[snapshot] support multiple contexts in the same snapshot.

src/snapshot/snapshot-common.cc

Index: src/snapshot/snapshot-common.cc
diff --git a/src/snapshot/snapshot-common.cc b/src/snapshot/snapshot-common.cc
index 655563ccd9cafbf7aaf3d43aa5a91601376cec56..5eac4af6eeb08f42e75cdf2359bfa315f88d192b 100644
--- a/src/snapshot/snapshot-common.cc
+++ b/src/snapshot/snapshot-common.cc
@@ -18,8 +18,7 @@ namespace internal {
 
 #ifdef DEBUG
 bool Snapshot::SnapshotIsValid(v8::StartupData* snapshot_blob) {
-  return !Snapshot::ExtractStartupData(snapshot_blob).is_empty() &&
-         !Snapshot::ExtractContextData(snapshot_blob).is_empty();
+  return Snapshot::ExtractNumContexts(snapshot_blob) > 0;
 }
 #endif  // DEBUG
 
@@ -61,15 +60,16 @@ bool Snapshot::Initialize(Isolate* isolate) {
   return success;
 }
 
-
 MaybeHandle<Context> Snapshot::NewContextFromSnapshot(
-    Isolate* isolate, Handle<JSGlobalProxy> global_proxy) {
+    Isolate* isolate, Handle<JSGlobalProxy> global_proxy,
+    size_t context_index) {
   if (!isolate->snapshot_available()) return Handle<Context>();
   base::ElapsedTimer timer;
   if (FLAG_profile_deserialization) timer.Start();
 
   const v8::StartupData* blob = isolate->snapshot_blob();
-  Vector<const byte> context_data = ExtractContextData(blob);
+  Vector<const byte> context_data =
+      ExtractContextData(blob, static_cast<int>(context_index));
   SnapshotData snapshot_data(context_data);
   Deserializer deserializer(&snapshot_data);
 
@@ -81,133 +81,163 @@ MaybeHandle<Context> Snapshot::NewContextFromSnapshot(
   if (FLAG_profile_deserialization) {
     double ms = timer.Elapsed().InMillisecondsF();
     int bytes = context_data.length();
-    PrintF("[Deserializing context (%d bytes) took %0.3f ms]\n", bytes, ms);
+    PrintF("[Deserializing context #%zu (%d bytes) took %0.3f ms]\n",
+           context_index, bytes, ms);
   }
   return Handle<Context>::cast(result);
 }
 
+void UpdateMaxRequirementPerPage(
+    uint32_t* requirements,
+    Vector<const SerializedData::Reservation> reservations) {
+  int space = 0;
+  uint32_t current_requirement = 0;
+  for (const auto& reservation : reservations) {
+    current_requirement += reservation.chunk_size();
+    if (reservation.is_last()) {
+      requirements[space] = std::max(requirements[space], current_requirement);
+      current_requirement = 0;
+      space++;
+    }
+  }
+  DCHECK_EQ(i::Serializer::kNumberOfSpaces, space);
+}
+
 void CalculateFirstPageSizes(const SnapshotData* startup_snapshot,
-                             const SnapshotData* context_snapshot,
+                             const List<SnapshotData*>* context_snapshots,
                              uint32_t* sizes_out) {
-  Vector<const SerializedData::Reservation> startup_reservations =
-      startup_snapshot->Reservations();
-  Vector<const SerializedData::Reservation> context_reservations =
-      context_snapshot->Reservations();
-  int startup_index = 0;
-  int context_index = 0;
-
   if (FLAG_profile_deserialization) {
     int startup_total = 0;
-    int context_total = 0;
-    for (const auto& reservation : startup_reservations) {
+    PrintF("Deserialization will reserve:\n");
+    for (const auto& reservation : startup_snapshot->Reservations()) {
       startup_total += reservation.chunk_size();
     }
-    for (const auto& reservation : context_reservations) {
-      context_total += reservation.chunk_size();
+    PrintF("%10d bytes per isolate\n", startup_total);
+    for (int i = 0; i < context_snapshots->length(); i++) {
+      int context_total = 0;
+      for (const auto& reservation : context_snapshots->at(i)->Reservations()) {
+        context_total += reservation.chunk_size();
+      }
+      PrintF("%10d bytes per context #%d\n", context_total, i);
     }
-    PrintF(
-        "Deserialization will reserve:\n"
-        "%10d bytes per isolate\n"
-        "%10d bytes per context\n",
-        startup_total, context_total);
   }
 
