From 891ec6d69ff375900d0c8a1cf8b806a8318c9a1f Mon Sep 17 00:00:00 2001
From: David Majnemer <david.majnemer@gmail.com>
Date: Wed, 17 Dec 2014 01:54:33 +0000
Subject: [PATCH] InstSimplify: shl nsw/nuw undef, %V -> undef

We can always choose an value for undef which might cause %V to shift
out an important bit except for one case, when %V is zero.

However, shl behaves like an identity function when the right hand side
is zero.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224405 91177308-0d34-0410-b5e6-96231b3b80d8
---
 lib/Analysis/InstructionSimplify.cpp  | 20 +++++++------------
 test/Transforms/InstSimplify/undef.ll | 28 +++++++++++++++++++++++++++
 2 files changed, 35 insertions(+), 13 deletions(-)

diff --git a/lib/Analysis/InstructionSimplify.cpp b/lib/Analysis/InstructionSimplify.cpp
index 408768e8515..1db7ab7bf03 100644
--- a/lib/Analysis/InstructionSimplify.cpp
+++ b/lib/Analysis/InstructionSimplify.cpp
@@ -1338,6 +1338,11 @@ static Value *SimplifyRightShift(unsigned Opcode, Value *Op0, Value *Op1,
   if (Op0 == Op1)
     return Constant::getNullValue(Op0->getType());
 
+  // undef >> X -> 0
+  // undef >> X -> undef (if it's exact)
+  if (match(Op0, m_Undef()))
+    return isExact ? Op0 : Constant::getNullValue(Op0->getType());
+
   // The low bit cannot be shifted out of an exact shift if it is set.
   if (isExact) {
     unsigned BitWidth = Op0->getType()->getScalarSizeInBits();
@@ -1360,8 +1365,9 @@ static Value *SimplifyShlInst(Value *Op0, Value *Op1, bool isNSW, bool isNUW,
     return V;
 
   // undef << X -> 0
+  // undef << X -> undef if (if it's NSW/NUW)
   if (match(Op0, m_Undef()))
-    return Constant::getNullValue(Op0->getType());
+    return isNSW || isNUW ? Op0 : Constant::getNullValue(Op0->getType());
 
   // (X >> A) << A -> X
   Value *X;
@@ -1386,12 +1392,6 @@ static Value *SimplifyLShrInst(Value *Op0, Value *Op1, bool isExact,
                                     MaxRecurse))
       return V;
 
-  // undef >>l X -> 0
-  // undef >>l X -> undef (if it's exact)
-  if (match(Op0, m_Undef()))
-    return isExact ? UndefValue::get(Op0->getType())
-                   : Constant::getNullValue(Op0->getType());
-
   // (X << A) >> A -> X
   Value *X;
   if (match(Op0, m_NUWShl(m_Value(X), m_Specific(Op1))))
@@ -1422,12 +1422,6 @@ static Value *SimplifyAShrInst(Value *Op0, Value *Op1, bool isExact,
   if (match(Op0, m_AllOnes()))
     return Op0;
 
-  // undef >>a X -> 0
-  // undef >>a X -> undef (if it's exact)
-  if (match(Op0, m_Undef()))
-    return isExact ? UndefValue::get(Op0->getType())
-                   : Constant::getNullValue(Op0->getType());
-
   // (X << A) >> A -> X
   Value *X;
   if (match(Op0, m_NSWShl(m_Value(X), m_Specific(Op1))))
diff --git a/test/Transforms/InstSimplify/undef.ll b/test/Transforms/InstSimplify/undef.ll
index 70a07b8a892..bf643171cc1 100644
--- a/test/Transforms/InstSimplify/undef.ll
+++ b/test/Transforms/InstSimplify/undef.ll
@@ -216,3 +216,31 @@ define i32 @test27() {
   %b = shl i32 0, undef
   ret i32 %b
 }
+
+; CHECK-LABEL: @test28
+; CHECK: ret i32 undef
+define i32 @test28(i32 %a) {
+  %b = shl nsw i32 undef, %a
+  ret i32 %b
+}
+
+; CHECK-LABEL: @test29
+; CHECK: ret i32 undef
+define i32 @test29(i32 %a) {
+  %b = shl nuw i32 undef, %a
+  ret i32 %b
+}
+
+; CHECK-LABEL: @test30
+; CHECK: ret i32 undef
+define i32 @test30(i32 %a) {
+  %b = shl nsw nuw i32 undef, %a
+  ret i32 %b
+}
+
+; CHECK-LABEL: @test31
+; CHECK: ret i32 0
+define i32 @test31(i32 %a) {
+  %b = shl i32 undef, %a
+  ret i32 %b
+}