auto *ReductionTy = cast<VectorType>(ReductionSrc->getType());
+ Type *ResultTy = I.getType();
+
+ TTI::TargetCostKind CostKind = TTI::TCK_RecipThroughput;
+ InstructionCost OldCost =
+ TTI.getCastInstrCost(Instruction::Trunc, ReductionTy, TruncTy,
+ TTI::CastContextHint::None, CostKind, Trunc) +
+ TTI.getArithmeticReductionCost(ReductionOpc, ReductionTy, std::nullopt,
+ CostKind);
+ InstructionCost NewCost =
+ TTI.getArithmeticReductionCost(ReductionOpc, TruncTy, std::nullopt,
+ CostKind) +
+ TTI.getCastInstrCost(Instruction::Trunc, ResultTy,
+ ReductionTy->getScalarType(),
+ TTI::CastContextHint::None, CostKind);
+
+ if (OldCost < NewCost || !NewCost.isValid())
+ return false;
+
+ Value *NewReduction = Builder.CreateIntrinsic(
+ TruncTy->getScalarType(), II->getIntrinsicID(), {TruncSrc});
+ Value *NewTruncation = Builder.CreateTrunc(NewReduction, ResultTy);
+ replaceValue(I, *NewTruncation);
+ return true;
+}
+
/// This method looks for groups of shuffles acting on binops, of the form:
/// %x = shuffle ...
/// %y = shuffle ...
@@ -1917,6 +1979,7 @@ bool VectorCombine::run() {
switch (Opcode) {
case Instruction::Call:
MadeChange |= foldShuffleFromReductions(I);
+ MadeChange |= foldTruncFromReductions(I);
break;
case Instruction::ICmp:
case Instruction::FCmp:
diff --git a/llvm/test/Transforms/VectorCombine/X86/reduction-of-truncations.ll b/llvm/test/Transforms/VectorCombine/X86/reduction-of-truncations.ll
new file mode 100644
index 00000000000000..54d8250cdec816
--- /dev/null
+++ b/llvm/test/Transforms/VectorCombine/X86/reduction-of-truncations.ll
@@ -0,0 +1,91 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -S --passes=vector-combine -mtriple=x86_64-- -mcpu=x86-64 | FileCheck %s --check-prefixes=CHECK,X64
+; RUN: opt < %s -S --passes=vector-combine -mtriple=x86_64-- -mcpu=x86-64-v2 | FileCheck %s --check-prefixes=CHECK,X64
+; RUN: opt < %s -S --passes=vector-combine -mtriple=x86_64-- -mcpu=x86-64-v3 | FileCheck %s --check-prefixes=CHECK,X64
+; RUN: opt < %s -S --passes=vector-combine -mtriple=x86_64-- -mcpu=x86-64-v4 | FileCheck %s --check-prefixes=CHECK,AVX512
+
+;
+; Fold reduce(trunc(X)) -> trunc(reduce(X)) if more cost efficient
+;
+
+; Cheap AVX512 v8i64 -> v8i32 truncation
+define i32 @reduce_add_trunc_v8i64_i32(<8 x i64> %a0) {
+; X64-LABEL: @reduce_add_trunc_v8i64_i32(
+; X64-NEXT: [[TMP1:%.*]] = call i64 @llvm.vector.reduce.add.v8i64(<8 x i64> [[A0:%.*]])
+; X64-NEXT: [[RED:%.*]] = trunc i64 [[TMP1]] to i32
+; X64-NEXT: ret i32 [[RED]]
+;
+; AVX512-LABEL: @reduce_add_trunc_v8i64_i32(
+; AVX512-NEXT: [[TR:%.*]] = trunc <8 x i64> [[A0:%.*]] to <8 x i32>
+; AVX512-NEXT: [[RED:%.*]] = tail call i32 @llvm.vector.reduce.add.v8i32(<8 x i32> [[TR]])
+; AVX512-NEXT: ret i32 [[RED]]
+;
+ %tr = trunc <8 x i64> %a0 to <8 x i32>
