Improve dead phi elimination.

This change splits the existing phi elimination into two phases:
1. Remove redundant phis
2. Remove dead phis with a fixed point iteration.

The new approach allows us to remove dead phis that are connected
in a cycle.

Review URL: http://codereview.chromium.org/6624061

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@7085 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/hydrogen-instructions.cc

@@ -933,6 +933,14 @@ void HPhi::AddInput(HValue* value) {
 }
 
 
+bool HPhi::HasRealUses() {
+  for (int i = 0; i < uses()->length(); i++) {
+    if (!uses()->at(i)->IsPhi()) return true;
+  }
+  return false;
+}
+
+
 HValue* HPhi::GetRedundantReplacement() {
   HValue* candidate = NULL;
   int count = OperandCount();

src/hydrogen-instructions.h

@@ -461,9 +461,9 @@ class HValue: public ZoneObject {
   int id() const { return id_; }
   void set_id(int id) { id_ = id; }
 
-  const ZoneList<HValue*>* uses() const { return &uses_; }
+  ZoneList<HValue*>* uses() { return &uses_; }
 
-  virtual bool EmitAtUses() const { return false; }
+  virtual bool EmitAtUses() { return false; }
   Representation representation() const { return representation_; }
   void ChangeRepresentation(Representation r) {
     // Representation was already set and is allowed to be changed.
@@ -1800,7 +1800,8 @@ class HPhi: public HValue {
   explicit HPhi(int merged_index)
       : inputs_(2),
         merged_index_(merged_index),
-        phi_id_(-1) {
+        phi_id_(-1),
+        is_live_(false) {
     for (int i = 0; i < Representation::kNumRepresentations; i++) {
       non_phi_uses_[i] = 0;
       indirect_uses_[i] = 0;
@@ -1834,6 +1835,7 @@ class HPhi: public HValue {
   virtual HValue* OperandAt(int index) { return inputs_[index]; }
   HValue* GetRedundantReplacement();
   void AddInput(HValue* value);
+  bool HasRealUses();
 
   bool IsReceiver() { return merged_index_ == 0; }
 
@@ -1872,6 +1874,8 @@ class HPhi: public HValue {
     return indirect_uses_[Representation::kDouble];
   }
   int phi_id() { return phi_id_; }
+  bool is_live() { return is_live_; }
+  void set_is_live(bool b) { is_live_ = b; }
 
  protected:
   virtual void DeleteFromGraph();
@@ -1886,6 +1890,7 @@ class HPhi: public HValue {
   int non_phi_uses_[Representation::kNumRepresentations];
   int indirect_uses_[Representation::kNumRepresentations];
   int phi_id_;
+  bool is_live_;
 };
 
 
@@ -1916,7 +1921,7 @@ class HConstant: public HTemplateInstruction<0> {
     return Representation::None();
   }
 
-  virtual bool EmitAtUses() const { return !representation().IsDouble(); }
+  virtual bool EmitAtUses() { return !representation().IsDouble(); }
   virtual void PrintDataTo(StringStream* stream);
   virtual HType CalculateInferredType();
   bool IsInteger() const { return handle_->IsSmi(); }
@@ -2191,7 +2196,7 @@ class HCompare: public HBinaryOperation {
 
   void SetInputRepresentation(Representation r);
 
-  virtual bool EmitAtUses() const {
+  virtual bool EmitAtUses() {
     return !HasSideEffects() && (uses()->length() <= 1);
   }
 
@@ -2232,7 +2237,7 @@ class HCompareJSObjectEq: public HBinaryOperation {
     SetFlag(kUseGVN);
   }
 
-  virtual bool EmitAtUses() const {
+  virtual bool EmitAtUses() {
     return !HasSideEffects() && (uses()->length() <= 1);
   }
 
@@ -2255,7 +2260,7 @@ class HUnaryPredicate: public HUnaryOperation {
     SetFlag(kUseGVN);
   }
 
-  virtual bool EmitAtUses() const {
+  virtual bool EmitAtUses() {
     return !HasSideEffects() && (uses()->length() <= 1);
   }
 
@@ -2315,7 +2320,7 @@ class HIsConstructCall: public HTemplateInstruction<0> {
     SetFlag(kUseGVN);
   }
 
-  virtual bool EmitAtUses() const {
+  virtual bool EmitAtUses() {
     return !HasSideEffects() && (uses()->length() <= 1);
   }
 
@@ -2437,7 +2442,7 @@ class HInstanceOf: public HTemplateInstruction<3> {
   HValue* left() { return OperandAt(1); }
   HValue* right() { return OperandAt(2); }
 
