[interpreter] source positions should not be emitted for dead code.

R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/1668863002

Cr-Commit-Position: refs/heads/master@{#33775}

src/interpreter/bytecode-array-builder.cc

@@ -138,7 +138,10 @@ Handle<BytecodeArray> BytecodeArrayBuilder::ToBytecodeArray() {
 template <size_t N>
 void BytecodeArrayBuilder::Output(Bytecode bytecode, uint32_t(&operands)[N]) {
   // Don't output dead code.
-  if (exit_seen_in_block_) return;
+  if (exit_seen_in_block_) {
+    source_position_table_builder_.RevertPosition(bytecodes()->size());
+    return;
+  }
 
   int operand_count = static_cast<int>(N);
   DCHECK_EQ(Bytecodes::NumberOfOperands(bytecode), operand_count);
@@ -206,7 +209,10 @@ void BytecodeArrayBuilder::Output(Bytecode bytecode, uint32_t operand0) {
 
 void BytecodeArrayBuilder::Output(Bytecode bytecode) {
   // Don't output dead code.
-  if (exit_seen_in_block_) return;
+  if (exit_seen_in_block_) {
+    source_position_table_builder_.RevertPosition(bytecodes()->size());
+    return;
+  }
 
   DCHECK_EQ(Bytecodes::NumberOfOperands(bytecode), 0);
   last_bytecode_start_ = bytecodes()->size();
@@ -886,7 +892,10 @@ void BytecodeArrayBuilder::PatchJump(
 BytecodeArrayBuilder& BytecodeArrayBuilder::OutputJump(Bytecode jump_bytecode,
                                                        BytecodeLabel* label) {
   // Don't emit dead code.
-  if (exit_seen_in_block_) return *this;
+  if (exit_seen_in_block_) {
+    source_position_table_builder_.RevertPosition(bytecodes()->size());
+    return *this;
+  }
 
   // Check if the value in accumulator is boolean, if not choose an
   // appropriate JumpIfToBoolean bytecode.
@@ -1210,14 +1219,14 @@ size_t BytecodeArrayBuilder::GetConstantPoolEntry(Handle<Object> object) {
 
 void BytecodeArrayBuilder::SetStatementPosition(Statement* stmt) {
   if (stmt->position() == RelocInfo::kNoPosition) return;
-  source_position_table_builder_.AddStatementPosition(
-      static_cast<int>(bytecodes_.size()), stmt->position());
+  source_position_table_builder_.AddStatementPosition(bytecodes_.size(),
+                                                      stmt->position());
 }
 
 void BytecodeArrayBuilder::SetExpressionPosition(Expression* expr) {
   if (expr->position() == RelocInfo::kNoPosition) return;
-  source_position_table_builder_.AddExpressionPosition(
-      static_cast<int>(bytecodes_.size()), expr->position());
+  source_position_table_builder_.AddExpressionPosition(bytecodes_.size(),
+                                                       expr->position());
 }
 
 bool BytecodeArrayBuilder::TemporaryRegisterIsLive(Register reg) const {

src/interpreter/source-position-table.cc

@@ -15,20 +15,32 @@ namespace interpreter {
 class IsStatementField : public BitField<bool, 0, 1> {};
 class SourcePositionField : public BitField<int, 1, 30> {};
 
-void SourcePositionTableBuilder::AddStatementPosition(int bytecode_offset,
+void SourcePositionTableBuilder::AddStatementPosition(size_t bytecode_offset,
                                                       int source_position) {
-  AssertMonotonic(bytecode_offset);
+  int offset = static_cast<int>(bytecode_offset);
+  AssertMonotonic(offset);
   uint32_t encoded = IsStatementField::encode(true) |
                      SourcePositionField::encode(source_position);
-  entries_.push_back({bytecode_offset, encoded});
+  entries_.push_back({offset, encoded});
 }
 
-void SourcePositionTableBuilder::AddExpressionPosition(int bytecode_offset,
+void SourcePositionTableBuilder::AddExpressionPosition(size_t bytecode_offset,
                                                        int source_position) {
-  AssertMonotonic(bytecode_offset);
+  int offset = static_cast<int>(bytecode_offset);
+  AssertMonotonic(offset);
   uint32_t encoded = IsStatementField::encode(false) |
                      SourcePositionField::encode(source_position);
-  entries_.push_back({bytecode_offset, encoded});
+  entries_.push_back({offset, encoded});
+}
+
+void SourcePositionTableBuilder::RevertPosition(size_t bytecode_offset) {
+  int offset = static_cast<int>(bytecode_offset);
+  // If we already added a source position table entry, but the bytecode array
+  // builder ended up not outputting a bytecode for the corresponding bytecode
+  // offset, we have to remove that entry.
+  if (entries_.size() > 0 && entries_.back().bytecode_offset == offset) {
+    entries_.pop_back();
+  }
 }
 
 Handle<FixedArray> SourcePositionTableBuilder::ToFixedArray() {

src/interpreter/source-position-table.h

@@ -24,8 +24,9 @@ class SourcePositionTableBuilder {
   explicit SourcePositionTableBuilder(Isolate* isolate, Zone* zone)
       : isolate_(isolate), entries_(zone) {}
 
-  void AddStatementPosition(int bytecode_offset, int source_position);
-  void AddExpressionPosition(int bytecode_offset, int source_position);
+  void AddStatementPosition(size_t bytecode_offset, int source_position);
+  void AddExpressionPosition(size_t bytecode_offset, int source_position);
+  void RevertPosition(size_t bytecode_offset);
   Handle<FixedArray> ToFixedArray();
 
  private:

test/mjsunit/ignition/dead-code-source-position.js

+// Copyright 2016 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+function f() {
+  for (f(x) in []) { f(new f()) }
+}
+
+f();