+ %red = tail call i32 @llvm.vector.reduce.add.v8i32(<8 x i32> %tr)
+ ret i32 %red
+}
+declare i32 @llvm.vector.reduce.add.v8i32(<8 x i32>)
+
+; No legal vXi8 multiplication so vXi16 is always cheaper
+define i8 @reduce_mul_trunc_v16i16_i8(<16 x i16> %a0) {
+; CHECK-LABEL: @reduce_mul_trunc_v16i16_i8(
+; CHECK-NEXT: [[TMP1:%.*]] = call i16 @llvm.vector.reduce.mul.v16i16(<16 x i16> [[A0:%.*]])
+; CHECK-NEXT: [[RED:%.*]] = trunc i16 [[TMP1]] to i8
+; CHECK-NEXT: ret i8 [[RED]]
+;
+ %tr = trunc <16 x i16> %a0 to <16 x i8>
+ %red = tail call i8 @llvm.vector.reduce.mul.v16i8(<16 x i8> %tr)
+ ret i8 %red
+}
+declare i8 @llvm.vector.reduce.mul.v16i8(<16 x i8>)
+
+define i8 @reduce_or_trunc_v8i32_i8(<8 x i32> %a0) {
+; CHECK-LABEL: @reduce_or_trunc_v8i32_i8(
+; CHECK-NEXT: [[TMP1:%.*]] = call i32 @llvm.vector.reduce.or.v8i32(<8 x i32> [[A0:%.*]])
+; CHECK-NEXT: [[RED:%.*]] = trunc i32 [[TMP1]] to i8
+; CHECK-NEXT: ret i8 [[RED]]
+;
+ %tr = trunc <8 x i32> %a0 to <8 x i8>
+ %red = tail call i8 @llvm.vector.reduce.or.v8i32(<8 x i8> %tr)
+ ret i8 %red
+}
+declare i32 @llvm.vector.reduce.or.v8i8(<8 x i8>)
+
+define i8 @reduce_xor_trunc_v16i64_i8(<16 x i64> %a0) {
+; CHECK-LABEL: @reduce_xor_trunc_v16i64_i8(
+; CHECK-NEXT: [[TMP1:%.*]] = call i64 @llvm.vector.reduce.xor.v16i64(<16 x i64> [[A0:%.*]])
+; CHECK-NEXT: [[RED:%.*]] = trunc i64 [[TMP1]] to i8
+; CHECK-NEXT: ret i8 [[RED]]
+;
+ %tr = trunc <16 x i64> %a0 to <16 x i8>
+ %red = tail call i8 @llvm.vector.reduce.xor.v16i8(<16 x i8> %tr)
+ ret i8 %red
+}
+declare i8 @llvm.vector.reduce.xor.v16i8(<16 x i8>)
+
+; Negative Test: vXi16 multiply is much cheaper than vXi64
+define i16 @reduce_mul_trunc_v8i64_i16(<8 x i64> %a0) {
+; CHECK-LABEL: @reduce_mul_trunc_v8i64_i16(
+; CHECK-NEXT: [[TR:%.*]] = trunc <8 x i64> [[A0:%.*]] to <8 x i16>
+; CHECK-NEXT: [[RED:%.*]] = tail call i16 @llvm.vector.reduce.mul.v8i16(<8 x i16> [[TR]])
+; CHECK-NEXT: ret i16 [[RED]]
+;
+ %tr = trunc <8 x i64> %a0 to <8 x i16>
+ %red = tail call i16 @llvm.vector.reduce.mul.v8i16(<8 x i16> %tr)
+ ret i16 %red
+}
+declare i16 @llvm.vector.reduce.mul.v8i16(<8 x i16>)
+
+; Negative Test: min/max reductions can't use pre-truncated types.
+define i8 @reduce_smin_trunc_v16i16_i8(<16 x i16> %a0) {
+; CHECK-LABEL: @reduce_smin_trunc_v16i16_i8(
+; CHECK-NEXT: [[TR:%.*]] = trunc <16 x i16> [[A0:%.*]] to <16 x i8>
+; CHECK-NEXT: [[RED:%.*]] = tail call i8 @llvm.vector.reduce.smin.v16i8(<16 x i8> [[TR]])
+; CHECK-NEXT: ret i8 [[RED]]
+;
+ %tr = trunc <16 x i16> %a0 to <16 x i8>
+ %red = tail call i8 @llvm.vector.reduce.smin.v16i8(<16 x i8> %tr)
+ ret i8 %red
+}
+declare i8 @llvm.vector.reduce.smin.v16i8(<16 x i8>)
+
``````````
</details>
https://github.com/llvm/llvm-project/pull/81852
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