Don't emit code for instructions that are hiding behind an HSoftDeoptimize

src/hydrogen.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 1223 matching lines @@
       block->AssignCommonDominator(block->predecessors()->first());
       block->AssignLoopSuccessorDominators();
     } else {
       for (int j = blocks_[i]->predecessors()->length() - 1; j >= 0; --j) {
         blocks_[i]->AssignCommonDominator(blocks_[i]->predecessors()->at(j));
       }
     }
   }
 }
 
+
 // Mark all blocks that are dominated by an unconditional soft deoptimize to
 // prevent code motion across those blocks.
 void HGraph::PropagateDeoptimizingMark() {
   HPhase phase("H_Propagate deoptimizing mark", this);
   MarkAsDeoptimizingRecursively(entry_block());
+  NullifyUnreachableInstructions();
 }
 
+
 void HGraph::MarkAsDeoptimizingRecursively(HBasicBlock* block) {
   for (int i = 0; i < block->dominated_blocks()->length(); ++i) {
     HBasicBlock* dominated = block->dominated_blocks()->at(i);
     if (block->IsDeoptimizing()) dominated->MarkAsDeoptimizing();
     MarkAsDeoptimizingRecursively(dominated);
   }
 }
 
+
+void HGraph::NullifyUnreachableInstructions() {

[danno, 2013/01/10 12:30:05]

Can you skip this pass all together if there are no soft deopts and a situation
hasn't occurred during MarkAsDeoptimizingRecursively() where
all_predecessors_deoptimizing is true? 

[Jakob Kummerow, 2013/01/16 11:35:02]

I can skip the entire phase when there are no soft deopts -- done.

MarkAsDeoptimizingRecursively, on the other hand, does not currently look at
predecessors, and even if it did, that wouldn't allow us to skip the second
pass, because all_predecessors_deoptimizing is just one of the conditions that
set nullify = true.

+  int block_count = blocks_.length();
+  for (int i = 0; i < block_count; ++i) {
+    HBasicBlock* block = blocks_.at(i);
+    bool nullify = false;
+    const ZoneList<HBasicBlock*>* predecessors = block->predecessors();
+    int predecessors_length = predecessors->length();
+    bool all_predecessors_deoptimizing = (predecessors_length > 0);
+    for (int j = 0; j < predecessors_length; ++j) {
+      if (!predecessors->at(j)->IsDeoptimizing()) {
+        all_predecessors_deoptimizing = false;
+        break;
+      }
+    }
+    if (all_predecessors_deoptimizing) nullify = true;
+    for (HInstruction* instr = block->first(); instr != NULL;
+         instr = instr->next()) {
+      // Leave the basic structure of the graph intact.
+      if (instr->IsBlockEntry()) continue;
+      if (instr->IsControlInstruction()) continue;
+      if (instr->IsSimulate()) continue;
+      if (instr->IsEnterInlined()) continue;
+      if (instr->IsLeaveInlined()) continue;
+      if (nullify) {
+        HInstruction* last_dummy = NULL;
+        for (int j = 0; j < instr->OperandCount(); ++j) {
+          HValue* operand = instr->OperandAt(j);
+          // Insert an HDummyUse for each operand, unless the operand
+          // is an HDummyUse itself. If it's even from the same block,
+          // remember it as a potential replacement for the instruction.
+          if (operand->IsDummyUse()) {
+            if (operand->block() == instr->block() &&
+                last_dummy == NULL) {
+              last_dummy = HInstruction::cast(operand);
+            }
+            continue;
+          }
+          HDummyUse* dummy = new(zone()) HDummyUse(operand);
+          dummy->InsertBefore(instr);
+          last_dummy = dummy;
+        }
+        if (last_dummy == NULL) last_dummy = GetConstant1();
+        instr->DeleteAndReplaceWith(last_dummy);
+        continue;
+      }
+      if (instr->IsSoftDeoptimize()) {
+        ASSERT(block->IsDeoptimizing());
+        nullify = true;
+      }
+    }
+  }
+}
+
+
 void HGraph::EliminateRedundantPhis() {
   HPhase phase("H_Redundant phi elimination", this);
 
   // Worklist of phis that can potentially be eliminated. Initialized with
   // all phi nodes. When elimination of a phi node modifies another phi node
   // the modified phi node is added to the worklist.
   ZoneList<HPhi*> worklist(blocks_.length(), zone());
   for (int i = 0; i < blocks_.length(); ++i) {
     worklist.AddAll(*blocks_[i]->phis(), zone());
   }
@@ skipping 2681 matching lines @@
   current_block()->AddPhi(instr);
 }
 
 
 void HOptimizedGraphBuilder::PushAndAdd(HInstruction* instr) {
   Push(instr);
   AddInstruction(instr);
 }
 
 
+void HOptimizedGraphBuilder::AddSoftDeoptimize() {
+  if (FLAG_always_opt) return;
+  if (current_block()->IsDeoptimizing()) return;
+  AddInstruction(new(zone()) HSoftDeoptimize());
+  current_block()->MarkAsDeoptimizing();
+}
+
+
 template <class Instruction>
 HInstruction* HOptimizedGraphBuilder::PreProcessCall(Instruction* call) {
   int count = call->argument_count();
   ZoneList<HValue*> arguments(count, zone());
   for (int i = 0; i < count; ++i) {
     arguments.Add(Pop(), zone());
   }
 
   while (!arguments.is_empty()) {
     AddInstruction(new(zone()) HPushArgument(arguments.RemoveLast()));
@@ skipping 2149 matching lines @@
     int offset = (index * kPointerSize) + FixedArray::kHeaderSize;
     return new(zone()) HLoadNamedField(object, false, offset);
   }
 }
 