+  uint32_t startup_requirements[i::Serializer::kNumberOfSpaces];
+  uint32_t context_requirements[i::Serializer::kNumberOfSpaces];
   for (int space = 0; space < i::Serializer::kNumberOfSpaces; space++) {
-    bool single_chunk = true;
-    while (!startup_reservations[startup_index].is_last()) {
-      single_chunk = false;
-      startup_index++;
-    }
-    while (!context_reservations[context_index].is_last()) {
-      single_chunk = false;
-      context_index++;
-    }
+    startup_requirements[space] = 0;
+    context_requirements[space] = 0;
+  }
 
-    uint32_t required = kMaxUInt32;
-    if (single_chunk) {
-      // If both the startup snapshot data and the context snapshot data on
-      // this space fit in a single page, then we consider limiting the size
-      // of the first page. For this, we add the chunk sizes and some extra
-      // allowance. This way we achieve a smaller startup memory footprint.
-      required = (startup_reservations[startup_index].chunk_size() +
-                  2 * context_reservations[context_index].chunk_size()) +
-                 Page::kObjectStartOffset;
-      // Add a small allowance to the code space for small scripts.
-      if (space == CODE_SPACE) required += 32 * KB;
-    }
+  UpdateMaxRequirementPerPage(startup_requirements,
+                              startup_snapshot->Reservations());
+  for (const auto& context_snapshot : *context_snapshots) {
+    UpdateMaxRequirementPerPage(context_requirements,
+                                context_snapshot->Reservations());
+  }
+
+  for (int space = 0; space < i::Serializer::kNumberOfSpaces; space++) {
+    // If the space requirement for a page is less than a page size, we consider
+    // limiting the size of the first page in order to save memory on startup.
+    uint32_t required = startup_requirements[space] +
+                        2 * context_requirements[space] +
+                        Page::kObjectStartOffset;
+    // Add a small allowance to the code space for small scripts.
+    if (space == CODE_SPACE) required += 32 * KB;
 
     if (space >= FIRST_PAGED_SPACE && space <= LAST_PAGED_SPACE) {
       uint32_t max_size =
           MemoryAllocator::PageAreaSize(static_cast<AllocationSpace>(space));
-      sizes_out[space - FIRST_PAGED_SPACE] = Min(required, max_size);
-    } else {
-      DCHECK(single_chunk);
+      sizes_out[space - FIRST_PAGED_SPACE] = std::min(required, max_size);
     }
-    startup_index++;
-    context_index++;
   }
-
-  DCHECK_EQ(startup_reservations.length(), startup_index);
-  DCHECK_EQ(context_reservations.length(), context_index);
 }
 
 v8::StartupData Snapshot::CreateSnapshotBlob(
-    const StartupSerializer* startup_serializer,
-    const PartialSerializer* context_serializer) {
-  SnapshotData startup_snapshot(startup_serializer);
-  SnapshotData context_snapshot(context_serializer);
-  Vector<const byte> startup_data = startup_snapshot.RawData();
-  Vector<const byte> context_data = context_snapshot.RawData();
+    const SnapshotData* startup_snapshot,
+    const List<SnapshotData*>* context_snapshots) {
+  int num_contexts = context_snapshots->length();
+  int startup_snapshot_offset = StartupSnapshotOffset(num_contexts);
+  int total_length = startup_snapshot_offset;
+  total_length += startup_snapshot->RawData().length();
+  for (const auto& context_snapshot : *context_snapshots) {
+    total_length += context_snapshot->RawData().length();
+  }
 
   uint32_t first_page_sizes[kNumPagedSpaces];
-
-  CalculateFirstPageSizes(&startup_snapshot, &context_snapshot,
+  CalculateFirstPageSizes(startup_snapshot, context_snapshots,
                           first_page_sizes);
 
