A simpler scheme for garbage collection of ureachable phi inputs.

runtime/vm/flow_graph_optimizer.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_optimizer.h"
 
 #include "vm/bit_vector.h"
 #include "vm/cha.h"
 #include "vm/flow_graph_builder.h"
 #include "vm/hash_map.h"
@@ skipping 2365 matching lines @@
 void ConstantPropagator::Transform() {
   // We will recompute dominators, block ordering, block ids, block last
   // instructions, previous pointers, predecessors, etc. after eliminating
   // unreachable code.  We do not maintain those properties during the
   // transformation.
   for (BlockIterator b = graph_->reverse_postorder_iterator();
        !b.Done();
        b.Advance()) {
     BlockEntryInstr* block = b.Current();
     if (!reachable_->Contains(block->preorder_number())) {
+      // Remove phi inputs corresponding to unreachable predecessor blocks.
+      // Predecessors will be recomputed (in block id order) after removing
+      // unreachable code so we merely have to keep the phi inputs in order.
+      GotoInstr* jump = block->last_instruction()->AsGoto();
+      if (jump != NULL) {
+        JoinEntryInstr* join = jump->successor();
+        ZoneGrowableArray<PhiInstr*>* phis = join->phis();
+        if (phis != NULL) {
+          intptr_t pred_idx = join->IndexOfPredecessor(block);
+          ASSERT(pred_idx >= 0);
+          for (intptr_t phi_idx = 0; phi_idx < phis->length(); ++phi_idx) {
+            PhiInstr* phi = (*phis)[phi_idx];
+            if (phi == NULL) continue;
+            phi->RemoveInputAt(pred_idx);
+          }
+        }
+      }
       continue;
     }
     for (ForwardInstructionIterator i(block); !i.Done(); i.Advance()) {
       Definition* defn = i.Current()->AsDefinition();
       BranchInstr* branch = i.Current()->AsBranch();

[Vyacheslav Egorov (Google), 2012/09/21 12:52:59]

I find it very confusing that branch case in inside the loop while it can only
occur for the last instruction in the block.

[Kevin Millikin (Google), 2012/09/21 13:08:04]

I'll move it out.

       if (defn != NULL) {
         if (IsConstant(defn->constant_value())) {
           if (!defn->IsConstant() &&
               !defn->IsPushArgument() &&
               !defn->IsStoreLocal() &&
               !defn->IsStoreIndexed() &&
               !defn->IsStoreInstanceField() &&
               !defn->IsStoreStaticField() &&
               !defn->IsStoreVMField()) {
             // TODO(kmillikin): propagate constants to replace instructions
             // without side effects.
           }
         }
       } else if (branch != NULL) {
         TargetEntryInstr* if_true = branch->true_successor();
         TargetEntryInstr* if_false = branch->false_successor();
         JoinEntryInstr* join = NULL;
         Instruction* next = NULL;
 
         if (!reachable_->Contains(if_true->preorder_number())) {
           ASSERT(reachable_->Contains(if_false->preorder_number()));
           ASSERT(branch->comparison()->IsStrictCompare());
           ASSERT(if_false->parallel_move() == NULL);
           ASSERT(if_false->loop_info() == NULL);
-          join = new JoinEntryInstr(if_false->try_index());
+          join =
+              new JoinEntryInstr(if_false->block_id(), if_false->try_index());
           next = if_false->next();
         } else if (!reachable_->Contains(if_false->preorder_number())) {
           ASSERT(branch->comparison()->IsStrictCompare());
           ASSERT(if_true->parallel_move() == NULL);
           ASSERT(if_true->loop_info() == NULL);
-          join = new JoinEntryInstr(if_true->try_index());
+          join = new JoinEntryInstr(if_true->block_id(), if_true->try_index());
           next = if_true->next();
         }
 
         if (join != NULL) {
           // Replace the branch with a jump to the reachable successor.
           // Drop the comparison, which does not have side effects as long
           // as it is a strict compare (the only one we can determine is
           // constant with the current analysis).
           GotoInstr* jump = new GotoInstr(join);
           // Removing the branch from the graph will leave the iterator in a
           // state where current is detached from the graph.  Since current
           // has no successors and neither does its replacement, that's
           // safe.
           Instruction* previous = branch->previous();
           branch->set_previous(NULL);
           previous->set_next(jump);
           // Replace the false target entry with the new join entry. We will
           // recompute the dominators after this pass.
           join->set_next(next);
         }
       }
     }
   }
   graph_->DiscoverBlocks();
   GrowableArray<BitVector*> dominance_frontier;
   graph_->ComputeDominators(&dominance_frontier);
-
-  // Garbage collect phi inputs corresponding to unreachable predecessors.
-  // This is required because we assume that predecessor and phi indexes
-  // align.  Note that this does not necessarily eliminate all useless phis
-  // (e.g., it does not eliminate phis that were originally inserted solely
-  // due to an assignment on the now-unreachable path).
-  for (BlockIterator it = graph_->reverse_postorder_iterator();
-       !it.Done();
-       it.Advance()) {
-    JoinEntryInstr* join = it.Current()->AsJoinEntry();
-    if (join != NULL) join->EliminateUnreachablePhiInputs();
-  }
-
   graph_->ComputeUseLists();
 }
 
 
 }  // namespace dart