From d12e82e5239c63cb04e27533b34f376713847f95 Mon Sep 17 00:00:00 2001
From: Benjamin Kramer <benny.kra@googlemail.com>
Date: Thu, 4 Oct 2012 16:11:49 +0000
Subject: [PATCH] SimplifyCFG: Enhance the "remove CFG edge that leads to null
 pointer dereference" optimization to also handle instructions with multiple
 uses.

We conservatively only check the first use to avoid walking long use chains.
This catches the common case of having both a load and a store to a pointer
supplied by a PHI node.

llvm-svn: 165232
---
 llvm/lib/Transforms/Utils/SimplifyCFG.cpp          |  5 ++--
 .../Transforms/SimplifyCFG/phi-undef-loadstore.ll  | 28 ++++++++++++++++++++++
 2 files changed, 31 insertions(+), 2 deletions(-)

diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index 6da791b..0edf7ca 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -3786,11 +3786,12 @@ static bool passingValueIsAlwaysUndefined(Value *V, Instruction *I) {
   if (!C)
     return false;
 
-  if (!I->hasOneUse()) // Only look at single-use instructions, for compile time
+  if (I->use_empty())
     return false;
 
   if (C->isNullValue()) {
-    Instruction *Use = I->use_back();
+    // Only look at the first use, avoid hurting compile time with long uselists
+    User *Use = *I->use_begin();
 
     // Now make sure that there are no instructions in between that can alter
     // control flow (eg. calls)
diff --git a/llvm/test/Transforms/SimplifyCFG/phi-undef-loadstore.ll b/llvm/test/Transforms/SimplifyCFG/phi-undef-loadstore.ll
index 65d888e..028fb07 100644
--- a/llvm/test/Transforms/SimplifyCFG/phi-undef-loadstore.ll
+++ b/llvm/test/Transforms/SimplifyCFG/phi-undef-loadstore.ll
@@ -85,3 +85,31 @@ if.end7:                                          ; preds = %if.else, %if.then4,
 ; CHECK: if.end7:
 ; CHECK: phi i32* [ %a, %if.then ], [ null, %if.then4 ], [ null, %if.else ]
 }
+
+define i32 @test4(i32* %a, i32 %b, i32* %c, i32 %d) nounwind {
+entry:
+  %tobool = icmp eq i32 %b, 0
+  br i1 %tobool, label %if.else, label %if.then
+
+if.then:                                          ; preds = %entry
+  tail call void @bar() nounwind
+  br label %if.end7
+
+if.else:                                          ; preds = %entry
+  %tobool3 = icmp eq i32 %d, 0
+  br i1 %tobool3, label %if.end7, label %if.then4
+
+if.then4:                                         ; preds = %if.else
+  tail call void @bar() nounwind
+  br label %if.end7
+
+if.end7:                                          ; preds = %if.else, %if.then4, %if.then
+  %x.0 = phi i32* [ %a, %if.then ], [ null, %if.then4 ], [ null, %if.else ]
+  %gep = getelementptr i32* %x.0, i32 10
+  %tmp9 = load i32* %gep
+  %tmp10 = or i32 %tmp9, 1
+  store i32 %tmp10, i32* %gep
+  ret i32 %tmp9
+; CHECK: @test4
+; CHECK-NOT: phi
+}
-- 
2.7.4