Revert "[compiler] Make BigInts bg-serialized"

This reverts commit 317462be.

Reason for revert: Landed prematurely.

Original change's description:
> [compiler] Make BigInts bg-serialized
>
> BigInts are immutable after initialization, thus an acquire-release
> synchronization point is sufficient to read safely from the background
> thread. This CL introduces the `length` field as that sync point.
>
> Bug: v8:7790
> Change-Id: I977f30836b311c3851896dd29e708f78a090f547
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2854745
> Auto-Submit: Jakob Gruber <jgruber@chromium.org>
> Reviewed-by: Michael Stanton <mvstanton@chromium.org>
> Commit-Queue: Michael Stanton <mvstanton@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#74245}

Tbr: mvstanton@chromium.org
Bug: v8:7790
Change-Id: I0fab1a8e161c75267aead3735708e26620686fa6
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2854749
Commit-Queue: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74247}

src/compiler/heap-refs.cc

@@ -1098,17 +1098,14 @@ class AllocationSiteData : public HeapObjectData {
 class BigIntData : public HeapObjectData {
  public:
   BigIntData(JSHeapBroker* broker, ObjectData** storage, Handle<BigInt> object,
-             ObjectDataKind kind)
-      : HeapObjectData(broker, storage, object, kind) {
-    // Read length first as an artificial synchronization point.
-    object->synchronized_length();
-    as_uint64_ = object->AsUint64(nullptr);
-  }
+             ObjectDataKind kind = ObjectDataKind::kSerializedHeapObject)
+      : HeapObjectData(broker, storage, object, kind),
+        as_uint64_(object->AsUint64(nullptr)) {}
 
   uint64_t AsUint64() const { return as_uint64_; }
 
  private:
-  uint64_t as_uint64_;
+  const uint64_t as_uint64_;
 };
 
 // Only used in JSNativeContextSpecialization.

src/compiler/heap-refs.h

@@ -110,7 +110,7 @@ enum class RefSerializationKind {
   V(AccessorInfo, RefSerializationKind::kNeverSerialized)                 \
   V(AllocationSite, RefSerializationKind::kSerialized)                    \
   V(ArrayBoilerplateDescription, RefSerializationKind::kNeverSerialized)  \
-  V(BigInt, RefSerializationKind::kBackgroundSerialized)                  \
+  V(BigInt, RefSerializationKind::kPossiblyBackgroundSerialized)          \
   V(CallHandlerInfo, RefSerializationKind::kNeverSerialized)              \
   V(Cell, RefSerializationKind::kNeverSerialized)                         \
   V(Code, RefSerializationKind::kNeverSerialized)                         \

src/objects/bigint.cc

@@ -418,6 +418,7 @@ void MutableBigInt::Canonicalize(MutableBigInt result) {
       // of the object changed significantly.
       heap->CreateFillerObjectAt(new_end, size_delta, ClearRecordedSlots::kNo);
     }
+    result.synchronized_set_length(new_length);
 
     // Canonicalize -0n.
     if (new_length == 0) {
@@ -425,11 +426,6 @@ void MutableBigInt::Canonicalize(MutableBigInt result) {
       // TODO(jkummerow): If we cache a canonical 0n, return that here.
     }
   }
-  // This is the synchronization point which safely allows concurrent reads of
-  // BigInt instances. It *must* be reached on every path that results in a
-  // published BigInt. No writes to the BigInt instance are allowed after this
-  // point.
-  result.synchronized_set_length(new_length);
   DCHECK_IMPLIES(result.length() > 0,
                  result.digit(result.length() - 1) != 0);  // MSD is non-zero.
 }

src/objects/bigint.h

@@ -32,17 +32,12 @@ class ValueSerializer;
 // Most code should be using BigInts instead.
 class BigIntBase : public PrimitiveHeapObject {
  public:
-  // The `length` field (or rather its underlying storage location) is the
-  // synchronization point that allows concurrent interactions with BigInt
-  // instances. The invariants are:
-  // - Instances are immutable after initialization, see MakeImmutable.
-  // - The last write to an instance is a release-store of `length`.
   inline int length() const {
     int32_t bitfield = RELAXED_READ_INT32_FIELD(*this, kBitfieldOffset);
     return LengthBits::decode(static_cast<uint32_t>(bitfield));
   }
 
-  // For use by the GC and Turbofan.
+  // For use by the GC.
   inline int synchronized_length() const {
     int32_t bitfield = ACQUIRE_READ_INT32_FIELD(*this, kBitfieldOffset);
     return LengthBits::decode(static_cast<uint32_t>(bitfield));