From 895316336e4a774a47b1a4daada070e0ac97f46e Mon Sep 17 00:00:00 2001
From: David Majnemer <david.majnemer@gmail.com>
Date: Wed, 10 Dec 2014 09:14:55 +0000
Subject: [PATCH] ConstantFold: div undef, 0 should fold to undef, not zero

Dividing by zero yields an undefined value.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223924 91177308-0d34-0410-b5e6-96231b3b80d8
---
 lib/IR/ConstantFold.cpp               | 28 ++++++++++++++++++---------
 test/Transforms/InstSimplify/undef.ll |  7 +++++++
 2 files changed, 26 insertions(+), 9 deletions(-)

diff --git a/lib/IR/ConstantFold.cpp b/lib/IR/ConstantFold.cpp
index 4cb22bc97a7..cd409083afc 100644
--- a/lib/IR/ConstantFold.cpp
+++ b/lib/IR/ConstantFold.cpp
@@ -27,12 +27,14 @@
 #include "llvm/IR/GlobalVariable.h"
 #include "llvm/IR/Instructions.h"
 #include "llvm/IR/Operator.h"
+#include "llvm/IR/PatternMatch.h"
 #include "llvm/Support/Compiler.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/ManagedStatic.h"
 #include "llvm/Support/MathExtras.h"
 #include <limits>
 using namespace llvm;
+using namespace llvm::PatternMatch;
 
 //===----------------------------------------------------------------------===//
 //                ConstantFold*Instruction Implementations
@@ -923,19 +925,27 @@ Constant *llvm::ConstantFoldBinaryInstruction(unsigned Opcode,
       // X * undef -> 0       otherwise
       return Constant::getNullValue(C1->getType());
     }
-    case Instruction::UDiv:
     case Instruction::SDiv:
+    case Instruction::UDiv:
+      // X / undef -> undef
+      if (match(C1, m_Zero()))
+        return C2;
+      // undef / 0 -> undef
       // undef / 1 -> undef
-      if (Opcode == Instruction::UDiv || Opcode == Instruction::SDiv)
-        if (ConstantInt *CI2 = dyn_cast<ConstantInt>(C2))
-          if (CI2->isOne())
-            return C1;
-      // FALL THROUGH
+      if (match(C2, m_Zero()) || match(C2, m_One()))
+        return C1;
+      // undef / X -> 0       otherwise
+      return Constant::getNullValue(C1->getType());
     case Instruction::URem:
     case Instruction::SRem:
-      if (!isa<UndefValue>(C2))                    // undef / X -> 0
-        return Constant::getNullValue(C1->getType());
-      return C2;                                   // X / undef -> undef
+      // X % undef -> undef
+      if (match(C2, m_Undef()))
+        return C2;
+      // undef % 0 -> undef
+      if (match(C2, m_Zero()))
+        return C1;
+      // undef % X -> 0       otherwise
+      return Constant::getNullValue(C1->getType());
     case Instruction::Or:                          // X | undef -> -1
       if (isa<UndefValue>(C1) && isa<UndefValue>(C2)) // undef | undef -> undef
         return C1;
diff --git a/test/Transforms/InstSimplify/undef.ll b/test/Transforms/InstSimplify/undef.ll
index 3742d7450af..5787badfd14 100644
--- a/test/Transforms/InstSimplify/undef.ll
+++ b/test/Transforms/InstSimplify/undef.ll
@@ -188,3 +188,10 @@ define i32 @test23(i32 %a) {
   %b = lshr exact i32 undef, %a
   ret i32 %b
 }
+
+; CHECK-LABEL: @test24
+; CHECK: ret i32 undef
+define i32 @test24(i32 %a) {
+  %b = udiv i32 undef, 0
+  ret i32 %b
+}
-- 
2.34.1