RemoveFromGraph on definitions asserts empty use lists.

runtime/vm/flow_graph_builder.cc

Index: runtime/vm/flow_graph_builder.cc
diff --git a/runtime/vm/flow_graph_builder.cc b/runtime/vm/flow_graph_builder.cc
index ea3c452243c0acafec742c862d1b071c96e608ad..28c563ef91ca767b804f718ed464fb836a93cbc5 100644
--- a/runtime/vm/flow_graph_builder.cc
+++ b/runtime/vm/flow_graph_builder.cc
@@ -2710,6 +2710,12 @@ void FlowGraphBuilder::RenameRecursive(BlockEntryInstr* block_entry,
       if ((as_bind != NULL) &&
           (as_bind->computation()->IsLoadLocal() ||
            as_bind->computation()->IsStoreLocal())) {
+        // Assert exactly one use.
+        ASSERT(as_bind->use_list() == v);
+        ASSERT(as_bind->use_list()->next_use() == NULL);
+        // Remove the use, its defintion and copy the environment value.
+        v->RemoveFromUseList();
+        as_bind->RemoveFromGraph();
         current->SetInputAt(i, CopyValue(input_value));
       }
     }
@@ -2747,11 +2753,16 @@ void FlowGraphBuilder::RenameRecursive(BlockEntryInstr* block_entry,
             }
           }
         }
-        // Update expression stack and remove from graph.
+        // Update expression stack or remove from graph.
         if (bind->is_used()) {
+          // Assert exactly one use.
+          ASSERT(bind->use_list() != NULL);
+          ASSERT(bind->use_list()->next_use() == NULL);
           env->Add(CopyValue((*env)[index]));
+          // We remove load/store instructions when we find their use in 2a.
+        } else {
+          it.RemoveCurrentFromGraph();
         }
-        it.RemoveCurrentFromGraph();
       } else {
         // Not a load or store.
         if (bind->is_used()) {
@@ -2790,10 +2801,7 @@ void FlowGraphBuilder::RenameRecursive(BlockEntryInstr* block_entry,
         PhiInstr* phi = (*successor->phis())[i];
         if (phi != NULL) {
           // Rename input operand and make a copy if it is a UseVal.
-          Value* new_val = (*env)[i]->IsUse()
-              ? new UseVal((*env)[i]->AsUse()->definition())
-              : (*env)[i];
-          phi->SetInputAt(pred_index, new_val);
+          phi->SetInputAt(pred_index, CopyValue((*env)[i]));
         }
       }
     }