[interpreter] Simplify BytecodeBranchAnalysis to minimum.

This simplifies the branch analysis we perform on the bytecode stream
down to the bare minimum that we need to build graphs. Note that we
still record all branch targets, even though only the backwards ones
would be needed, but this is essentially for free and might be useful
eventually.

R=oth@chromium.org

Review URL: https://codereview.chromium.org/1646873004

Cr-Commit-Position: refs/heads/master@{#33635}

src/compiler/bytecode-branch-analysis.cc

@@ -11,47 +11,13 @@ 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);
-
-  // The offsets of bytecodes that refer to this bytecode as
-  // a back-edge predecessor.
-  const ZoneVector<int>* back_edge_offsets() { return &back_edge_offsets_; }
-
-  // The offsets of bytecodes that refer to this bytecode as
-  // a forwards-edge predecessor.
-  const ZoneVector<int>* fore_edge_offsets() { return &fore_edge_offsets_; }
-
- private:
-  ZoneVector<int> back_edge_offsets_;
-  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);
-  }
-}
-
-
 BytecodeBranchAnalysis::BytecodeBranchAnalysis(
     Handle<BytecodeArray> bytecode_array, Zone* zone)
-    : branch_infos_(zone),
-      bytecode_array_(bytecode_array),
+    : bytecode_array_(bytecode_array),
+      is_backward_target_(bytecode_array->length(), zone),
+      is_forward_target_(bytecode_array->length(), zone),
       zone_(zone) {}
 
-
 void BytecodeBranchAnalysis::Analyze() {
   interpreter::BytecodeArrayIterator iterator(bytecode_array());
   while (!iterator.done()) {
@@ -64,38 +30,12 @@ void BytecodeBranchAnalysis::Analyze() {
   }
 }
 
-
-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;
-  }
-}
-
 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));
+  if (source_offset < target_offset) {
+    is_forward_target_.Add(target_offset);
   } else {
-    branch_info = iterator->second;
+    is_backward_target_.Add(target_offset);
   }
-  branch_info->AddBranch(source_offset, target_offset);
 }
 
 }  // namespace compiler

src/compiler/bytecode-branch-analysis.h

@@ -7,7 +7,6 @@
 
 #include "src/bit-vector.h"
 #include "src/handles.h"
-#include "src/zone-containers.h"
 
 namespace v8 {
 namespace internal {
@@ -16,15 +15,13 @@ class BytecodeArray;
 
 namespace compiler {
 
-class BytecodeBranchInfo;
-
-// A class for identifying the branch targets and their branch sites
-// within a bytecode array. This information can be used to construct
-// the local control flow logic for high-level IR graphs built from
-// bytecode.
+// A class for identifying branch targets within a bytecode array.
+// This information can be used to construct the local control flow
+// logic for high-level IR graphs built from bytecode.
 //
-// NB This class relies on the only backwards branches in bytecode
-// being jumps back to loop headers.
+// N.B. If this class is used to determine loop headers, then such a
+// usage relies on the only backwards branches in bytecode being jumps
+// back to loop headers.
 class BytecodeBranchAnalysis BASE_EMBEDDED {
  public:
   BytecodeBranchAnalysis(Handle<BytecodeArray> bytecode_array, Zone* zone);
@@ -37,31 +34,24 @@ class BytecodeBranchAnalysis BASE_EMBEDDED {
   // Returns true if there are any forward branches to the bytecode at
   // |offset|.
   bool forward_branches_target(int offset) const {
-    const ZoneVector<int>* sites = ForwardBranchesTargetting(offset);
-    return sites != nullptr && sites->size() > 0;
+    return is_forward_target_.Contains(offset);
   }
 
   // Returns true if there are any backward branches to the bytecode
   // at |offset|.
   bool backward_branches_target(int offset) const {
-    const ZoneVector<int>* sites = BackwardBranchesTargetting(offset);
-    return sites != nullptr && sites->size() > 0;
+    return is_backward_target_.Contains(offset);
   }
 
  private:
   void AddBranch(int origin_offset, int target_offset);
 
-  // Offsets of bytecodes having a backward branch to the bytecode at |offset|.
-  const ZoneVector<int>* BackwardBranchesTargetting(int offset) const;
-
-  // Offsets of bytecodes having a forward branch to the bytecode at |offset|.
-  const ZoneVector<int>* ForwardBranchesTargetting(int offset) const;
-
   Zone* zone() const { return zone_; }
   Handle<BytecodeArray> bytecode_array() const { return bytecode_array_; }
 
-  ZoneMap<int, BytecodeBranchInfo*> branch_infos_;
   Handle<BytecodeArray> bytecode_array_;
+  BitVector is_backward_target_;
+  BitVector is_forward_target_;
   Zone* zone_;
 
   DISALLOW_COPY_AND_ASSIGN(BytecodeBranchAnalysis);