RemoveFromGraph on definitions asserts empty use lists.

runtime/vm/flow_graph_builder.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/flow_graph_builder.h"
 
 #include "vm/ast_printer.h"
 #include "vm/bit_vector.h"
 #include "vm/code_descriptors.h"
 #include "vm/dart_entry.h"
@@ skipping 2692 matching lines @@
       ASSERT(env->length() > variable_count());
 
       Value* input_value = env->Last();
       ASSERT(input_value->IsUse());
       env->RemoveLast();
 
       BindInstr* as_bind = v->AsUse()->definition()->AsBind();
       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));
       }
     }
 
     // Drop pushed arguments for calls.
     for (intptr_t j = 0; j < current->ArgumentCount(); j++) {
       env->RemoveLast();
     }
 
     // 2b. Handle LoadLocal and StoreLocal.
@@ skipping 17 matching lines @@
 
           Value* value = (*env)[index];
           if (value->IsUse()) {
             PhiInstr* phi = value->AsUse()->definition()->AsPhi();
             if ((phi != NULL) && !phi->is_alive()) {
               phi->mark_alive();
               live_phis->Add(phi);
             }
           }
         }
-        // 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()) {
           // Assign fresh SSA temporary and update expression stack.
           bind->set_ssa_temp_index(alloc_ssa_temp_index());
           env->Add(new UseVal(bind));
         }
       }
     }
 
@@ skipping 18 matching lines @@
       block_entry->last_instruction()->SuccessorAt(0)->IsJoinEntry()) {
     JoinEntryInstr* successor =
         block_entry->last_instruction()->SuccessorAt(0)->AsJoinEntry();
     intptr_t pred_index = successor->IndexOfPredecessor(block_entry);
     ASSERT(pred_index >= 0);
     if (successor->phis() != NULL) {
       for (intptr_t i = 0; i < successor->phis()->length(); ++i) {
         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]));
         }
       }
     }
   }
 }
 
 
 void FlowGraphBuilder::MarkLivePhis(GrowableArray<PhiInstr*>* live_phis) {
   while (!live_phis->is_empty()) {
     PhiInstr* phi = live_phis->Last();
@@ skipping 16 matching lines @@
   intptr_t len = OS::SNPrint(NULL, 0, kFormat, function_name, reason) + 1;
   char* chars = Isolate::Current()->current_zone()->Alloc<char>(len);
   OS::SNPrint(chars, len, kFormat, function_name, reason);
   const Error& error = Error::Handle(
       LanguageError::New(String::Handle(String::New(chars))));
   Isolate::Current()->long_jump_base()->Jump(1, error);
 }
 
 
 }  // namespace dart