[Interpreter] Local flow control in the bytecode graph builder.

src/compiler/bytecode-basic-block-analysis.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/compiler/bytecode-basic-block-analysis.h"
+
+#include "src/interpreter/bytecode-array-iterator.h"
+
+namespace v8 {
+namespace internal {
+namespace compiler {
+
+namespace {
+
+int GetJumpAbsoluteOffset(const interpreter::BytecodeArrayIterator& iterator) {
+  interpreter::Bytecode bytecode = iterator.current_bytecode();
+  if (interpreter::Bytecodes::IsJumpImmediate(bytecode)) {
+    int relative_offset = iterator.GetImmediateOperand(0);
+    return iterator.current_offset() + relative_offset;
+  } else if (interpreter::Bytecodes::IsJumpConstant(bytecode)) {
+    Smi* smi = Smi::cast(*iterator.GetConstantForIndexOperand(0));
+    return iterator.current_offset() + smi->value();
+  } else {
+    UNREACHABLE();
+    return -1;
+  }
+}
+
+}  // namespace
+
+
+BytecodeBasicBlockAnalysis::BytecodeBasicBlockAnalysis(
+    Zone* zone, Handle<BytecodeArray> bytecode_array)
+    : zone_(zone),
+      block_map_(zone),
+      rpo_order_(zone),
+      dependency_order_(zone),
+      bytecode_array_(bytecode_array) {}
+
+
+const ZoneVector<BytecodeBasicBlock*>* BytecodeBasicBlockAnalysis::Analyze() {
+  CreateBasicBlocks();
+  LinkBasicBlocks();
+  AssignRpoOrder();
+  FindDominators();
+  FindDependencyOrder();
+  return &dependency_order_;
+}
+
+
+void BytecodeBasicBlockAnalysis::CreateBasicBlock(int start, int end) {
+  DCHECK(block_map_.find(start) == block_map_.end());
+  BytecodeBasicBlock* new_block =
+      new (zone()) BytecodeBasicBlock(zone(), start, end);
+  block_map_.insert(std::make_pair(start, new_block));
+}
+
+
+void BytecodeBasicBlockAnalysis::SplitBasicBlock(int offset) {
+  // Find the basic blocking containing offset.
+  auto offset_block_pair = block_map_.lower_bound(offset);
+  if (offset_block_pair->first > offset) {

[rmcilroy, 2015/12/08 13:25:07]

nit - comment would be good here

[oth, 2015/12/09 11:26:45]

Done.

+    offset_block_pair--;
+  }
+  DCHECK(offset_block_pair->first <= offset);
+
+  if (offset_block_pair->first < offset) {

[rmcilroy, 2015/12/08 13:25:07]

nit - offset_block_pair->first != offset (for clarity) ?

[oth, 2015/12/09 11:26:44]

Done.

+    // Only split if offset does not point to an existing block start.
+    int split_end = offset_block_pair->second->end();
+    offset_block_pair->second->set_end(offset);
+    CreateBasicBlock(offset, split_end);
+  }
+}
+
+
+void BytecodeBasicBlockAnalysis::CreateBasicBlocks() {
+  interpreter::BytecodeArrayIterator iterator(bytecode_array_);
+  ZoneVector<int> split_offsets(zone());

[rmcilroy, 2015/12/08 13:25:07]

nit - jump_targets would be clearer here IMO

[oth, 2015/12/09 11:26:45]

Done.

+
+  // Create the exit block.
+  CreateBasicBlock(bytecode_array_->length(), kMaxInt);
+
+  // Find and allocate basic blocks in the bytecode.
+  while (!iterator.done()) {
+    int block_start = iterator.current_offset();
+    interpreter::Bytecode bytecode = iterator.current_bytecode();
+    while (!interpreter::Bytecodes::IsLocalControlFlow(bytecode) &&
+           iterator.current_offset() + iterator.current_size() <
+               bytecode_array_->length()) {

[rmcilroy, 2015/12/08 13:25:07]

Can you not call !iterator.done() instead of "iterator.current_offset() +
iterator.current_size() < bytecode_array_->length()" ?

[oth, 2015/12/09 11:26:45]

Yes, flattened into a single loop. 

+      iterator.Advance();
+      bytecode = iterator.current_bytecode();
+    }
+    CreateBasicBlock(block_start,
+                     iterator.current_offset() + iterator.current_size());
+    if (interpreter::Bytecodes::IsJump(bytecode)) {
+      split_offsets.push_back(GetJumpAbsoluteOffset(iterator));
+    }
+    iterator.Advance();
+  }
+
+  for (auto split_offset = split_offsets.begin();

[rmcilroy, 2015/12/08 13:25:07]

Comment that you are splitting basic blocks based on incoming jumps here.

[oth, 2015/12/09 11:26:45]

Done.

