From f128dbc036c1a27881665eb7b1836af31c0764a5 Mon Sep 17 00:00:00 2001
From: Evgeniy Stepanov <eugeni.stepanov@gmail.com>
Date: Fri, 22 Mar 2013 08:43:04 +0000
Subject: [PATCH] Fix llvm::removeUnreachableBlocks to handle unreachable
 loops.

llvm-svn: 177713
---
 lib/Transforms/Utils/Local.cpp                | 21 +++++++------------
 .../MemorySanitizer/unreachable.ll            | 16 ++++++++++++++
 2 files changed, 24 insertions(+), 13 deletions(-)

diff --git a/lib/Transforms/Utils/Local.cpp b/lib/Transforms/Utils/Local.cpp
index a54ee08b676..be80d34d960 100644
--- a/lib/Transforms/Utils/Local.cpp
+++ b/lib/Transforms/Utils/Local.cpp
@@ -985,22 +985,17 @@ bool llvm::removeUnreachableBlocks(Function &F) {
     if (Reachable.count(I))
       continue;
 
-    // Remove the block as predecessor of all its reachable successors.
-    // Unreachable successors don't matter as they'll soon be removed, too.
     for (succ_iterator SI = succ_begin(I), SE = succ_end(I); SI != SE; ++SI)
       if (Reachable.count(*SI))
         (*SI)->removePredecessor(I);
-
-    // Zap all instructions in this basic block.
-    while (!I->empty()) {
-      Instruction &Inst = I->back();
-      if (!Inst.use_empty())
-        Inst.replaceAllUsesWith(UndefValue::get(Inst.getType()));
-      I->getInstList().pop_back();
-    }
-
-    --I;
-    llvm::next(I)->eraseFromParent();
+    I->dropAllReferences();
   }
+
+  for (Function::iterator I = llvm::next(F.begin()), E=F.end(); I != E;)
+    if (!Reachable.count(I))
+      I = F.getBasicBlockList().erase(I);
+    else
+      ++I;
+
   return true;
 }
diff --git a/test/Instrumentation/MemorySanitizer/unreachable.ll b/test/Instrumentation/MemorySanitizer/unreachable.ll
index 66a9575d3f5..c8130717c7d 100644
--- a/test/Instrumentation/MemorySanitizer/unreachable.ll
+++ b/test/Instrumentation/MemorySanitizer/unreachable.ll
@@ -21,3 +21,19 @@ exit:
 ; CHECK: @Func
 ; CHECK: store i32 0, {{.*}} @__msan_retval_tls
 ; CHECK: ret i32 42
+
+
+define i32 @UnreachableLoop() nounwind uwtable {
+entry:
+  ret i32 0
+
+zzz:
+  br label %xxx
+
+xxx:
+  br label %zzz
+}
+
+; CHECK: @UnreachableLoop
+; CHECK: store i32 0, {{.*}} @__msan_retval_tls
+; CHECK: ret i32 0