From ea9bcfc70719c1ecc18828781ef4cd3bce8c8e2f Mon Sep 17 00:00:00 2001
From: David Majnemer <david.majnemer@gmail.com>
Date: Wed, 10 Dec 2014 21:38:05 +0000
Subject: [PATCH] ConstantFold: an undef shift amount results in undef

X shifted by undef results in undef because the undef value can
represent values greater than the width of the operands.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223968 91177308-0d34-0410-b5e6-96231b3b80d8
---
 lib/IR/ConstantFold.cpp               | 27 ++++++++++++++-------------
 test/Transforms/InstSimplify/undef.ll | 21 +++++++++++++++++++++
 2 files changed, 35 insertions(+), 13 deletions(-)

diff --git a/lib/IR/ConstantFold.cpp b/lib/IR/ConstantFold.cpp
index cd409083afc..49ef3024033 100644
--- a/lib/IR/ConstantFold.cpp
+++ b/lib/IR/ConstantFold.cpp
@@ -951,21 +951,22 @@ Constant *llvm::ConstantFoldBinaryInstruction(unsigned Opcode,
         return C1;
       return Constant::getAllOnesValue(C1->getType()); // undef | X -> ~0
     case Instruction::LShr:
-      if (isa<UndefValue>(C2) && isa<UndefValue>(C1))
-        return C1;                                  // undef lshr undef -> undef
-      return Constant::getNullValue(C1->getType()); // X lshr undef -> 0
-                                                    // undef lshr X -> 0
+      // X >>l undef -> undef
+      if (isa<UndefValue>(C2))
+        return C2;
+      // undef >>l X -> 0
+      return Constant::getNullValue(C1->getType());
     case Instruction::AShr:
-      if (!isa<UndefValue>(C2))                     // undef ashr X --> all ones
-        return Constant::getAllOnesValue(C1->getType());
-      else if (isa<UndefValue>(C1)) 
-        return C1;                                  // undef ashr undef -> undef
-      else
-        return C1;                                  // X ashr undef --> X
+      // X >>a undef -> undef
+      if (isa<UndefValue>(C2))
+        return C2;
+      // undef >>a X -> all ones
+      return Constant::getAllOnesValue(C1->getType());
     case Instruction::Shl:
-      if (isa<UndefValue>(C2) && isa<UndefValue>(C1))
-        return C1;                                  // undef shl undef -> undef
-      // undef << X -> 0   or   X << undef -> 0
+      // X << undef -> undef
+      if (isa<UndefValue>(C2))
+        return C2;
+      // undef << X -> 0
       return Constant::getNullValue(C1->getType());
     }
   }
diff --git a/test/Transforms/InstSimplify/undef.ll b/test/Transforms/InstSimplify/undef.ll
index 5787badfd14..d72ff8568e6 100644
--- a/test/Transforms/InstSimplify/undef.ll
+++ b/test/Transforms/InstSimplify/undef.ll
@@ -195,3 +195,24 @@ define i32 @test24(i32 %a) {
   %b = udiv i32 undef, 0
   ret i32 %b
 }
+
+; CHECK-LABEL: @test25
+; CHECK: ret i32 undef
+define i32 @test25(i32 %a) {
+  %b = lshr i32 0, undef
+  ret i32 %b
+}
+
+; CHECK-LABEL: @test26
+; CHECK: ret i32 undef
+define i32 @test26(i32 %a) {
+  %b = ashr i32 0, undef
+  ret i32 %b
+}
+
+; CHECK-LABEL: @test27
+; CHECK: ret i32 undef
+define i32 @test27(i32 %a) {
+  %b = shl i32 0, undef
+  ret i32 %b
+}
-- 
2.34.1