[llvm] 4b4699a - [InstCombine] Don't cover up poison elements for shifts when folding shuffles thru binops (#141303)
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Sat May 24 05:47:22 PDT 2025
Author: Luke Lau
Date: 2025-05-24T13:47:18+01:00
New Revision: 4b4699a13c9d05fa828b06bf398f1432ff3d9fb8
URL: https://github.com/llvm/llvm-project/commit/4b4699a13c9d05fa828b06bf398f1432ff3d9fb8
DIFF: https://github.com/llvm/llvm-project/commit/4b4699a13c9d05fa828b06bf398f1432ff3d9fb8.diff
LOG: [InstCombine] Don't cover up poison elements for shifts when folding shuffles thru binops (#141303)
As noted in the TODO, we don't need to cover up the poison elements
placed in the unused lanes for shifts, since it's not UB unlike div/rem.
New poison elements are only introduced in cases like
ShMask = <1,1,2,2> and C = <5,5,6,6> --> NewC = <poison,5,6,poison>
And the resulting shuffle won't use the poison lanes.
Added:
Modified:
llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
llvm/test/Transforms/InstCombine/vec_shuffle-inseltpoison.ll
llvm/test/Transforms/InstCombine/vec_shuffle.ll
llvm/test/Transforms/LoopVectorize/AArch64/deterministic-type-shrinkage.ll
Removed:
################################################################################
diff --git a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
index c1608b1866a5d..2a96f4beb5f57 100644
--- a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -2257,11 +2257,9 @@ Instruction *InstCombinerImpl::foldVectorBinop(BinaryOperator &Inst) {
}
if (MayChange) {
Constant *NewC = ConstantVector::get(NewVecC);
- // It may not be safe to execute a binop on a vector with poison elements
- // because the entire instruction can be folded to undef or create poison
- // that did not exist in the original code.
- // TODO: The shift case should not be necessary.
- if (Inst.isIntDivRem() || (Inst.isShift() && ConstOp1))
+ // Lanes of NewC not used by the shuffle will be poison which will cause
+ // UB for div/rem. Mask them with a safe constant.
+ if (Inst.isIntDivRem())
NewC = getSafeVectorConstantForBinop(Opcode, NewC, ConstOp1);
// Op(shuffle(V1, Mask), C) -> shuffle(Op(V1, NewC), Mask)
diff --git a/llvm/test/Transforms/InstCombine/vec_shuffle-inseltpoison.ll b/llvm/test/Transforms/InstCombine/vec_shuffle-inseltpoison.ll
index 74d07fea9793a..9aa050e8cd500 100644
--- a/llvm/test/Transforms/InstCombine/vec_shuffle-inseltpoison.ll
+++ b/llvm/test/Transforms/InstCombine/vec_shuffle-inseltpoison.ll
@@ -880,7 +880,7 @@ define <2 x i32> @shl_splat_constant0(<2 x i32> %x) {
define <2 x i32> @shl_splat_constant1(<2 x i32> %x) {
; CHECK-LABEL: @shl_splat_constant1(
-; CHECK-NEXT: [[TMP1:%.*]] = shl <2 x i32> [[X:%.*]], <i32 5, i32 0>
+; CHECK-NEXT: [[TMP1:%.*]] = shl <2 x i32> [[X:%.*]], <i32 5, i32 poison>
; CHECK-NEXT: [[R:%.*]] = shufflevector <2 x i32> [[TMP1]], <2 x i32> poison, <2 x i32> zeroinitializer
; CHECK-NEXT: ret <2 x i32> [[R]]
;
