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

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 2324 matching lines @@
       }
     }
   }
 
   if (for_optimized && use_ssa) {
     GrowableArray<BitVector*> dominance_frontier;
     ComputeDominators(&preorder_block_entries_, &parent, &dominance_frontier);
     InsertPhis(preorder_block_entries_,
                assigned_vars,
                dominance_frontier);
-    Rename();
+
+    GrowableArray<PhiInstr*> live_phis;
+
+    // Rename uses to reference inserted phis where appropriate.
+    // Collect phis that reach a non-environment use.
+    Rename(&live_phis);
+
+    // Propagate alive mark transitively from alive phis.
+    MarkLivePhis(&live_phis);
   }
   if (FLAG_print_flow_graph || (Dart::flow_graph_writer() != NULL)) {
     intptr_t length = postorder_block_entries_.length();
     GrowableArray<BlockEntryInstr*> reverse_postorder(length);
     for (intptr_t i = length - 1; i >= 0; --i) {
       reverse_postorder.Add(postorder_block_entries_[i]);
     }
     if (FLAG_print_flow_graph) {
       // Print flow graph to stdout.
       FlowGraphPrinter printer(function, reverse_postorder);
@@ skipping 180 matching lines @@
             work[index] = var_index;
             worklist.Add(block);
           }
         }
       }
     }
   }
 }
 
 
-void FlowGraphBuilder::Rename() {
+void FlowGraphBuilder::Rename(GrowableArray<PhiInstr*>* live_phis) {
   // TODO(fschneider): Support catch-entry.
   if (graph_entry_->SuccessorCount() > 1) {
     Bailout("Catch-entry support in SSA.");
   }
 
   // Initialize start environment.
   GrowableArray<Value*> start_env(variable_count());
   for (intptr_t i = 0; i < parameter_count(); ++i) {
     ParameterInstr* param = new ParameterInstr(i);
     param->set_ssa_temp_index(alloc_ssa_temp_index());  // New SSA temp.
     start_env.Add(new UseVal(param));
   }
 
   // All locals are initialized with #null.
   Value* null_value = new ConstantVal(Object::ZoneHandle());
   while (start_env.length() < variable_count()) {
     start_env.Add(null_value);
   }
   graph_entry_->set_start_env(
       new Environment(start_env, non_copied_parameter_count_));
 
   BlockEntryInstr* normal_entry = graph_entry_->SuccessorAt(0);
   ASSERT(normal_entry != NULL);  // Must have entry.
   GrowableArray<Value*> env(variable_count());
   env.AddArray(start_env);
-  RenameRecursive(normal_entry, &env);
+  RenameRecursive(normal_entry, &env, live_phis);
 }
 
 
 // Helper to a copy a value iff it is a UseVal.
 static Value* CopyValue(Value* value) {
   return value->IsUse()
       ? new UseVal(value->AsUse()->definition())
       : value;
 }
 
 
 void FlowGraphBuilder::RenameRecursive(BlockEntryInstr* block_entry,
-                                       GrowableArray<Value*>* env) {
+                                       GrowableArray<Value*>* env,
+                                       GrowableArray<PhiInstr*>* live_phis) {
   // 1. Process phis first.
   if (block_entry->IsJoinEntry()) {
     JoinEntryInstr* join = block_entry->AsJoinEntry();
     if (join->phis() != NULL) {
       for (intptr_t i = 0; i < join->phis()->length(); ++i) {
         PhiInstr* phi = (*join->phis())[i];
         if (phi != NULL) {
           (*env)[i] = new UseVal(phi);
           phi->set_ssa_temp_index(alloc_ssa_temp_index());  // New SSA temp.
         }
       }
     }
   }
 
   // 2. Process normal instructions.
   for (ForwardInstructionIterator it(block_entry); !it.Done(); it.Advance()) {
     Instruction* current = it.Current();
     // Attach current environment to the instruction. First, each instruction
     // gets a full copy of the environment. Later we optimize this by
     // eliminating unnecessary environments.
     current->set_env(new Environment(*env, non_copied_parameter_count_));
 
