[InstCombine] Fix miscompile bug in canEvaluateShuffled

Summary:
Add restrictions in canEvaluateShuffled to prevent that we for example
transform

  %0 = insertelement <2 x i16> undef, i16 %a, i32 0
  %1 = srem <2 x i16> %0, <i16 2, i16 1>
  %2 = shufflevector <2 x i16> %1, <2 x i16> undef, <2 x i32> <i32 undef, i32 0>

into

   %1 = insertelement <2 x i16> undef, i16 %a, i32 1
   %2 = srem <2 x i16> %1, <i16 undef, i16 2>

as having an undef denominator makes the srem undefined (for all
vector elements).

Fixes: https://bugs.llvm.org/show_bug.cgi?id=43689

Reviewers: spatel, lebedev.ri

Reviewed By: spatel, lebedev.ri

Subscribers: lebedev.ri, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D69038

llvm-svn: 375208

2 files changed, 37 insertions, 12 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp b/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
index b07aae466a3..13c38caa1e6 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp
@@ -1061,17 +1061,23 @@ static bool canEvaluateShuffled(Value *V, ArrayRef<int> Mask,
   if (Depth == 0) return false;
 
   switch (I->getOpcode()) {
+    case Instruction::UDiv:
+    case Instruction::SDiv:
+    case Instruction::URem:
+    case Instruction::SRem:
+      // Propagating an undefined shuffle mask element to integer div/rem is not
+      // allowed because those opcodes can create immediate undefined behavior
+      // from an undefined element in an operand.
+      if (llvm::any_of(Mask, [](int M){ return M == -1; }))
+        return false;
+      LLVM_FALLTHROUGH;
     case Instruction::Add:
     case Instruction::FAdd:
     case Instruction::Sub:
     case Instruction::FSub:
     case Instruction::Mul:
     case Instruction::FMul:
-    case Instruction::UDiv:
-    case Instruction::SDiv:
     case Instruction::FDiv:
-    case Instruction::URem:
-    case Instruction::SRem:
     case Instruction::FRem:
     case Instruction::Shl:
     case Instruction::LShr:
@@ -1092,9 +1098,7 @@ static bool canEvaluateShuffled(Value *V, ArrayRef<int> Mask,
     case Instruction::FPExt:
     case Instruction::GetElementPtr: {
       // Bail out if we would create longer vector ops. We could allow creating
-      // longer vector ops, but that may result in more expensive codegen. We
-      // would also need to limit the transform to avoid undefined behavior for
-      // integer div/rem.
+      // longer vector ops, but that may result in more expensive codegen.
       Type *ITy = I->getType();
       if (ITy->isVectorTy() && Mask.size() > ITy->getVectorNumElements())
         return false;
diff --git a/llvm/test/Transforms/InstCombine/shufflevector-div-rem.ll b/llvm/test/Transforms/InstCombine/shufflevector-div-rem.ll
index ee63a25653b..8364aab0aa2 100644
--- a/llvm/test/Transforms/InstCombine/shufflevector-div-rem.ll
+++ b/llvm/test/Transforms/InstCombine/shufflevector-div-rem.ll
@@ -8,7 +8,12 @@
 ; extracting the second element in the vector).
 define i16 @test_srem_orig(i16 %a, i1 %cmp) {
 ; CHECK-LABEL: @test_srem_orig(
-; CHECK-NEXT:    ret i16 1
+; CHECK-NEXT:    [[SPLATINSERT:%.*]] = insertelement <2 x i16> undef, i16 [[A:%.*]], i32 0
+; CHECK-NEXT:    [[TMP1:%.*]] = srem <2 x i16> [[SPLATINSERT]], <i16 2, i16 1>
+; CHECK-NEXT:    [[SPLAT_OP:%.*]] = shufflevector <2 x i16> [[TMP1]], <2 x i16> undef, <2 x i32> <i32 undef, i32 0>
+; CHECK-NEXT:    [[T2:%.*]] = select i1 [[CMP:%.*]], <2 x i16> <i16 undef, i16 1>, <2 x i16> [[SPLAT_OP]]
+; CHECK-NEXT:    [[T3:%.*]] = extractelement <2 x i16> [[T2]], i32 1
+; CHECK-NEXT:    ret i16 [[T3]]
 ;
   %splatinsert = insertelement <2 x i16> undef, i16 %a, i32 0
   %splat = shufflevector <2 x i16> %splatinsert, <2 x i16> undef, <2 x i32> zeroinitializer
@@ -24,7 +29,11 @@ define i16 @test_srem_orig(i16 %a, i1 %cmp) {
 ; "evaluateInDifferentElementOrder".
 define <2 x i16> @test_srem(i16 %a, i1 %cmp) {
 ; CHECK-LABEL: @test_srem(
-; CHECK-NEXT:    ret <2 x i16> <i16 77, i16 99>
+; CHECK-NEXT:    [[SPLATINSERT:%.*]] = insertelement <2 x i16> undef, i16 [[A:%.*]], i32 0
+; CHECK-NEXT:    [[T1:%.*]] = srem <2 x i16> [[SPLATINSERT]], <i16 2, i16 1>
+; CHECK-NEXT:    [[SPLAT_OP:%.*]] = shufflevector <2 x i16> [[T1]], <2 x i16> undef, <2 x i32> <i32 undef, i32 0>
+; CHECK-NEXT:    [[T2:%.*]] = select i1 [[CMP:%.*]], <2 x i16> <i16 77, i16 99>, <2 x i16> [[SPLAT_OP]]
+; CHECK-NEXT:    ret <2 x i16> [[T2]]
 ;
   %splatinsert = insertelement <2 x i16> undef, i16 %a, i32 0
   %t1 = srem <2 x i16> %splatinsert, <i16 2, i16 1>
@@ -35,7 +44,11 @@ define <2 x i16> @test_srem(i16 %a, i1 %cmp) {
 
