[interpreter] Introduce bytecode generation pipeline.

src/interpreter/bytecode-peephole-optimizer.cc

+// Copyright 2015 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.
+
+#include "src/interpreter/bytecode-peephole-optimizer.h"
+
+#include "src/interpreter/constant-array-builder.h"
+#include "src/objects-inl.h"
+#include "src/objects.h"
+
+namespace v8 {
+namespace internal {
+namespace interpreter {
+
+BytecodePeepholeOptimizer::BytecodePeepholeOptimizer(
+    ConstantArrayBuilder* constant_array_builder,
+    BytecodePipelineStage* next_stage)
+    : constant_array_builder_(constant_array_builder),
+      next_stage_(next_stage),
+      last_(Bytecode::kNop),

[rmcilroy, 2016/05/12 12:15:13]

nit - as mentioned offline, would be nice to use kInvalid as invalid bytecode
(and get rid of last_is_valid_ field) once you have addressed the TODO for never
emiting kInvalid. Don't block this CL on that though.

[oth, 2016/05/12 14:59:52]

Done.

+      last_is_valid_(false),
+      last_is_discardable_(false) {}
+
+void BytecodePeepholeOptimizer::InvalidateLast() { last_is_valid_ = false; }
+
+bool BytecodePeepholeOptimizer::LastIsValid() const { return last_is_valid_; }
+
+void BytecodePeepholeOptimizer::SetLast(const BytecodeNode* const node) {
+  memcpy(&last_, node, sizeof(*node));
+  last_is_valid_ = true;
+  last_is_discardable_ = true;
+}
+
+// override
+size_t BytecodePeepholeOptimizer::FlushForOffset() {
+  size_t buffered_size = next_stage_->FlushForOffset();
+  if (LastIsValid()) {
+    if (last_.bytecode() == Bytecode::kNop &&
+        !last_.source_info().is_statement()) {

[rmcilroy, 2016/05/12 12:15:13]

nit - please add a comment here as to why the Nop can be invalidated if it
doesn't have a statement position.

[oth, 2016/05/12 14:59:52]

Done.

+      InvalidateLast();
+    } else {
+      buffered_size += last_.Size();
+      last_is_discardable_ = false;
+    }
+  }
+  return buffered_size;
+}
+
+// override
+void BytecodePeepholeOptimizer::FlushBasicBlock() {
+  if (LastIsValid()) {
+    next_stage_->Write(&last_);
+    InvalidateLast();
+  }
+  last_is_discardable_ = false;
+  next_stage_->FlushBasicBlock();
+}
+
+// override
+void BytecodePeepholeOptimizer::Write(BytecodeNode* node) {
+  // Attempt optimization if there is an earlier node to optimize with.
+  if (LastIsValid()) {
+    node = Optimize(node);
+    // Only output if optimization did not invalidate earlier node.

[rmcilroy, 2016/05/12 12:15:13]

nit - // Only output the last node if it wasn't invalidated by the optimization.

[oth, 2016/05/12 14:59:52]

Done.

+    if (LastIsValid()) {
+      next_stage_->Write(&last_);
+      InvalidateLast();
+    }
+  }
+
+  if (node != nullptr) {
+    SetLast(node);
+  }
+}
+
+Handle<Object> BytecodePeepholeOptimizer::GetConstantForIndexOperand(
+    const BytecodeNode* const node, int index) const {
+  DCHECK_LE(index, node->operand_count());
+  DCHECK_EQ(Bytecodes::GetOperandType(node->bytecode(), 0), OperandType::kIdx);
+  uint32_t index_operand = node->operand(0);
+  return constant_array_builder_->At(index_operand);
+}
+
+bool BytecodePeepholeOptimizer::LastBytecodePutsNameInAccumulator() const {
+  DCHECK(LastIsValid());
+  return (last_.bytecode() == Bytecode::kTypeOf ||
+          last_.bytecode() == Bytecode::kToName ||
+          (last_.bytecode() == Bytecode::kLdaConstant &&
+           GetConstantForIndexOperand(&last_, 0)->IsName()));

[rmcilroy, 2016/05/12 12:15:13]

I wish we could put this in bytecodes.h but don't see an easy way to do it with
having to get a constant from the constant pool :(.

[oth, 2016/05/12 14:59:52]

Acknowledged.

+}
+
+void BytecodePeepholeOptimizer::UpdateCurrentBytecode(BytecodeNode* current) {
+  // Conditional jumps with boolean conditions are emiitted in
+  // ToBoolean form by the bytecode array builder,
+  // i.e. JumpIfToBooleanTrue rather JumpIfTrue. The ToBoolean element
+  // can be removed if the previous bytecode put a boolean value in
+  // the accumulator.
+  if (Bytecodes::IsJumpIfToBoolean(current->bytecode()) &&
+      Bytecodes::WritesBooleanToAccumulator(last_.bytecode())) {
+    Bytecode jump = Bytecodes::GetJumpWithoutToBoolean(current->bytecode());
+    current->set_bytecode(jump, current->operand(0), current->operand_scale());
+  }
+}
+
+bool BytecodePeepholeOptimizer::CanElideCurrent(
+    const BytecodeNode* const current) const {
+  if (Bytecodes::IsLdarOrStar(last_.bytecode()) &&
+      Bytecodes::IsLdarOrStar(current->bytecode()) &&
+      current->operand(0) == last_.operand(0)) {
+    // Ldar and Star make the accumulator and register hold equivalent
+    // values. Only the first bytecode is needed if there's a sequence
+    // of back-to-back of Ldar and Star bytecodes with the same operand.

[rmcilroy, 2016/05/12 12:15:13]

nit - remove second "of"

[oth, 2016/05/12 14:59:52]

Done.

+    return true;
+  } else if (current->bytecode() == Bytecode::kToName &&
+             LastBytecodePutsNameInAccumulator()) {
+    // If the previous bytecode ensured a name was in the accumulator,
+    // the type coercion ToName() can be elided.
+    return true;
+  } else {
+    return false;
+  }
+}
+
+bool BytecodePeepholeOptimizer::CanElideLast(
+    const BytecodeNode* const current) const {
+  if (!last_is_discardable_) {
+    return false;
+  }
+
+  if (last_.bytecode() == Bytecode::kNop) {
+    // Nop are placeholders for holding source position information
+    // and can be elided.
+    return true;
+  } else if (Bytecodes::IsAccumulatorLoadWithoutEffects(current->bytecode()) &&
+             Bytecodes::IsAccumulatorLoadWithoutEffects(last_.bytecode())) {
+    // The accumulator is invisible to the debugger. If there is a sequence of
+    // consecutive accumulator loads (that don't have side effects) then only
+    // the final load is potentially visible.
+    return true;
+  } else {
+    return false;
+  }
+}
+
+BytecodeNode* BytecodePeepholeOptimizer::Optimize(BytecodeNode* current) {
+  UpdateCurrentBytecode(current);
+
+  if (CanElideCurrent(current)) {
+    if (current->source_info().is_valid()) {
+      current->set_bytecode(Bytecode::kNop);
+    } else {
+      current = nullptr;
+    }
+  } else if (CanElideLast(current)) {
+    if (last_.source_info().is_valid()) {
+      current->source_info().Update(last_.source_info());
+    }
+    InvalidateLast();
+  }
+  return current;
+}
+
+}  // namespace interpreter
+}  // namespace internal
+}  // namespace v8