ConstantFold, InstSimplify: undef >>a x can be either -1 or 0, choose 0

Zero is usually a nicer constant to have than -1.

llvm-svn: 223969

3 files changed, 7 insertions, 6 deletions

diff --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp
index 1c5a917cbec..408768e8515 100644
--- a/llvm/lib/Analysis/InstructionSimplify.cpp
+++ b/llvm/lib/Analysis/InstructionSimplify.cpp
@@ -1422,11 +1422,11 @@ static Value *SimplifyAShrInst(Value *Op0, Value *Op1, bool isExact,
   if (match(Op0, m_AllOnes()))
     return Op0;
 
-  // undef >>a X -> all ones
+  // undef >>a X -> 0
   // undef >>a X -> undef (if it's exact)
   if (match(Op0, m_Undef()))
     return isExact ? UndefValue::get(Op0->getType())
-                   : Constant::getAllOnesValue(Op0->getType());
+                   : Constant::getNullValue(Op0->getType());
 
   // (X << A) >> A -> X
   Value *X;
diff --git a/llvm/lib/IR/ConstantFold.cpp b/llvm/lib/IR/ConstantFold.cpp
index 49ef3024033..a05c594ac8e 100644
--- a/llvm/lib/IR/ConstantFold.cpp
+++ b/llvm/lib/IR/ConstantFold.cpp
@@ -960,8 +960,9 @@ Constant *llvm::ConstantFoldBinaryInstruction(unsigned Opcode,
       // X >>a undef -> undef
       if (isa<UndefValue>(C2))
         return C2;
-      // undef >>a X -> all ones
-      return Constant::getAllOnesValue(C1->getType());
+      // TODO: undef >>a X -> undef if the shift is exact
+      // undef >>a X -> 0
+      return Constant::getNullValue(C1->getType());
     case Instruction::Shl:
       // X << undef -> undef
       if (isa<UndefValue>(C2))
diff --git a/llvm/test/Transforms/InstCombine/shift.ll b/llvm/test/Transforms/InstCombine/shift.ll
index 1acc0a56430..0b5b5deb68c 100644
--- a/llvm/test/Transforms/InstCombine/shift.ll
+++ b/llvm/test/Transforms/InstCombine/shift.ll
@@ -84,14 +84,14 @@ define <4 x i32> @test5a_non_splat_vector(<4 x i32> %A) {
 
 define i32 @test5b() {
 ; CHECK-LABEL: @test5b(
-; CHECK: ret i32 -1
+; CHECK: ret i32 0
         %B = ashr i32 undef, 2  ;; top two bits must be equal, so not undef
         ret i32 %B
 }
 
 define i32 @test5b2(i32 %A) {
 ; CHECK-LABEL: @test5b2(
-; CHECK: ret i32 -1
+; CHECK: ret i32 0
         %B = ashr i32 undef, %A  ;; top %A bits must be equal, so not undef
         ret i32 %B
 }