@@ -902,7 +902,7 @@ define <2 x i32> @ashr_splat_constant0(<2 x i32> %x) {
define <2 x i32> @ashr_splat_constant1(<2 x i32> %x) {
; CHECK-LABEL: @ashr_splat_constant1(
-; CHECK-NEXT: [[TMP1:%.*]] = ashr <2 x i32> [[X:%.*]], <i32 5, i32 0>
+; CHECK-NEXT: [[TMP1:%.*]] = ashr <2 x i32> [[X:%.*]], <i32 5, i32 poison>
; CHECK-NEXT: [[R:%.*]] = shufflevector <2 x i32> [[TMP1]], <2 x i32> poison, <2 x i32> zeroinitializer
; CHECK-NEXT: ret <2 x i32> [[R]]
;
@@ -924,7 +924,7 @@ define <2 x i32> @lshr_splat_constant0(<2 x i32> %x) {
define <2 x i32> @lshr_splat_constant1(<2 x i32> %x) {
; CHECK-LABEL: @lshr_splat_constant1(
-; CHECK-NEXT: [[TMP1:%.*]] = lshr <2 x i32> [[X:%.*]], <i32 5, i32 0>
+; CHECK-NEXT: [[TMP1:%.*]] = lshr <2 x i32> [[X:%.*]], <i32 5, i32 poison>
; CHECK-NEXT: [[R:%.*]] = shufflevector <2 x i32> [[TMP1]], <2 x i32> poison, <2 x i32> zeroinitializer
; CHECK-NEXT: ret <2 x i32> [[R]]
;
@@ -1161,7 +1161,7 @@ entry:
define <4 x i16> @shl_constant_mask_undef(<4 x i16> %in) {
; CHECK-LABEL: @shl_constant_mask_undef(
; CHECK-NEXT: entry:
-; CHECK-NEXT: [[TMP0:%.*]] = shl <4 x i16> [[IN:%.*]], <i16 10, i16 0, i16 0, i16 0>
+; CHECK-NEXT: [[TMP0:%.*]] = shl <4 x i16> [[IN:%.*]], <i16 10, i16 0, i16 poison, i16 poison>
; CHECK-NEXT: [[SHL:%.*]] = shufflevector <4 x i16> [[TMP0]], <4 x i16> poison, <4 x i32> <i32 0, i32 poison, i32 1, i32 1>
; CHECK-NEXT: ret <4 x i16> [[SHL]]
;
diff --git a/llvm/test/Transforms/InstCombine/vec_shuffle.ll b/llvm/test/Transforms/InstCombine/vec_shuffle.ll
index eb88fbadab956..90ad88089054e 100644
--- a/llvm/test/Transforms/InstCombine/vec_shuffle.ll
+++ b/llvm/test/Transforms/InstCombine/vec_shuffle.ll
@@ -929,7 +929,7 @@ define <2 x i32> @shl_splat_constant0(<2 x i32> %x) {
define <2 x i32> @shl_splat_constant1(<2 x i32> %x) {
; CHECK-LABEL: @shl_splat_constant1(
-; CHECK-NEXT: [[TMP1:%.*]] = shl <2 x i32> [[X:%.*]], <i32 5, i32 0>
+; CHECK-NEXT: [[TMP1:%.*]] = shl <2 x i32> [[X:%.*]], <i32 5, i32 poison>
; CHECK-NEXT: [[R:%.*]] = shufflevector <2 x i32> [[TMP1]], <2 x i32> poison, <2 x i32> zeroinitializer
; CHECK-NEXT: ret <2 x i32> [[R]]
;
@@ -951,7 +951,7 @@ define <2 x i32> @ashr_splat_constant0(<2 x i32> %x) {
define <2 x i32> @ashr_splat_constant1(<2 x i32> %x) {
; CHECK-LABEL: @ashr_splat_constant1(
-; CHECK-NEXT: [[TMP1:%.*]] = ashr <2 x i32> [[X:%.*]], <i32 5, i32 0>
+; CHECK-NEXT: [[TMP1:%.*]] = ashr <2 x i32> [[X:%.*]], <i32 5, i32 poison>
; CHECK-NEXT: [[R:%.*]] = shufflevector <2 x i32> [[TMP1]], <2 x i32> poison, <2 x i32> zeroinitializer
; CHECK-NEXT: ret <2 x i32> [[R]]
;
@@ -973,7 +973,7 @@ define <2 x i32> @lshr_splat_constant0(<2 x i32> %x) {
define <2 x i32> @lshr_splat_constant1(<2 x i32> %x) {
; CHECK-LABEL: @lshr_splat_constant1(
-; CHECK-NEXT: [[TMP1:%.*]] = lshr <2 x i32> [[X:%.*]], <i32 5, i32 0>
+; CHECK-NEXT: [[TMP1:%.*]] = lshr <2 x i32> [[X:%.*]], <i32 5, i32 poison>
; CHECK-NEXT: [[R:%.*]] = shufflevector <2 x i32> [[TMP1]], <2 x i32> poison, <2 x i32> zeroinitializer
