Use a worklist to avoid a sneaky iterator invalidation.

The iterator could be invalidated when it's recursively deleting a whole bunch
of constant expressions in a constant initializer.

Note: This was only reproducible if `opt' was run on a `.bc' file. If `opt' was
run on a `.ll' file, it wouldn't crash. This is why the test first pushes the
`.ll' file through `llvm-as' before feeding it to `opt'.

PR15440

llvm-svn: 178531

llvm/lib/Transforms/IPO/GlobalOpt.cpp

@@ -470,8 +470,9 @@ static bool CleanupPointerRootUsers(GlobalVariable *GV,
 static bool CleanupConstantGlobalUsers(Value *V, Constant *Init,
                                        DataLayout *TD, TargetLibraryInfo *TLI) {
   bool Changed = false;
-  for (Value::use_iterator UI = V->use_begin(), E = V->use_end(); UI != E;) {
-    User *U = *UI++;
+  SmallVector<User*, 8> WorkList(V->use_begin(), V->use_end());
+  while (!WorkList.empty()) {
+    User *U = WorkList.pop_back_val();
 
     if (LoadInst *LI = dyn_cast<LoadInst>(U)) {
       if (Init) {
@@ -534,7 +535,6 @@ static bool CleanupConstantGlobalUsers(Value *V, Constant *Init,
       // us, and if they are all dead, nuke them without remorse.
       if (SafeToDestroyConstant(C)) {
         C->destroyConstant();
-        // This could have invalidated UI, start over from scratch.
         CleanupConstantGlobalUsers(V, Init, TD, TLI);
         return true;
       }