     // 2a. Handle uses:
     // Update expression stack environment for each use.
     // For each use of a LoadLocal or StoreLocal: Replace it with the value
     // from the environment.
-    for (intptr_t i = 0; i < current->InputCount(); ++i) {
+    for (intptr_t i = current->InputCount() - 1; i >= 0; --i) {
       Value* v = current->InputAt(i);
       if (!v->IsUse()) continue;
       // Update expression stack.
       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()) {
-        Computation* comp = as_bind->computation();
-        intptr_t index = comp->AsLoadLocal()->local().BitIndexIn(
-            non_copied_parameter_count_);
-        current->SetInputAt(i, CopyValue((*env)[index]));
-      }
-      if ((as_bind != NULL) && as_bind->computation()->IsStoreLocal()) {
-        // For each use of a StoreLocal: Replace it with the value from the
-        // environment.
-        Computation* comp = as_bind->computation();
-        intptr_t index = comp->AsStoreLocal()->local().BitIndexIn(
-            non_copied_parameter_count_);
-        current->SetInputAt(i, CopyValue((*env)[index]));
+      if ((as_bind != NULL) &&
+          (as_bind->computation()->IsLoadLocal() ||
+           as_bind->computation()->IsStoreLocal())) {
+        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.
     // For each LoadLocal: Remove it from the graph.
     // For each StoreLocal: Remove it from the graph and update the environment.
     BindInstr* bind = current->AsBind();
     if (bind != NULL) {
       LoadLocalComp* load = bind->computation()->AsLoadLocal();
       StoreLocalComp* store = bind->computation()->AsStoreLocal();
       if ((load != NULL) || (store != NULL)) {
         intptr_t index;
         if (store != NULL) {
           index = store->local().BitIndexIn(non_copied_parameter_count_);
           // Update renaming environment.
           (*env)[index] = store->value();
         } else {
           // The graph construction ensures we do not have an unused LoadLocal
           // computation.
           ASSERT(bind->is_used());
           index = load->local().BitIndexIn(non_copied_parameter_count_);
+
+          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.
         if (bind->is_used()) {
           env->Add(CopyValue((*env)[index]));
         }
         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));
         }
       }
     }
 
     // 2c. Handle pushed argument.
     PushArgumentInstr* push = current->AsPushArgument();
     if (push != NULL) {
       env->Add(push->value());
     }
   }
 
   // 3. Process dominated blocks.
   for (intptr_t i = 0; i < block_entry->dominated_blocks().length(); ++i) {
     BlockEntryInstr* block = block_entry->dominated_blocks()[i];
     GrowableArray<Value*> new_env(env->length());
     new_env.AddArray(*env);
-    RenameRecursive(block, &new_env);
+    RenameRecursive(block, &new_env, live_phis);
   }
 
   // 4. Process successor block. We have edge-split form, so that only blocks
   // with one successor can have a join block as successor.
   if ((block_entry->last_instruction()->SuccessorCount() == 1) &&
       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);
         }
       }
     }
   }
 }
 
 
+void FlowGraphBuilder::MarkLivePhis(GrowableArray<PhiInstr*>* live_phis) {
+  while (!live_phis->is_empty()) {
+    PhiInstr* phi = live_phis->Last();
+    live_phis->RemoveLast();
+    for (intptr_t i = 0; i < phi->InputCount(); i++) {
+      Value* val = phi->InputAt(i);
+      if (!val->IsUse()) continue;
+      PhiInstr* used_phi = val->AsUse()->definition()->AsPhi();
+      if ((used_phi != NULL) && !used_phi->is_alive()) {
+        used_phi->mark_alive();
+        live_phis->Add(used_phi);
+      }
+    }
+  }
+}
+
 void FlowGraphBuilder::Bailout(const char* reason) {
   const char* kFormat = "FlowGraphBuilder Bailout: %s %s";
   const char* function_name = parsed_function_.function().ToCString();
   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