[heap] Store FreeSpace size in multiples of tagged words

LeszekSwirski · V8 LUCI CQ · commit 146962dda8d2 · 2025-05-30T06:21:09.000-07:00
Since FreeSpace has to be aligned to Tagged words, we can support larger free spaces by storing the size in words rather than bytes. Bug: 417413670 Change-Id: I19ef4921e00a5ec23d39ff4aa5b379b36fc86e0a Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/6596680 Commit-Queue: Leszek Swirski <leszeks@chromium.org> Reviewed-by: Michael Lippautz <mlippautz@chromium.org> Auto-Submit: Leszek Swirski <leszeks@chromium.org> Cr-Commit-Position: refs/heads/main@{#100590}
diff --git a/src/compiler/access-builder.cc b/src/compiler/access-builder.cc
@@ -896,15 +896,6 @@ FieldAccess AccessBuilder::ForNameRawHashField() {
   return access;
 }
 
-// static
-FieldAccess AccessBuilder::ForFreeSpaceSize() {
-  FieldAccess access = {kTaggedBase,         offsetof(FreeSpace, size_),
-                        MaybeHandle<Name>(), OptionalMapRef(),
-                        Type::SignedSmall(), MachineType::TaggedSigned(),
-                        kNoWriteBarrier};
-  return access;
-}
-
 // static
 FieldAccess AccessBuilder::ForStringLength() {
   FieldAccess access = {kTaggedBase,
diff --git a/src/compiler/access-builder.h b/src/compiler/access-builder.h
@@ -267,9 +267,6 @@ class V8_EXPORT_PRIVATE AccessBuilder final
   // Provides access to Name::raw_hash_field() field.
   static FieldAccess ForNameRawHashField();
 
-  // Provides access to FreeSpace::size() field
-  static FieldAccess ForFreeSpaceSize();
-
   // Provides access to String::length() field.
   static FieldAccess ForStringLength();
 
diff --git a/src/diagnostics/objects-debug.cc b/src/diagnostics/objects-debug.cc
@@ -388,8 +388,8 @@ void HeapObject::VerifyCodePointer(Isolate* isolate, Tagged<Object> p) {
 void FreeSpace::FreeSpaceVerify(Isolate* isolate) {
   CHECK(IsFreeSpace(this));
   {
-    Tagged<Object> size = size_.Relaxed_Load();
-    CHECK(IsSmi(size));
+    Tagged<Object> size_in_tagged = size_in_tagged_.Relaxed_Load();
+    CHECK(IsSmi(size_in_tagged));
   }
 }
 
diff --git a/src/objects/free-space-inl.h b/src/objects/free-space-inl.h
@@ -20,14 +20,18 @@ namespace v8 {
 namespace internal {
 
 int FreeSpace::size(RelaxedLoadTag) const {
-  return size_.Relaxed_Load().value();
+  return size_in_tagged_.Relaxed_Load().value() * kTaggedSize;
 }
 
 // static
 inline void FreeSpace::SetSize(const WritableFreeSpace& writable_free_space,
                                int size, RelaxedStoreTag tag) {
-  writable_free_space.WriteHeaderSlot<Smi, offsetof(FreeSpace, size_)>(
-      Smi::FromInt(size), tag);
+  // For size <= 2 * kTaggedSize, we expect to use one/two pointer filler maps.
+  DCHECK_GT(size, 2 * kTaggedSize);
+  DCHECK_EQ(size % kTaggedSize, 0);
+  writable_free_space
+      .WriteHeaderSlot<Smi, offsetof(FreeSpace, size_in_tagged_)>(
+          Smi::FromInt(size / kTaggedSize), tag);
 }
 
 int FreeSpace::Size() { return size(kRelaxedLoad); }
diff --git a/src/objects/free-space.h b/src/objects/free-space.h
@@ -51,11 +51,10 @@ class FreeSpace : public HeapObjectLayout {
 
  private:
   friend class Heap;
-  friend class compiler::AccessBuilder;
 
   inline bool IsValid() const;
 
-  TaggedMember<Smi> size_;
+  TaggedMember<Smi> size_in_tagged_;
 #ifdef V8_EXTERNAL_CODE_SPACE
   TaggedMember<Smi> next_;
 #else

Original file line number	Diff line number	Diff line change
`@@ -388,8 +388,8 @@ void HeapObject::VerifyCodePointer(Isolate* isolate, Tagged<Object> p) {`
`388`	`388`	`void FreeSpace::FreeSpaceVerify(Isolate* isolate) {`
`389`	`389`	`CHECK(IsFreeSpace(this));`
`390`	`390`	`{`
`391`		`- Tagged<Object> size = size_.Relaxed_Load();`
`392`		`- CHECK(IsSmi(size));`
	`391`	`+ Tagged<Object> size_in_tagged = size_in_tagged_.Relaxed_Load();`
	`392`	`+ CHECK(IsSmi(size_in_tagged));`
`393`	`393`	`}`
`394`	`394`	`}`
`395`	`395`