[cpu-profiler] Save space in the SourcePositionTable by using a vector.

This map is often quite small and holds small items (ints) so wastes
quite a bit of overhead in the backing tree representation.

This CL changes the std::map to a sorted vector of pairs. This reduces
the size significantly (2.13 MiB -> 598 KiB on the node server example).

Bug: v8:7719
Change-Id: Ic829693f007732ae145fae02850a1ed913cd941e
Reviewed-on: https://chromium-review.googlesource.com/1064233
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Reviewed-by: Alexei Filippov <alph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53278}

src/profiler/profile-generator.cc

@@ -21,23 +21,25 @@ namespace internal {
 void SourcePositionTable::SetPosition(int pc_offset, int line) {
   DCHECK_GE(pc_offset, 0);
   DCHECK_GT(line, 0);  // The 1-based number of the source line.
-  if (GetSourceLineNumber(pc_offset) != line) {
-    auto result = pc_offset_to_line_map_.emplace(pc_offset, line);
-    // Check that a new element was inserted.
-    USE(result);
-    DCHECK(result.second);
+  // Check that we are inserting in ascending order, so that the vector remains
+  // sorted.
+  DCHECK(pc_offsets_to_lines_.empty() ||
+         pc_offsets_to_lines_.back().pc_offset < pc_offset);
+  if (pc_offsets_to_lines_.empty() ||
+      pc_offsets_to_lines_.back().line_number != line) {
+    pc_offsets_to_lines_.push_back({pc_offset, line});
   }
 }
 
 int SourcePositionTable::GetSourceLineNumber(int pc_offset) const {
-  if (pc_offset_to_line_map_.empty()) {
+  if (pc_offsets_to_lines_.empty()) {
     return v8::CpuProfileNode::kNoLineNumberInfo;
   }
-  auto it = pc_offset_to_line_map_.upper_bound(pc_offset);
-  if (it != pc_offset_to_line_map_.begin()) {
-    return (--it)->second;
-  }
-  return it->second;
+  auto it =
+      std::upper_bound(pc_offsets_to_lines_.begin(), pc_offsets_to_lines_.end(),
+                       PCOffsetAndLineNumber{pc_offset, 0});
+  if (it != pc_offsets_to_lines_.begin()) --it;
+  return it->line_number;
 }
 
 const char* const CodeEntry::kEmptyResourceName = "";

src/profiler/profile-generator.h

@@ -32,7 +32,17 @@ class SourcePositionTable : public Malloced {
   int GetSourceLineNumber(int pc_offset) const;
 
  private:
-  std::map<int, int> pc_offset_to_line_map_;
+  struct PCOffsetAndLineNumber {
+    bool operator<(const PCOffsetAndLineNumber& other) const {
+      return pc_offset < other.pc_offset;
+    }
+    int pc_offset;
+    int line_number;
+  };
+  // This is logically a map, but we store it as a vector of pairs, sorted by
+  // the pc offset, so that we can save space and look up items using binary
+  // search.
+  std::vector<PCOffsetAndLineNumber> pc_offsets_to_lines_;
   DISALLOW_COPY_AND_ASSIGN(SourcePositionTable);
 };
 

test/cctest/test-cpu-profiler.cc

@@ -2424,49 +2424,43 @@ TEST(NativeFrameStackTrace) {
 }
 
 TEST(SourcePositionTable) {
-  std::unique_ptr<i::SourcePositionTable> info(new SourcePositionTable());
+  i::SourcePositionTable info;
 