+       split_offset != split_offsets.end(); split_offset++) {
+    SplitBasicBlock(*split_offset);
+  }
+}
+
+
+void BytecodeBasicBlockAnalysis::LinkBasicBlocks() {
+  interpreter::BytecodeArrayIterator iterator(bytecode_array_);
+  while (!iterator.done()) {
+    // Find the last last bytecode in each block.
+    auto offset_block_pair = block_map_.find(iterator.current_offset());
+
+    DCHECK(offset_block_pair != block_map_.end());
+    BytecodeBasicBlock* current_block = offset_block_pair->second;
+    DCHECK_NE(current_block, exit());
+    while (iterator.current_offset() +
+               interpreter::Bytecodes::Size(iterator.current_bytecode()) !=

[rmcilroy, 2015/12/08 13:25:07]

iterator.current_bytecode_size()  ?

[oth, 2015/12/09 11:26:44]

Removed current_bytecode from BytecodeArrayIterator.

With the revision to CreateBasicBlocks, this method just passes over the basic
blocks and not the bytecode.

+           current_block->end()) {
+      DCHECK_LT(iterator.current_offset(), current_block->end());
+      iterator.Advance();
+    }
+    current_block->set_last_bytecode_offset(iterator.current_offset());
+
+    // Connect up links with other blocks.
+    interpreter::Bytecode bytecode = iterator.current_bytecode();
+    if (interpreter::Bytecodes::IsConditionalJump(bytecode)) {
+      int target_offset = GetJumpAbsoluteOffset(iterator);
+      current_block->end_with_conditional_jump(
+          block_map_[target_offset], block_map_[current_block->end()]);
+    } else if (interpreter::Bytecodes::IsJump(bytecode)) {
+      int target_offset = GetJumpAbsoluteOffset(iterator);
+      current_block->end_with_block(block_map_[target_offset]);
+    } else if (!interpreter::Bytecodes::IsLocalControlFlow(bytecode)) {
+      current_block->end_with_block(block_map_[current_block->end()]);
+    } else {
+      DCHECK_EQ(bytecode, interpreter::Bytecode::kReturn);
+      current_block->end_with_block(exit());
+    }
+    iterator.Advance();
+  }
+}
+
+
+void BytecodeBasicBlockAnalysis::AssignRpoOrder() {
+  // Assign reverse post-order id to aid with finding dominators.
+  ZoneStack<BytecodeBasicBlock*> pending(zone());
+  ZoneStack<BytecodeBasicBlock*> ordered(zone());
+
+  pending.push(block_map_[0]);
+  while (!pending.empty()) {
+    BytecodeBasicBlock* current = pending.top();
+    pending.pop();
+    if (current->rpo_id() != BytecodeBasicBlock::kUnassignedId) {
+      continue;
+    }
+
+    ordered.push(current);
+    current->set_rpo_id(0);
+    DCHECK((current->if_true() != nullptr || current->if_false() != nullptr) ||
+           current->start() == bytecode_array_->length());
+
+    if (current->if_true() != nullptr &&
+        current->if_true()->rpo_id() == BytecodeBasicBlock::kUnassignedId) {
+      pending.push(current->if_true());
+    }
+    if (current->if_false() != nullptr &&
+        current->if_false()->rpo_id() == BytecodeBasicBlock::kUnassignedId) {
+      pending.push(current->if_false());
+    }
+  }
+
+  // Non-reachable blocks are not assigned an RPO id and need to be pruned.
+  // This happens quite rarely, e.g. all paths in a switch return from function:
+  //   function f(x) {
+  //     switch (x) {
+  //     case 'a': return 1;
+  //     default: return 0;
+  //     }
+  //     return undefined; // Potentially inserted by bytecode generator.
+  //   }
+  if (ordered.size() != block_map_.size()) {
+    int unreachable = 0;
+    for (auto iter = block_map_.begin(); iter != block_map_.end(); iter++) {
+      BytecodeBasicBlock* block = iter->second;
+      if (block->incoming_count() == 0 && block->start() > 0) {
+        if (block->if_true() != nullptr) {
+          auto it = std::find(block->if_true()->incoming_.begin(),
+                              block->if_true()->incoming_.end(), block);
+          block->if_true()->incoming_.erase(it);
+        }
+        if (block->if_false() != nullptr) {
+          auto it = std::find(block->if_false()->incoming_.begin(),
+                              block->if_false()->incoming_.end(), block);
+          block->if_false()->incoming_.erase(it);
+        }
+        unreachable++;
+      }
+    }
+    // Start has no incoming, but this is expected.
+    DCHECK_EQ(unreachable, block_map_.size() - ordered.size());
+  }
+
+  // Pop post-order entries and back insert into rpo_order_ vector.
+  int id = static_cast<int>(ordered.size());
+  rpo_order_.resize(ordered.size());
+  while (!ordered.empty()) {
+    BytecodeBasicBlock* current = ordered.top();
+    current->set_rpo_id(--id);
+    rpo_order_[id] = current;
+    ordered.pop();
+  }
+}
+
+
+void BytecodeBasicBlockAnalysis::FindDominators() {
+  // Naive implementation of finding dominators O(N^2).  Lower cost
+  // algorithms exist, but have additional implementation complexity
+  // and higher fixed cost. This typically takes a maximum of 3
+  // iterations to converge and in practice N is small.
+  DCHECK(!rpo_order_.empty());
+  DCHECK(rpo_order_[0]->dominator_rpo_ids() == nullptr);
+  for (size_t i = 0; i < rpo_order_.size(); i++) {
+    rpo_order_[i]->set_dominator_rpo_ids(
+        new (zone()) BitVector(static_cast<int>(rpo_order_.size()), zone()));
+    if (i == 0) {