 
 HInstruction* HOptimizedGraphBuilder::BuildLoadNamedGeneric(
     HValue* object,
     Handle<String> name,
     Property* expr) {
-  if (expr->IsUninitialized() && !FLAG_always_opt) {
-    AddInstruction(new(zone()) HSoftDeoptimize);
-    current_block()->MarkAsDeoptimizing();
+  if (expr->IsUninitialized()) {
+    AddSoftDeoptimize();
   }
   HValue* context = environment()->LookupContext();
   return new(zone()) HLoadNamedGeneric(context, object, name);
 }
 
 
 HInstruction* HOptimizedGraphBuilder::BuildCallGetter(
     HValue* object,
     Handle<Map> map,
     Handle<JSFunction> getter,
@@ skipping 1871 matching lines @@
 
 void HOptimizedGraphBuilder::VisitSub(UnaryOperation* expr) {
   CHECK_ALIVE(VisitForValue(expr->expression()));
   HValue* value = Pop();
   HValue* context = environment()->LookupContext();
   HInstruction* instr =
       new(zone()) HMul(context, value, graph()->GetConstantMinus1());
   TypeInfo info = oracle()->UnaryType(expr);
   Representation rep = ToRepresentation(info);
   if (info.IsUninitialized()) {
-    AddInstruction(new(zone()) HSoftDeoptimize);
-    current_block()->MarkAsDeoptimizing();
+    AddSoftDeoptimize();
     info = TypeInfo::Unknown();
   }
   HBinaryOperation::cast(instr)->set_observed_input_representation(rep, rep);
   return ast_context()->ReturnInstruction(instr, expr->id());
 }
 
 
 void HOptimizedGraphBuilder::VisitBitNot(UnaryOperation* expr) {
   CHECK_ALIVE(VisitForValue(expr->expression()));
   HValue* value = Pop();
   TypeInfo info = oracle()->UnaryType(expr);
   if (info.IsUninitialized()) {
-    AddInstruction(new(zone()) HSoftDeoptimize);
-    current_block()->MarkAsDeoptimizing();
+    AddSoftDeoptimize();
   }
   HInstruction* instr = new(zone()) HBitNot(value);
   return ast_context()->ReturnInstruction(instr, expr->id());
 }
 
 
 void HOptimizedGraphBuilder::VisitNot(UnaryOperation* expr) {
   if (ast_context()->IsTest()) {
     TestContext* context = TestContext::cast(ast_context());
     VisitForControl(expr->expression(),
@@ skipping 335 matching lines @@
     HValue* right) {
   HValue* context = environment()->LookupContext();
   TypeInfo left_info, right_info, result_info, combined_info;
   oracle()->BinaryType(expr, &left_info, &right_info, &result_info);
   Representation left_rep = ToRepresentation(left_info);
   Representation right_rep = ToRepresentation(right_info);
   Representation result_rep = ToRepresentation(result_info);
   if (left_info.IsUninitialized()) {
     // Can't have initialized one but not the other.
     ASSERT(right_info.IsUninitialized());
-    AddInstruction(new(zone()) HSoftDeoptimize);
-    current_block()->MarkAsDeoptimizing();
+    AddSoftDeoptimize();
     left_info = right_info = TypeInfo::Unknown();
   }
   HInstruction* instr = NULL;
   switch (expr->op()) {
     case Token::ADD:
       if (left_info.IsString() && right_info.IsString()) {
         AddInstruction(new(zone()) HCheckNonSmi(left));
         AddInstruction(HCheckInstanceType::NewIsString(left, zone()));
         AddInstruction(new(zone()) HCheckNonSmi(right));
         AddInstruction(HCheckInstanceType::NewIsString(right, zone()));
@@ skipping 293 matching lines @@
   }
 
   TypeInfo left_type, right_type, overall_type_info;
   oracle()->CompareType(expr, &left_type, &right_type, &overall_type_info);
   Representation combined_rep = ToRepresentation(overall_type_info);
   Representation left_rep = ToRepresentation(left_type);
   Representation right_rep = ToRepresentation(right_type);
   // Check if this expression was ever executed according to type feedback.
   // Note that for the special typeof/null/undefined cases we get unknown here.
   if (overall_type_info.IsUninitialized()) {
-    AddInstruction(new(zone()) HSoftDeoptimize);
-    current_block()->MarkAsDeoptimizing();
+    AddSoftDeoptimize();
     overall_type_info = left_type = right_type = TypeInfo::Unknown();
   }
 
   CHECK_ALIVE(VisitForValue(expr->left()));
   CHECK_ALIVE(VisitForValue(expr->right()));
 
   HValue* context = environment()->LookupContext();
   HValue* right = Pop();
   HValue* left = Pop();
   Token::Value op = expr->op();
@@ skipping 1449 matching lines @@
     }
   }
 
 #ifdef DEBUG
   if (graph_ != NULL) graph_->Verify(false);  // No full verify.
   if (allocator_ != NULL) allocator_->Verify();
 #endif
 }
 
 } }  // namespace v8::internal