llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-llvm-transforms
Author: Michele Scandale (michele-scandale)
<details>
<summary>Changes</summary>
Folding a select-like `shufflevector` into a floating point binary operators can only be done if the result is preserved for both case. In particular, if the common operand of the `shufflevector` and the floating point binary operator can be a NaN, then the transformation won't preserve the result value.
---
Full diff: https://github.com/llvm/llvm-project/pull/85452.diff
3 Files Affected:
- (modified) llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp (+16-3)
- (modified) llvm/test/Transforms/InstCombine/shuffle_select-inseltpoison.ll (+3-3)
- (modified) llvm/test/Transforms/InstCombine/shuffle_select.ll (+3-3)
``````````diff
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp b/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
index 3c4c0f35eb6d48..e3252633153930 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
@@ -2135,7 +2135,8 @@ static Instruction *foldSelectShuffleOfSelectShuffle(ShuffleVectorInst &Shuf) {
return new ShuffleVectorInst(X, Y, NewMask);
}
-static Instruction *foldSelectShuffleWith1Binop(ShuffleVectorInst &Shuf) {
+static Instruction *foldSelectShuffleWith1Binop(ShuffleVectorInst &Shuf,
+ const SimplifyQuery &SQ) {
assert(Shuf.isSelect() && "Must have select-equivalent shuffle");
// Are we shuffling together some value and that same value after it has been
@@ -2159,6 +2160,18 @@ static Instruction *foldSelectShuffleWith1Binop(ShuffleVectorInst &Shuf) {
if (!IdC)
return nullptr;
+ Value *X = Op0IsBinop ? Op1 : Op0;
+
+ // Prevent folding in the case the non-binop operand might have NaN values.
+ // If X can have NaN elements then we have that the floating point math
+ // operation in the transformed code may not preserve the exact NaN
+ // bit-pattern -- e.g. `fadd sNaN, 0.0 -> qNaN`.
+ // This makes the transformation incorrect since the original program would
+ // have preserved the exact NaN bit-pattern.
+ // Avoid the folding if X can have NaN elements.
+ if (Shuf.getType()->isFPOrFPVectorTy() && !isKnownNeverNaN(X, 0, SQ))
+ return nullptr;
+
// Shuffle identity constants into the lanes that return the original value.
// Example: shuf (mul X, {-1,-2,-3,-4}), X, {0,5,6,3} --> mul X, {-1,1,1,-4}
// Example: shuf X, (add X, {-1,-2,-3,-4}), {0,1,6,7} --> add X, {0,0,-3,-4}
@@ -2175,7 +2188,6 @@ static Instruction *foldSelectShuffleWith1Binop(ShuffleVectorInst &Shuf) {
// shuf (bop X, C), X, M --> bop X, C'
// shuf X, (bop X, C), M --> bop X, C'
- Value *X = Op0IsBinop ? Op1 : Op0;
Instruction *NewBO = BinaryOperator::Create(BOpcode, X, NewC);
NewBO->copyIRFlags(BO);
@@ -2241,7 +2253,8 @@ Instruction *InstCombinerImpl::foldSelectShuffle(ShuffleVectorInst &Shuf) {
if (Instruction *I = foldSelectShuffleOfSelectShuffle(Shuf))
return I;
- if (Instruction *I = foldSelectShuffleWith1Binop(Shuf))
+ if (Instruction *I = foldSelectShuffleWith1Binop(
+ Shuf, getSimplifyQuery().getWithInstruction(&Shuf)))
return I;
BinaryOperator *B0, *B1;
diff --git a/llvm/test/Transforms/InstCombine/shuffle_select-inseltpoison.ll b/llvm/test/Transforms/InstCombine/shuffle_select-inseltpoison.ll
index 44ec77e471bb41..9b4f80d3150758 100644
--- a/llvm/test/Transforms/InstCombine/shuffle_select-inseltpoison.ll
+++ b/llvm/test/Transforms/InstCombine/shuffle_select-inseltpoison.ll
@@ -336,7 +336,7 @@ define <4 x i32> @srem(<4 x i32> %v) {
; Try FP ops/types.
