From 73059bd1f10f28f574304c98038b91103cfdb68c Mon Sep 17 00:00:00 2001
From: David Majnemer <david.majnemer@gmail.com>
Date: Thu, 18 Dec 2014 23:54:43 +0000
Subject: [PATCH] ConstantFold: Shifting undef by zero results in undef

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224553 91177308-0d34-0410-b5e6-96231b3b80d8
---
 lib/IR/ConstantFold.cpp               |  9 +++++++++
 test/Transforms/InstSimplify/undef.ll | 21 +++++++++++++++++++++
 2 files changed, 30 insertions(+)

diff --git a/lib/IR/ConstantFold.cpp b/lib/IR/ConstantFold.cpp
index 31e962af0b..9176bf2aee 100644
--- a/lib/IR/ConstantFold.cpp
+++ b/lib/IR/ConstantFold.cpp
@@ -956,12 +956,18 @@ Constant *llvm::ConstantFoldBinaryInstruction(unsigned Opcode,
       // X >>l undef -> undef
       if (isa<UndefValue>(C2))
         return C2;
+      // undef >>l 0 -> undef
+      if (match(C2, m_Zero()))
+        return C1;
       // undef >>l X -> 0
       return Constant::getNullValue(C1->getType());
     case Instruction::AShr:
       // X >>a undef -> undef
       if (isa<UndefValue>(C2))
         return C2;
+      // undef >>a 0 -> undef
+      if (match(C2, m_Zero()))
+        return C1;
       // TODO: undef >>a X -> undef if the shift is exact
       // undef >>a X -> 0
       return Constant::getNullValue(C1->getType());
@@ -969,6 +975,9 @@ Constant *llvm::ConstantFoldBinaryInstruction(unsigned Opcode,
       // X << undef -> undef
       if (isa<UndefValue>(C2))
         return C2;
+      // undef << 0 -> undef
+      if (match(C2, m_Zero()))
+        return C1;
       // undef << X -> 0
       return Constant::getNullValue(C1->getType());
     }
diff --git a/test/Transforms/InstSimplify/undef.ll b/test/Transforms/InstSimplify/undef.ll
index bf643171cc..e8b49b6397 100644
--- a/test/Transforms/InstSimplify/undef.ll
+++ b/test/Transforms/InstSimplify/undef.ll
@@ -244,3 +244,24 @@ define i32 @test31(i32 %a) {
   %b = shl i32 undef, %a
   ret i32 %b
 }
+
+; CHECK-LABEL: @test32
+; CHECK: ret i32 undef
+define i32 @test32(i32 %a) {
+  %b = shl i32 undef, 0
+  ret i32 %b
+}
+
+; CHECK-LABEL: @test33
+; CHECK: ret i32 undef
+define i32 @test33(i32 %a) {
+  %b = ashr i32 undef, 0
+  ret i32 %b
+}
+
+; CHECK-LABEL: @test34
+; CHECK: ret i32 undef
+define i32 @test34(i32 %a) {
+  %b = lshr i32 undef, 0
+  ret i32 %b
+}
-- 
2.39.2