[InstCombine] Avoid incorrect folding of select into phi nodes when incoming element is a vector type

Summary:
We are incorrectly folding selects into phi nodes when the incoming value of a phi
node is a constant vector. This optimization is done in `FoldOpIntoPhi` when the
select condition is a phi node with constant incoming values.
Without the fix, we are miscompiling (i.e. incorrectly folding the
select into the phi node) when the vector contains non-zero
elements.
This patch fixes the miscompile and we will correctly fold based on the
select vector operand (see added test cases).

Reviewers: majnemer, sanjoy, spatel

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D31189

llvm-svn: 298845

2 files changed, 43 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
index 9cd22f65c99..020e8c9f046 100644
--- a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -908,7 +908,11 @@ Instruction *InstCombiner::FoldOpIntoPhi(Instruction &I) {
       // Beware of ConstantExpr:  it may eventually evaluate to getNullValue,
       // even if currently isNullValue gives false.
       Constant *InC = dyn_cast<Constant>(PN->getIncomingValue(i));
-      if (InC && !isa<ConstantExpr>(InC))
+      // For vector constants, we cannot use isNullValue to fold into
+      // FalseVInPred versus TrueVInPred. When we have individual nonzero
+      // elements in the vector, we will incorrectly fold InC to
+      // `TrueVInPred`.
+      if (InC && !isa<ConstantExpr>(InC) && !isa<VectorType>(InC->getType()))
         InV = InC->isNullValue() ? FalseVInPred : TrueVInPred;
       else
         InV = Builder->CreateSelect(PN->getIncomingValue(i),
diff --git a/llvm/test/Transforms/InstCombine/phi-select-constexpr.ll b/llvm/test/Transforms/InstCombine/phi-select-constexpr.ll
index 054e0691d47..272594d7f4f 100644
--- a/llvm/test/Transforms/InstCombine/phi-select-constexpr.ll
+++ b/llvm/test/Transforms/InstCombine/phi-select-constexpr.ll
@@ -9,6 +9,7 @@ entry:
 delay:
   br label %final
 
+; CHECK-LABEL: @foo
 ; CHECK-LABEL: final:
 ; CHECK: phi i32 [ 1, %entry ], [ select (i1 icmp eq (i32* @A, i32* @B), i32 2, i32 1), %delay ]
 final:
@@ -17,3 +18,40 @@ final:
   ret i32 %value
 }
 
+
+; test folding of select into phi for vectors.
+define <4 x i64> @vec1(i1 %which) {
+entry:
+  br i1 %which, label %final, label %delay
+
+delay:
+ br label %final
+
+final:
+; CHECK-LABEL: @vec1
+; CHECK-LABEL: final:
+; CHECK: %phinode = phi <4 x i64> [ zeroinitializer, %entry ], [ <i64 0, i64 0, i64 126, i64 127>, %delay ]
+; CHECK-NOT: select
+; CHECK: ret <4 x i64> %phinode
+ %phinode =  phi <4 x i1> [ <i1 true, i1 true, i1 true, i1 true>, %entry ], [ <i1 true, i1 true, i1 false, i1 false>, %delay ]
+ %sel = select <4 x i1> %phinode, <4 x i64> zeroinitializer, <4 x i64> <i64 124, i64 125, i64 126, i64 127>
+ ret <4 x i64> %sel
+}
+
+define <4 x i64> @vec2(i1 %which) {
+entry:
+  br i1 %which, label %final, label %delay
+
+delay:
+ br label %final
+
+final:
+; CHECK-LABEL: @vec2
+; CHECK-LABEL: final:
+; CHECK: %phinode = phi <4 x i64> [ <i64 124, i64 125, i64 126, i64 127>, %entry ], [ <i64 0, i64 125, i64 0, i64 127>, %delay ]
+; CHECK-NOT: select
+; CHECK: ret <4 x i64> %phinode
+ %phinode =  phi <4 x i1> [ <i1 false, i1 false, i1 false, i1 false>, %entry ], [ <i1 true, i1 false, i1 true, i1 false>, %delay ]
+ %sel = select <4 x i1> %phinode, <4 x i64> zeroinitializer, <4 x i64> <i64 124, i64 125, i64 126, i64 127>
+ ret <4 x i64> %sel
+}