-  int startup_length = startup_data.length();
-  int context_length = context_data.length();
-  int context_offset = ContextOffset(startup_length);
-
-  int length = context_offset + context_length;
-  char* data = new char[length];
-
+  char* data = new char[total_length];
   memcpy(data + kFirstPageSizesOffset, first_page_sizes,
          kNumPagedSpaces * kInt32Size);
-  memcpy(data + kStartupLengthOffset, &startup_length, kInt32Size);
-  memcpy(data + kStartupDataOffset, startup_data.begin(), startup_length);
-  memcpy(data + context_offset, context_data.begin(), context_length);
-  v8::StartupData result = {data, length};
-
+  memcpy(data + kNumberOfContextsOffset, &num_contexts, kInt32Size);
+  int payload_offset = StartupSnapshotOffset(num_contexts);
+  int payload_length = startup_snapshot->RawData().length();
+  memcpy(data + payload_offset, startup_snapshot->RawData().start(),
+         payload_length);
   if (FLAG_profile_deserialization) {
-    PrintF(
-        "Snapshot blob consists of:\n"
-        "%10d bytes for startup\n"
-        "%10d bytes for context\n",
-        startup_length, context_length);
+    PrintF("Snapshot blob consists of:\n%10d bytes for startup\n",
+           payload_length);
   }
+  payload_offset += payload_length;
+  for (int i = 0; i < num_contexts; i++) {
+    memcpy(data + ContextSnapshotOffsetOffset(i), &payload_offset, kInt32Size);
+    SnapshotData* context_snapshot = context_snapshots->at(i);
+    payload_length = context_snapshot->RawData().length();
+    memcpy(data + payload_offset, context_snapshot->RawData().start(),
+           payload_length);
+    if (FLAG_profile_deserialization) {
+      PrintF("%10d bytes for context #%d\n", payload_length, i);
+    }
+    payload_offset += payload_length;
+  }
+
+  v8::StartupData result = {data, total_length};
   return result;
 }
 
+int Snapshot::ExtractNumContexts(const v8::StartupData* data) {
+  CHECK_LT(kNumberOfContextsOffset, data->raw_size);
+  int num_contexts;
+  memcpy(&num_contexts, data->data + kNumberOfContextsOffset, kInt32Size);
+  return num_contexts;
+}
+
 Vector<const byte> Snapshot::ExtractStartupData(const v8::StartupData* data) {
-  DCHECK_LT(kIntSize, data->raw_size);
-  int startup_length;
-  memcpy(&startup_length, data->data + kStartupLengthOffset, kInt32Size);
-  DCHECK_LT(startup_length, data->raw_size);
+  int num_contexts = ExtractNumContexts(data);
+  int startup_offset = StartupSnapshotOffset(num_contexts);
+  CHECK_LT(startup_offset, data->raw_size);
+  int first_context_offset;
+  memcpy(&first_context_offset, data->data + ContextSnapshotOffsetOffset(0),
+         kInt32Size);
+  CHECK_LT(first_context_offset, data->raw_size);
+  int startup_length = first_context_offset - startup_offset;
   const byte* startup_data =
-      reinterpret_cast<const byte*>(data->data + kStartupDataOffset);
+      reinterpret_cast<const byte*>(data->data + startup_offset);
   return Vector<const byte>(startup_data, startup_length);
 }
 
+Vector<const byte> Snapshot::ExtractContextData(const v8::StartupData* data,
+                                                int index) {
+  int num_contexts = ExtractNumContexts(data);
+  CHECK_LT(index, num_contexts);
+
+  int context_offset;
+  memcpy(&context_offset, data->data + ContextSnapshotOffsetOffset(index),
+         kInt32Size);
+  int next_context_offset;
+  if (index == num_contexts - 1) {
+    next_context_offset = data->raw_size;
+  } else {
+    memcpy(&next_context_offset,
+           data->data + ContextSnapshotOffsetOffset(index + 1), kInt32Size);
+    CHECK_LT(next_context_offset, data->raw_size);
+  }
 
-Vector<const byte> Snapshot::ExtractContextData(const v8::StartupData* data) {
-  DCHECK_LT(kIntSize, data->raw_size);
-  int startup_length;
-  memcpy(&startup_length, data->data + kStartupLengthOffset, kIntSize);
-  int context_offset = ContextOffset(startup_length);
   const byte* context_data =
       reinterpret_cast<const byte*>(data->data + context_offset);
-  DCHECK_LT(context_offset, data->raw_size);
-  int context_length = data->raw_size - context_offset;
+  int context_length = next_context_offset - context_offset;
   return Vector<const byte>(context_data, context_length);
 }