[compiler] Make BigInts bg-serialized

BigInts are immutable after initialization, thus safe to access
concurrently if the Heap::IsPendingAllocation predicate returns false.

Bug: v8:7790
Change-Id: Icf3954b1e50af2132846eca5304c7947afb8f298
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2856535
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Reviewed-by: Michael Stanton <mvstanton@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74322}

src/compiler/heap-refs.cc

@@ -1084,7 +1084,7 @@ class AllocationSiteData : public HeapObjectData {
 class BigIntData : public HeapObjectData {
  public:
   BigIntData(JSHeapBroker* broker, ObjectData** storage, Handle<BigInt> object,
-             ObjectDataKind kind = ObjectDataKind::kSerializedHeapObject)
+             ObjectDataKind kind)
       : HeapObjectData(broker, storage, object, kind),
         as_uint64_(object->AsUint64(nullptr)) {}
 

src/compiler/heap-refs.h

@@ -108,7 +108,7 @@ enum class RefSerializationKind {
   V(AccessorInfo, RefSerializationKind::kNeverSerialized)                 \
   V(AllocationSite, RefSerializationKind::kSerialized)                    \
   V(ArrayBoilerplateDescription, RefSerializationKind::kNeverSerialized)  \
-  V(BigInt, RefSerializationKind::kPossiblyBackgroundSerialized)          \
+  V(BigInt, RefSerializationKind::kBackgroundSerialized)                  \
   V(CallHandlerInfo, RefSerializationKind::kNeverSerialized)              \
   V(Cell, RefSerializationKind::kNeverSerialized)                         \
   V(Code, RefSerializationKind::kNeverSerialized)                         \

src/compiler/representation-change.cc

@@ -1104,7 +1104,7 @@ Node* RepresentationChanger::GetWord64RepresentationFor(
       HeapObjectMatcher m(node);
       if (m.HasResolvedValue() && m.Ref(broker_).IsBigInt() &&
           use_info.truncation().IsUsedAsWord64()) {
-        auto bigint = m.Ref(broker_).AsBigInt();
+        BigIntRef bigint = m.Ref(broker_).AsBigInt();
         return jsgraph()->Int64Constant(
             static_cast<int64_t>(bigint.AsUint64()));
       }