; CHECK-NEXT: ret <2 x i32> [[R]]
;
@@ -1210,7 +1210,7 @@ entry:
define <4 x i16> @shl_constant_mask_undef(<4 x i16> %in) {
; CHECK-LABEL: @shl_constant_mask_undef(
; CHECK-NEXT: entry:
-; CHECK-NEXT: [[TMP0:%.*]] = shl <4 x i16> [[IN:%.*]], <i16 10, i16 0, i16 0, i16 0>
+; CHECK-NEXT: [[TMP0:%.*]] = shl <4 x i16> [[IN:%.*]], <i16 10, i16 0, i16 poison, i16 poison>
; CHECK-NEXT: [[SHL:%.*]] = shufflevector <4 x i16> [[TMP0]], <4 x i16> poison, <4 x i32> <i32 0, i32 poison, i32 1, i32 1>
; CHECK-NEXT: ret <4 x i16> [[SHL]]
;
diff --git a/llvm/test/Transforms/LoopVectorize/AArch64/deterministic-type-shrinkage.ll b/llvm/test/Transforms/LoopVectorize/AArch64/deterministic-type-shrinkage.ll
index c2502aac5b61d..3e3d88e3cea2c 100644
--- a/llvm/test/Transforms/LoopVectorize/AArch64/deterministic-type-shrinkage.ll
+++ b/llvm/test/Transforms/LoopVectorize/AArch64/deterministic-type-shrinkage.ll
@@ -164,8 +164,8 @@ define void @test_shrink_zext_in_preheader(ptr noalias %src, ptr noalias %dst, i
; CHECK-NEXT: [[TMP1:%.*]] = insertelement <16 x i16> poison, i16 [[TMP0]], i64 0
; CHECK-NEXT: [[TMP2:%.*]] = insertelement <16 x i16> poison, i16 [[B]], i64 0
; CHECK-NEXT: [[TMP3:%.*]] = mul <16 x i16> [[TMP1]], [[TMP2]]
-; CHECK-NEXT: [[TMP4:%.*]] = lshr <16 x i16> [[TMP3]], <i16 8, i16 0, i16 0, i16 0, i16 0, i16 0, i16 0, i16 0, i16 0, i16 0, i16 0, i16 0, i16 0, i16 0, i16 0, i16 0>
-; CHECK-NEXT: [[TMP5:%.*]] = trunc <16 x i16> [[TMP4]] to <16 x i8>
+; CHECK-NEXT: [[TMP4:%.*]] = lshr <16 x i16> [[TMP3]], <i16 8, i16 poison, i16 poison, i16 poison, i16 poison, i16 poison, i16 poison, i16 poison, i16 poison, i16 poison, i16 poison, i16 poison, i16 poison, i16 poison, i16 poison, i16 poison>
+; CHECK-NEXT: [[TMP5:%.*]] = trunc nuw <16 x i16> [[TMP4]] to <16 x i8>
; CHECK-NEXT: [[TMP6:%.*]] = shufflevector <16 x i8> [[TMP5]], <16 x i8> poison, <16 x i32> zeroinitializer
; CHECK-NEXT: br label [[VECTOR_BODY:%.*]]
; CHECK: vector.body:
@@ -187,8 +187,8 @@ define void @test_shrink_zext_in_preheader(ptr noalias %src, ptr noalias %dst, i
; CHECK-NEXT: [[TMP12:%.*]] = insertelement <8 x i16> poison, i16 [[TMP11]], i64 0
; CHECK-NEXT: [[TMP13:%.*]] = insertelement <8 x i16> poison, i16 [[B]], i64 0
; CHECK-NEXT: [[TMP14:%.*]] = mul <8 x i16> [[TMP12]], [[TMP13]]
-; CHECK-NEXT: [[TMP15:%.*]] = lshr <8 x i16> [[TMP14]], <i16 8, i16 0, i16 0, i16 0, i16 0, i16 0, i16 0, i16 0>
-; CHECK-NEXT: [[TMP16:%.*]] = trunc <8 x i16> [[TMP15]] to <8 x i8>
+; CHECK-NEXT: [[TMP15:%.*]] = lshr <8 x i16> [[TMP14]], <i16 8, i16 poison, i16 poison, i16 poison, i16 poison, i16 poison, i16 poison, i16 poison>
+; CHECK-NEXT: [[TMP16:%.*]] = trunc nuw <8 x i16> [[TMP15]] to <8 x i8>
; CHECK-NEXT: [[TMP17:%.*]] = shufflevector <8 x i8> [[TMP16]], <8 x i8> poison, <8 x i32> zeroinitializer
; CHECK-NEXT: br label [[VEC_EPILOG_VECTOR_BODY:%.*]]
; CHECK: vec.epilog.vector.body:
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