[snapshot] Clean up builtin serializer allocater

Recent changes in SerializerReferences (see https://crrev.com/c/1113930)
opened up these minor improvements to the builtin serializer allocator,
since we now have dedicated BuiltinReferences.

R=yangguo@chromium.org

Bug: v8:7887
Change-Id: Ie76a52a88ef4075c63c5eb2eaa81382c52325688
Reviewed-on: https://chromium-review.googlesource.com/1125844Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54207}

src/snapshot/builtin-serializer-allocator.cc

@@ -18,10 +18,11 @@ SerializerReference BuiltinSerializerAllocator::Allocate(AllocationSpace space,
   // system.  Instead of worrying about chunk indices and offsets, we simply
   // need to generate unique offsets here.
 
-  const auto ref = SerializerReference::BuiltinReference(virtual_chunk_offset_);
+  const auto ref =
+      SerializerReference::BuiltinReference(next_builtin_reference_index_);
 
-  virtual_chunk_size_ += size;
-  virtual_chunk_offset_ += kObjectAlignment;  // Needs to be aligned.
+  allocated_bytes_ += size;
+  next_builtin_reference_index_++;
 
   return ref;
 }
@@ -30,7 +31,7 @@ SerializerReference BuiltinSerializerAllocator::Allocate(AllocationSpace space,
 bool BuiltinSerializerAllocator::BackReferenceIsAlreadyAllocated(
     SerializerReference reference) const {
   DCHECK(reference.is_builtin_reference());
-  return reference.builtin_offset() < virtual_chunk_offset_;
+  return reference.builtin_index() < next_builtin_reference_index_;
 }
 #endif  // DEBUG
 
@@ -50,7 +51,7 @@ void BuiltinSerializerAllocator::OutputStatistics() {
   PrintF("\n");
 
   for (int space = FIRST_SPACE; space < kNumberOfSpaces; space++) {
-    uint32_t space_size = (space == CODE_SPACE) ? virtual_chunk_size_ : 0;
+    uint32_t space_size = (space == CODE_SPACE) ? allocated_bytes_ : 0;
     PrintF("%16d", space_size);
   }
   PrintF("\n");

src/snapshot/builtin-serializer-allocator.h

@@ -38,10 +38,8 @@ class BuiltinSerializerAllocator final {
   static constexpr int kNumberOfSpaces =
       SerializerDeserializer::kNumberOfSpaces;
 
-  // We need to track a faked offset to create back-references. The size is
-  // kept simply to display statistics.
-  uint32_t virtual_chunk_size_ = 0;
-  uint32_t virtual_chunk_offset_ = 0;
+  uint32_t allocated_bytes_ = 0;
+  uint32_t next_builtin_reference_index_ = 0;
 
   DISALLOW_COPY_AND_ASSIGN(BuiltinSerializerAllocator)
 };

src/snapshot/references.h

@@ -131,7 +131,7 @@ class SerializerReference {
            SpecialValueTypeBits::decode(bitfield_) == kBuiltinReference;
   }
 
-  uint32_t builtin_offset() const {
+  uint32_t builtin_index() const {
     DCHECK(is_builtin_reference());
     return value_;
   }