-  virtual bool EmitAtUses() const {
+  virtual bool EmitAtUses() {
     return !HasSideEffects() && (uses()->length() <= 1);
   }
 

src/hydrogen.cc

@@ -92,7 +92,7 @@ void HBasicBlock::AddPhi(HPhi* phi) {
 void HBasicBlock::RemovePhi(HPhi* phi) {
   ASSERT(phi->block() == this);
   ASSERT(phis_.Contains(phi));
-  ASSERT(phi->HasNoUses());
+  ASSERT(phi->HasNoUses() || !phi->is_live());
   phi->ClearOperands();
   phis_.RemoveElement(phi);
   phi->SetBlock(NULL);
@@ -703,8 +703,7 @@ void HGraph::AssignDominators() {
 
 
 void HGraph::EliminateRedundantPhis() {
-  HPhase phase("Phi elimination", this);
+  HPhase phase("Redundant phi elimination", this);
-  ZoneList<HValue*> uses_to_replace(2);
 
   // Worklist of phis that can potentially be eliminated. Initialized
   // with all phi nodes. When elimination of a phi node modifies
@@ -727,26 +726,57 @@ void HGraph::EliminateRedundantPhis() {
     if (value != NULL) {
       // Iterate through uses finding the ones that should be
       // replaced.
-      const ZoneList<HValue*>* uses = phi->uses();
+      ZoneList<HValue*>* uses = phi->uses();
-      for (int i = 0; i < uses->length(); ++i) {
+      while (!uses->is_empty()) {
-        HValue* use = uses->at(i);
+        HValue* use = uses->RemoveLast();
-        if (!use->block()->IsStartBlock()) {
+        if (use != NULL) {
-          uses_to_replace.Add(use);
+          phi->ReplaceAtUse(use, value);
+          if (use->IsPhi()) worklist.Add(HPhi::cast(use));
         }
       }
-      // Replace the uses and add phis modified to the work list.
-      for (int i = 0; i < uses_to_replace.length(); ++i) {
-        HValue* use = uses_to_replace[i];
-        phi->ReplaceAtUse(use, value);
-        if (use->IsPhi()) worklist.Add(HPhi::cast(use));
-      }
-      uses_to_replace.Rewind(0);
       block->RemovePhi(phi);
-    } else if (FLAG_eliminate_dead_phis && phi->HasNoUses() &&
+    }
-               !phi->IsReceiver()) {
+  }
+}
+
+
+void HGraph::EliminateUnreachablePhis() {
+  HPhase phase("Unreachable phi elimination", this);
+
+  // Initialize worklist.
+  ZoneList<HPhi*> phi_list(blocks_.length());
+  ZoneList<HPhi*> worklist(blocks_.length());
+  for (int i = 0; i < blocks_.length(); ++i) {
+    for (int j = 0; j < blocks_[i]->phis()->length(); j++) {
+      HPhi* phi = blocks_[i]->phis()->at(j);
+      phi_list.Add(phi);
       // We can't eliminate phis in the receiver position in the environment
       // because in case of throwing an error we need this value to
       // construct a stack trace.
+      if (phi->HasRealUses() || phi->IsReceiver())  {
+        phi->set_is_live(true);
+        worklist.Add(phi);
+      }
+    }
+  }
+
+  // Iteratively mark live phis.
+  while (!worklist.is_empty()) {
+    HPhi* phi = worklist.RemoveLast();
+    for (int i = 0; i < phi->OperandCount(); i++) {
+      HValue* operand = phi->OperandAt(i);
+      if (operand->IsPhi() && !HPhi::cast(operand)->is_live()) {
+        HPhi::cast(operand)->set_is_live(true);
+        worklist.Add(HPhi::cast(operand));
+      }
+    }
+  }
+
+  // Remove unreachable phis.
+  for (int i = 0; i < phi_list.length(); i++) {
+    HPhi* phi = phi_list[i];
+    if (!phi->is_live()) {
+      HBasicBlock* block = phi->block();
       block->RemovePhi(phi);
       block->RecordDeletedPhi(phi->merged_index());
     }
@@ -2174,6 +2204,7 @@ HGraph* HGraphBuilder::CreateGraph() {
   graph()->OrderBlocks();
   graph()->AssignDominators();
   graph()->EliminateRedundantPhis();
+  if (FLAG_eliminate_dead_phis) graph()->EliminateUnreachablePhis();
   if (!graph()->CollectPhis()) {
     Bailout("Phi-use of arguments object");
     return NULL;

src/hydrogen.h

@@ -234,6 +234,7 @@ class HGraph: public HSubgraph {
   void ComputeMinusZeroChecks();
   bool ProcessArgumentsObject();
   void EliminateRedundantPhis();
+  void EliminateUnreachablePhis();
   void Canonicalize();
   void OrderBlocks();
   void AssignDominators();