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

src/compiler/bytecode-branch-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-branch-analysis.h"
+
+#include "src/interpreter/bytecode-array-iterator.h"
+
+namespace v8 {
+namespace internal {
+namespace compiler {
+
+// The class contains all of the sites that contain
+// branches to a particular target (bytecode offset).
+class BytecodeBranchInfo final : public ZoneObject {
+ public:
+  explicit BytecodeBranchInfo(Zone* zone)
+      : back_edge_offsets_(zone), fore_edge_offsets_(zone) {}
+
+  void AddBranch(int source_offset, int target_offset);
+
+  const ZoneVector<int>* back_edge_offsets() { return &back_edge_offsets_; }
+  const ZoneVector<int>* fore_edge_offsets() { return &fore_edge_offsets_; }
+
+ private:
+  // The offsets of bytecodes that refer to this bytecode as
+  // a back-edge predecessor.

[rmcilroy, 2015/12/15 17:35:43]

nit - these comments would be better on the public getters

[oth, 2015/12/15 22:42:17]

Done.

+  ZoneVector<int> back_edge_offsets_;
+
+  // The offsets of bytecodes that refer to this bytecode as
+  // a forwards-edge predecessor.
+  ZoneVector<int> fore_edge_offsets_;
+
+  DISALLOW_COPY_AND_ASSIGN(BytecodeBranchInfo);
+};
+
+
+void BytecodeBranchInfo::AddBranch(int source_offset, int target_offset) {
+  if (source_offset < target_offset) {
+    fore_edge_offsets_.push_back(source_offset);
+  } else {
+    back_edge_offsets_.push_back(source_offset);
+  }
+}
+
+
+void BytecodeBranchAnalysis::Analyze() {
+  interpreter::BytecodeArrayIterator iterator(bytecode_array());
+  bool reachable = true;
+  while (!iterator.done()) {
+    interpreter::Bytecode bytecode = iterator.current_bytecode();
+    int current_offset = iterator.current_offset();
+    // All bytecode basic blocks are generated to be forward reachable
+    // and may also be backward reachable. Hence if there's a forward
+    // branch targetting here the code becomes reachable.
+    reachable = reachable || forward_branches_target(current_offset);
+    if (reachable) {
+      reachable_.Add(current_offset);
+      if (interpreter::Bytecodes::IsConditionalJump(bytecode)) {
+        // Only the branch is recorded, the forward path falls through
+        // and is handled as normal bytecode data flow.
+        AddBranch(current_offset, iterator.GetJumpTargetOffset());
+      } else if (interpreter::Bytecodes::IsJump(bytecode)) {
+        // Unless the branch targets the next bytecode it's not
+        // reachable. If it targets the next bytecode the check at the
+        // start of the loop will set the reachable flag.
+        AddBranch(current_offset, iterator.GetJumpTargetOffset());
+        reachable = false;
+      } else if (interpreter::Bytecodes::IsLocalControlFlow(bytecode)) {
+        DCHECK_EQ(bytecode, interpreter::Bytecode::kReturn);
+        reachable = false;
+      }
+    }
+    iterator.Advance();
+  }
+}
+
+
+const ZoneVector<int>* BytecodeBranchAnalysis::BackwardBranchesTargetting(
+    int offset) const {
+  auto iterator = branch_infos_.find(offset);
+  if (branch_infos_.end() != iterator) {
+    return iterator->second->back_edge_offsets();
+  } else {
+    return nullptr;
+  }
+}
+
+
+const ZoneVector<int>* BytecodeBranchAnalysis::ForwardBranchesTargetting(
+    int offset) const {
+  auto iterator = branch_infos_.find(offset);
+  if (branch_infos_.end() != iterator) {
+    return iterator->second->fore_edge_offsets();
+  } else {
+    return nullptr;
+  }

[rmcilroy, 2015/12/15 17:35:43]

nit - pull the repeated code out into a shared helper function. 

[oth, 2015/12/15 22:42:17]

Nit - ignoring this one, the delta is back_edge_offsets() vs
fore_edge_offsets().

+}
+
+
+bool BytecodeBranchAnalysis::IsLastBackwardBranchTo(int branch_target,
+                                                    int offset) const {
+  return BackwardBranchesTargetting(branch_target)->back() == offset;
+}
+
+
+void BytecodeBranchAnalysis::AddBranch(int source_offset, int target_offset) {
+  BytecodeBranchInfo* branch_info = nullptr;
+  auto iterator = branch_infos_.find(target_offset);
+  if (branch_infos_.end() == iterator) {
+    branch_info = new (zone()) BytecodeBranchInfo(zone());
+    branch_infos_.insert(std::make_pair(target_offset, branch_info));
+  } else {
+    branch_info = iterator->second;
+  }
+  branch_info->AddBranch(source_offset, target_offset);
+}
+
+
+}  // namespace compiler
+}  // namespace internal
+}  // namespace v8