Fix llvm::removeUnreachableBlocks to handle unreachable loops.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@177713 91177308-0d34-0410-b5e6-96231b3b80d8

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;
 }

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