[rmcilroy, 2015/12/08 13:25:07]

Could you add a comment on what this if/else block is doing

[oth, 2015/12/09 11:26:45]

Done and added a reference to the algorithm.

+      rpo_order_[i]->dominator_rpo_ids()->Add(0);
+    } else {
+      rpo_order_[i]->dominator_rpo_ids()->AddAll();
+      rpo_order_[i]->dominator_rpo_ids()->Remove(0);
+    }
+  }
+
+  BitVector accumulator(static_cast<int>(rpo_order_.size()), zone());
+  bool changed = true;
+  while (changed) {

[rmcilroy, 2015/12/08 13:25:07]

A couple of comments here would be good.

[oth, 2015/12/09 11:26:45]

Done.

+    changed = false;
+    for (size_t i = 1; i < rpo_order_.size(); i++) {
+      for (size_t j = 0; j < rpo_order_[i]->incoming()->size(); j++) {
+        auto incoming = rpo_order_[i]->incoming()->at(j);
+        if (j == 0) {
+          accumulator.CopyFrom(*incoming->dominator_rpo_ids());
+        } else {
+          accumulator.Intersect(*incoming->dominator_rpo_ids());
+        }
+      }
+      accumulator.Add(static_cast<int>(i));
+      if (!rpo_order_[i]->dominator_rpo_ids()->Equals(accumulator)) {
+        rpo_order_[i]->dominator_rpo_ids()->CopyFrom(accumulator);
+        changed = true;
+      }
+    }
+  }
+}
+
+
+void BytecodeBasicBlockAnalysis::FindDependencyOrder() {
+  // Compute the dependency order of basic blocks based on forward
+  // links.  In dependency order, all forward linking blocks into a
+  // block are satisfied before the block. This simplifies the
+  // handling merges.
+  BitVector queued(static_cast<int>(block_map_.size()), zone());
+  BitVector satisfied(static_cast<int>(block_map_.size()), zone());
+  ZoneQueue<int> to_visit(zone());
+
+  dependency_order_.resize(rpo_order_.size());
+  auto inserter = dependency_order_.begin();
+  to_visit.push(start()->rpo_id());
+  queued.Add(start()->rpo_id());
+  while (!to_visit.empty()) {
+    int rpo_id = to_visit.front();
+    queued.Remove(rpo_id);
+    to_visit.pop();
+    if (satisfied.Contains(rpo_id)) {
+      continue;
+    }
+
+    // Check whether incoming edges have satisfied dependencies.
+    BytecodeBasicBlock* current = rpo_order_[rpo_id];
+    bool unsatisfied = false;
+    for (size_t i = 0; i < current->incoming_count(); i++) {
+      const BytecodeBasicBlock* incoming = current->incoming(i);
+      // Enqueue unsatisfied dependencies from incoming edges provided
+      // they are not in the queue already and are not loop
+      // headers. The latter are not considered dependencies.
+      int incoming_rpo_id = incoming->rpo_id();
+      if (!satisfied.Contains(incoming_rpo_id) &&
+          !incoming->is_dominated_by(current)) {
+        if (!queued.Contains(incoming_rpo_id)) {
+          to_visit.push(incoming_rpo_id);
+          queued.Add(incoming_rpo_id);
+        }
+        unsatisfied = true;
+      }
+    }
+
+    if (unsatisfied) {
+      // Dependencies not satisfied, put block back on list to process.
+      DCHECK(!queued.Contains(rpo_id));
+      to_visit.push(rpo_id);
+      queued.Add(rpo_id);
+    } else {
+      // Dependencies satisfied, time to visit the blocks attached to
+      // out edges.
+      *inserter++ = current;
+      satisfied.Add(rpo_id);
+      BytecodeBasicBlock* nodes[2] = {current->if_true(), current->if_false()};
+      for (size_t i = 0; i < 2; i++) {
+        if (nodes[i] != nullptr && !satisfied.Contains(nodes[i]->rpo_id()) &&
+            !current->is_dominated_by(nodes[i])) {
+          if (!queued.Contains(nodes[i]->rpo_id())) {
+            to_visit.push(nodes[i]->rpo_id());
+            queued.Add(nodes[i]->rpo_id());
+          }
+        }
+      }
+    }
+  }
+  DCHECK(inserter == dependency_order_.end());
+}
+
+
+}  // namespace compiler
+}  // namespace internal
+}  // namespace v8