-define <4 x float> @fadd(<4 x float> %v) {
+define <4 x float> @fadd(<4 x float> nofpclass(nan) %v) {
; CHECK-LABEL: @fadd(
; CHECK-NEXT: [[S:%.*]] = fadd <4 x float> [[V:%.*]], <float 4.100000e+01, float 4.200000e+01, float -0.000000e+00, float -0.000000e+00>
; CHECK-NEXT: ret <4 x float> [[S]]
@@ -359,7 +359,7 @@ define <4 x double> @fsub(<4 x double> %v) {
; Propagate any FMF.
-define <4 x float> @fmul(<4 x float> %v) {
+define <4 x float> @fmul(<4 x float> nofpclass(nan) %v) {
; CHECK-LABEL: @fmul(
; CHECK-NEXT: [[S:%.*]] = fmul nnan ninf <4 x float> [[V:%.*]], <float 4.100000e+01, float 1.000000e+00, float 1.000000e+00, float 1.000000e+00>
; CHECK-NEXT: ret <4 x float> [[S]]
@@ -380,7 +380,7 @@ define <4 x double> @fdiv_constant_op0(<4 x double> %v) {
ret <4 x double> %s
}
-define <4 x double> @fdiv_constant_op1(<4 x double> %v) {
+define <4 x double> @fdiv_constant_op1(<4 x double> nofpclass(nan) %v) {
; CHECK-LABEL: @fdiv_constant_op1(
; CHECK-NEXT: [[S:%.*]] = fdiv reassoc <4 x double> [[V:%.*]], <double undef, double 1.000000e+00, double 4.300000e+01, double 4.400000e+01>
; CHECK-NEXT: ret <4 x double> [[S]]
diff --git a/llvm/test/Transforms/InstCombine/shuffle_select.ll b/llvm/test/Transforms/InstCombine/shuffle_select.ll
index a1b0d782b554f9..aae02a13dc8cd6 100644
--- a/llvm/test/Transforms/InstCombine/shuffle_select.ll
+++ b/llvm/test/Transforms/InstCombine/shuffle_select.ll
@@ -336,7 +336,7 @@ define <4 x i32> @srem(<4 x i32> %v) {
; Try FP ops/types.
-define <4 x float> @fadd(<4 x float> %v) {
+define <4 x float> @fadd(<4 x float> nofpclass(nan) %v) {
; CHECK-LABEL: @fadd(
; CHECK-NEXT: [[S:%.*]] = fadd <4 x float> [[V:%.*]], <float 4.100000e+01, float 4.200000e+01, float -0.000000e+00, float -0.000000e+00>
; CHECK-NEXT: ret <4 x float> [[S]]
@@ -359,7 +359,7 @@ define <4 x double> @fsub(<4 x double> %v) {
; Propagate any FMF.
-define <4 x float> @fmul(<4 x float> %v) {
+define <4 x float> @fmul(<4 x float> nofpclass(nan) %v) {
; CHECK-LABEL: @fmul(
; CHECK-NEXT: [[S:%.*]] = fmul nnan ninf <4 x float> [[V:%.*]], <float 4.100000e+01, float 1.000000e+00, float 1.000000e+00, float 1.000000e+00>
; CHECK-NEXT: ret <4 x float> [[S]]
@@ -380,7 +380,7 @@ define <4 x double> @fdiv_constant_op0(<4 x double> %v) {
ret <4 x double> %s
}
-define <4 x double> @fdiv_constant_op1(<4 x double> %v) {
+define <4 x double> @fdiv_constant_op1(<4 x double> nofpclass(nan) %v) {
; CHECK-LABEL: @fdiv_constant_op1(
; CHECK-NEXT: [[S:%.*]] = fdiv reassoc <4 x double> [[V:%.*]], <double undef, double 1.000000e+00, double 4.300000e+01, double 4.400000e+01>
; CHECK-NEXT: ret <4 x double> [[S]]
``````````
</details>
https://github.com/llvm/llvm-project/pull/85452