 define <2 x i16> @test_urem(i16 %a, i1 %cmp) {
 ; CHECK-LABEL: @test_urem(
-; CHECK-NEXT:    ret <2 x i16> <i16 77, i16 99>
+; CHECK-NEXT:    [[SPLATINSERT:%.*]] = insertelement <2 x i16> undef, i16 [[A:%.*]], i32 0
+; CHECK-NEXT:    [[T1:%.*]] = urem <2 x i16> [[SPLATINSERT]], <i16 3, i16 1>
+; CHECK-NEXT:    [[SPLAT_OP:%.*]] = shufflevector <2 x i16> [[T1]], <2 x i16> undef, <2 x i32> <i32 undef, i32 0>
+; CHECK-NEXT:    [[T2:%.*]] = select i1 [[CMP:%.*]], <2 x i16> <i16 77, i16 99>, <2 x i16> [[SPLAT_OP]]
+; CHECK-NEXT:    ret <2 x i16> [[T2]]
 ;
   %splatinsert = insertelement <2 x i16> undef, i16 %a, i32 0
   %t1 = urem <2 x i16> %splatinsert, <i16 3, i16 1>
@@ -46,7 +59,11 @@ define <2 x i16> @test_urem(i16 %a, i1 %cmp) {
 
 define <2 x i16> @test_sdiv(i16 %a, i1 %cmp) {
 ; CHECK-LABEL: @test_sdiv(
-; CHECK-NEXT:    ret <2 x i16> <i16 77, i16 99>
+; CHECK-NEXT:    [[SPLATINSERT:%.*]] = insertelement <2 x i16> undef, i16 [[A:%.*]], i32 0
+; CHECK-NEXT:    [[T1:%.*]] = sdiv <2 x i16> [[SPLATINSERT]], <i16 2, i16 1>
+; CHECK-NEXT:    [[SPLAT_OP:%.*]] = shufflevector <2 x i16> [[T1]], <2 x i16> undef, <2 x i32> <i32 undef, i32 0>
+; CHECK-NEXT:    [[T2:%.*]] = select i1 [[CMP:%.*]], <2 x i16> <i16 77, i16 99>, <2 x i16> [[SPLAT_OP]]
+; CHECK-NEXT:    ret <2 x i16> [[T2]]
 ;
   %splatinsert = insertelement <2 x i16> undef, i16 %a, i32 0
   %t1 = sdiv <2 x i16> %splatinsert, <i16 2, i16 1>
@@ -57,7 +74,11 @@ define <2 x i16> @test_sdiv(i16 %a, i1 %cmp) {
 
 define <2 x i16> @test_udiv(i16 %a, i1 %cmp) {
 ; CHECK-LABEL: @test_udiv(
-; CHECK-NEXT:    ret <2 x i16> <i16 77, i16 99>
+; CHECK-NEXT:    [[SPLATINSERT:%.*]] = insertelement <2 x i16> undef, i16 [[A:%.*]], i32 0
+; CHECK-NEXT:    [[T1:%.*]] = udiv <2 x i16> [[SPLATINSERT]], <i16 3, i16 1>
+; CHECK-NEXT:    [[SPLAT_OP:%.*]] = shufflevector <2 x i16> [[T1]], <2 x i16> undef, <2 x i32> <i32 undef, i32 0>
+; CHECK-NEXT:    [[T2:%.*]] = select i1 [[CMP:%.*]], <2 x i16> <i16 77, i16 99>, <2 x i16> [[SPLAT_OP]]
+; CHECK-NEXT:    ret <2 x i16> [[T2]]
 ;
   %splatinsert = insertelement <2 x i16> undef, i16 %a, i32 0
   %t1 = udiv <2 x i16> %splatinsert, <i16 3, i16 1>