Eliminate phis that do not reach any non-environment uses.

runtime/vm/flow_graph_allocator.cc

Index: runtime/vm/flow_graph_allocator.cc
diff --git a/runtime/vm/flow_graph_allocator.cc b/runtime/vm/flow_graph_allocator.cc
index ae1a3c255c01344014468583cb60856bf4a17519..f38871405d03813ae69bdd5abb0bbd82f2653a4e 100644
--- a/runtime/vm/flow_graph_allocator.cc
+++ b/runtime/vm/flow_graph_allocator.cc
@@ -80,12 +80,31 @@ FlowGraphAllocator::FlowGraphAllocator(
 }
 
 
+// Remove environments from the instructions which can't deoptimize.
+// Replace dead phis uses with null values in environments.
 void FlowGraphAllocator::EliminateEnvironmentUses() {
+  ConstantVal* null_value = new ConstantVal(Object::ZoneHandle());
+
   for (intptr_t i = 0; i < block_order_.length(); ++i) {
     BlockEntryInstr* block = block_order_[i];
     for (ForwardInstructionIterator it(block); !it.Done(); it.Advance()) {
       Instruction* current = it.Current();
-      if (!current->CanDeoptimize()) current->set_env(NULL);
+      if (current->CanDeoptimize()) {
+        ASSERT(current->env() != NULL);
+        GrowableArray<Value*>* values = current->env()->values_ptr();
+
+        for (intptr_t i = 0; i < values->length(); i++) {
+          UseVal* use = (*values)[i]->AsUse();
+          if (use == NULL) continue;
+
+          PhiInstr* phi = use->definition()->AsPhi();
+          if (phi == NULL) continue;
+
+          if (!phi->is_alive()) (*values)[i] = null_value;
+        }
+      } else {
+        current->set_env(NULL);
+      }
     }
   }
 }
@@ -138,7 +157,8 @@ void FlowGraphAllocator::ComputeInitialSets() {
       if (join->phis() != NULL) {
         for (intptr_t j = 0; j < join->phis()->length(); j++) {
           PhiInstr* phi = (*join->phis())[j];
-          if (phi == NULL) continue;
+          if ((phi == NULL) || !phi->is_alive()) continue;

[Kevin Millikin (Google), 2012/08/08 11:07:43]

It doesn't work to set (*join->phis())[j] = NULL when !phi->is_alive()?  That
seems better than leaving them in and skipping them everywhere.

+
           kill->Add(phi->ssa_temp_index());
           live_in->Remove(phi->ssa_temp_index());
 
@@ -556,6 +576,11 @@ Instruction* FlowGraphAllocator::ConnectOutgoingPhiMoves(
     PhiInstr* phi = (*phis)[phi_idx];
     if (phi == NULL) continue;
 
+    if (!phi->is_alive()) {
+      move_idx++;
+      continue;
+    }
+
     Value* val = phi->InputAt(pred_idx);
     MoveOperands* move = parallel_move->MoveOperandsAt(move_idx);
     if (val->IsUse()) {
@@ -602,6 +627,11 @@ void FlowGraphAllocator::ConnectIncomingPhiMoves(BlockEntryInstr* block) {
       PhiInstr* phi = (*phis)[phi_idx];
       if (phi == NULL) continue;
 
+      if (!phi->is_alive()) {
+        move_idx++;
+        continue;
+      }
+
       const intptr_t vreg = phi->ssa_temp_index();
       ASSERT(vreg != -1);