   // Newly created tables should return NoLineNumberInfo for any lookup.
   int no_info = v8::CpuProfileNode::kNoLineNumberInfo;
-  CHECK_EQ(no_info, info->GetSourceLineNumber(std::numeric_limits<int>::min()));
-  CHECK_EQ(no_info, info->GetSourceLineNumber(0));
-  CHECK_EQ(no_info, info->GetSourceLineNumber(1));
-  CHECK_EQ(no_info, info->GetSourceLineNumber(9));
-  CHECK_EQ(no_info, info->GetSourceLineNumber(10));
-  CHECK_EQ(no_info, info->GetSourceLineNumber(11));
-  CHECK_EQ(no_info, info->GetSourceLineNumber(19));
-  CHECK_EQ(no_info, info->GetSourceLineNumber(20));
-  CHECK_EQ(no_info, info->GetSourceLineNumber(21));
-  CHECK_EQ(no_info, info->GetSourceLineNumber(100));
-  CHECK_EQ(no_info, info->GetSourceLineNumber(std::numeric_limits<int>::max()));
-
-  info->SetPosition(10, 1);
-  info->SetPosition(20, 2);
+  CHECK_EQ(no_info, info.GetSourceLineNumber(std::numeric_limits<int>::min()));
+  CHECK_EQ(no_info, info.GetSourceLineNumber(0));
+  CHECK_EQ(no_info, info.GetSourceLineNumber(1));
+  CHECK_EQ(no_info, info.GetSourceLineNumber(9));
+  CHECK_EQ(no_info, info.GetSourceLineNumber(10));
+  CHECK_EQ(no_info, info.GetSourceLineNumber(11));
+  CHECK_EQ(no_info, info.GetSourceLineNumber(19));
+  CHECK_EQ(no_info, info.GetSourceLineNumber(20));
+  CHECK_EQ(no_info, info.GetSourceLineNumber(21));
+  CHECK_EQ(no_info, info.GetSourceLineNumber(100));
+  CHECK_EQ(no_info, info.GetSourceLineNumber(std::numeric_limits<int>::max()));
+
+  info.SetPosition(10, 1);
+  info.SetPosition(20, 2);
 
   // The only valid return values are 1 or 2 - every pc maps to a line number.
-  CHECK_EQ(1, info->GetSourceLineNumber(std::numeric_limits<int>::min()));
-  CHECK_EQ(1, info->GetSourceLineNumber(0));
-  CHECK_EQ(1, info->GetSourceLineNumber(1));
-  CHECK_EQ(1, info->GetSourceLineNumber(9));
-  CHECK_EQ(1, info->GetSourceLineNumber(10));
-  CHECK_EQ(1, info->GetSourceLineNumber(11));
-  CHECK_EQ(1, info->GetSourceLineNumber(19));
-  CHECK_EQ(2, info->GetSourceLineNumber(20));
-  CHECK_EQ(2, info->GetSourceLineNumber(21));
-  CHECK_EQ(2, info->GetSourceLineNumber(100));
-  CHECK_EQ(2, info->GetSourceLineNumber(std::numeric_limits<int>::max()));
+  CHECK_EQ(1, info.GetSourceLineNumber(std::numeric_limits<int>::min()));
+  CHECK_EQ(1, info.GetSourceLineNumber(0));
+  CHECK_EQ(1, info.GetSourceLineNumber(1));
+  CHECK_EQ(1, info.GetSourceLineNumber(9));
+  CHECK_EQ(1, info.GetSourceLineNumber(10));
+  CHECK_EQ(1, info.GetSourceLineNumber(11));
+  CHECK_EQ(1, info.GetSourceLineNumber(19));
+  CHECK_EQ(2, info.GetSourceLineNumber(20));
+  CHECK_EQ(2, info.GetSourceLineNumber(21));
+  CHECK_EQ(2, info.GetSourceLineNumber(100));
+  CHECK_EQ(2, info.GetSourceLineNumber(std::numeric_limits<int>::max()));
 
   // Test SetPosition behavior.
-  // Setting a position to the same value has no effect.
-  info->SetPosition(10, 1);
-  CHECK_EQ(1, info->GetSourceLineNumber(9));
-  CHECK_EQ(1, info->GetSourceLineNumber(10));
-  CHECK_EQ(1, info->GetSourceLineNumber(11));
-
-  info->SetPosition(25, 3);
-  CHECK_EQ(2, info->GetSourceLineNumber(21));
-  CHECK_EQ(3, info->GetSourceLineNumber(100));
-  CHECK_EQ(3, info->GetSourceLineNumber(std::numeric_limits<int>::max()));
+  info.SetPosition(25, 3);
+  CHECK_EQ(2, info.GetSourceLineNumber(21));
+  CHECK_EQ(3, info.GetSourceLineNumber(100));
+  CHECK_EQ(3, info.GetSourceLineNumber(std::numeric_limits<int>::max()));
 }
 
 TEST(MultipleProfilers) {