From f2aa160b357c98d8c37b69ea9775aa3e097d1319 Mon Sep 17 00:00:00 2001
From: Owen Anderson <resistor@mac.com>
Date: Wed, 2 Jul 2008 17:20:16 +0000
Subject: [PATCH] A better fix for PR2503 that doesn't pessimize GVN in the
 presence of unreachable blocks.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@53032 91177308-0d34-0410-b5e6-96231b3b80d8
---
 lib/Analysis/MemoryDependenceAnalysis.cpp     | 13 -------
 lib/Transforms/Scalar/GVN.cpp                 |  5 +++
 test/Transforms/GVN/2008-07-02-Unreachable.ll | 36 +++++++++++++++++++
 3 files changed, 41 insertions(+), 13 deletions(-)
 create mode 100644 test/Transforms/GVN/2008-07-02-Unreachable.ll

diff --git a/lib/Analysis/MemoryDependenceAnalysis.cpp b/lib/Analysis/MemoryDependenceAnalysis.cpp
index 1cd16bb06de..2012ab473c9 100644
--- a/lib/Analysis/MemoryDependenceAnalysis.cpp
+++ b/lib/Analysis/MemoryDependenceAnalysis.cpp
@@ -19,7 +19,6 @@
 #include "llvm/Instructions.h"
 #include "llvm/Function.h"
 #include "llvm/Analysis/AliasAnalysis.h"
-#include "llvm/Analysis/Dominators.h"
 #include "llvm/Support/CFG.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Target/TargetData.h"
@@ -83,7 +82,6 @@ void MemoryDependenceAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
   AU.setPreservesAll();
   AU.addRequiredTransitive<AliasAnalysis>();
   AU.addRequiredTransitive<TargetData>();
-  AU.addRequiredTransitive<DominatorTree>();
 }
 
 /// getCallSiteDependency - Private helper for finding the local dependencies
@@ -224,17 +222,6 @@ void MemoryDependenceAnalysis::nonLocalHelper(Instruction* query,
       continue;
     }
     
-    // Don't recur upwards if the current block is unreachable.
-    // Instead, mark it as having no dependency on this path,
-    // which will block optzns from occuring.  For this reason,
-    // eliminating unreachable blocks before running a memdep
-    // based optimization is recommended.
-    DominatorTree& DT = getAnalysis<DominatorTree>();
-    if (!DT.isReachableFromEntry(BB)) {
-      resp.insert(std::make_pair(BB, None));
-      continue;
-    }
-    
     // If we didn't find anything, recurse on the precessors of this block
     // Only do this for blocks with a small number of predecessors.
     bool predOnStack = false;
diff --git a/lib/Transforms/Scalar/GVN.cpp b/lib/Transforms/Scalar/GVN.cpp
index 5564dbde1bf..590227c27f9 100644
--- a/lib/Transforms/Scalar/GVN.cpp
+++ b/lib/Transforms/Scalar/GVN.cpp
@@ -808,6 +808,11 @@ Value *GVN::GetValueForBlock(BasicBlock *BB, LoadInst* orig,
   DenseMap<BasicBlock*, Value*>::iterator V = Phis.find(BB);
   if (V != Phis.end() && !top_level) return V->second;
   
+  if (!getAnalysis<DominatorTree>().isReachableFromEntry(BB)) {
+    Phis[BB] = UndefValue::get(orig->getType());
+    return UndefValue::get(orig->getType());
+  }
+  
   BasicBlock* singlePred = BB->getSinglePredecessor();
   if (singlePred) {
     Value *ret = GetValueForBlock(singlePred, orig, Phis);
diff --git a/test/Transforms/GVN/2008-07-02-Unreachable.ll b/test/Transforms/GVN/2008-07-02-Unreachable.ll
new file mode 100644
index 00000000000..0052daf7c57
--- /dev/null
+++ b/test/Transforms/GVN/2008-07-02-Unreachable.ll
@@ -0,0 +1,36 @@
+; RUN: llvm-as < %s | opt -gvn | llvm-dis | grep undef
+; PR2503
+target datalayout = "e-p:32:32:32-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:32:64-f32:32:32-f64:32:64-v64:64:64-v128:128:128-a0:0:64-f80:128:128"
+target triple = "i686-apple-darwin9.3.0"
+@g_3 = external global i8		; <i8*> [#uses=2]
+
+define i32 @func_1() nounwind  {
+entry:
+	br i1 false, label %ifelse, label %ifthen
+
+ifthen:		; preds = %entry
+	br label %ifend
+
+ifelse:		; preds = %entry
+	%tmp3 = load i8* @g_3		; <i8> [#uses=0]
+	br label %forcond.thread
+
+forcond.thread:		; preds = %ifelse
+	br label %afterfor
+
+forcond:		; preds = %forinc
+	br i1 false, label %afterfor, label %forbody
+
+forbody:		; preds = %forcond
+	br label %forinc
+
+forinc:		; preds = %forbody
+	br label %forcond
+
+afterfor:		; preds = %forcond, %forcond.thread
+	%tmp10 = load i8* @g_3		; <i8> [#uses=0]
+	br label %ifend
+
+ifend:		; preds = %afterfor, %ifthen
+	ret i32 0
+}