[objects] initialize SmallOrderedHashTable padding

This fixes the test SmallOrderedHashMapAllocate in
test-code-stub-assembler.cc, which uses memcmp to check
SmallOrderedHashMap objects for equality:
https://ci.chromium.org/p/v8/builders/ci/V8%20Linux%20-%20nosnap%20-%20debug/24470


Change-Id: I1c1ad2e8923322c856191fac30f109a4e671b98e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1667417
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62350}

src/codegen/code-stub-assembler.cc

@@ -3647,6 +3647,17 @@ TNode<CollectionType> CodeStubAssembler::AllocateSmallOrderedHashTable(
   StoreMapNoWriteBarrier(table_obj, small_ordered_hash_map);
   TNode<CollectionType> table = UncheckedCast<CollectionType>(table_obj);
 
+  {
+    // This store overlaps with the header fields stored below.
+    // Since it happens first, it effectively still just zero-initializes the
+    // padding.
+    constexpr int offset =
+        RoundDown<kTaggedSize>(CollectionType::PaddingOffset());
+    STATIC_ASSERT(offset + kTaggedSize == CollectionType::PaddingOffset() +
+                                              CollectionType::PaddingSize());
+    StoreObjectFieldNoWriteBarrier(table, offset, SmiConstant(0));
+  }
+
   // Initialize the SmallOrderedHashTable fields.
   StoreObjectByteNoWriteBarrier(
       table, CollectionType::NumberOfBucketsOffset(),

src/objects/ordered-hash-table.cc

@@ -508,6 +508,8 @@ void SmallOrderedHashTable<Derived>::Initialize(Isolate* isolate,
   SetNumberOfBuckets(num_buckets);
   SetNumberOfElements(0);
   SetNumberOfDeletedElements(0);
+  memset(reinterpret_cast<void*>(field_address(PaddingOffset())), 0,
+         PaddingSize());
 
   Address hashtable_start = GetHashTableStartAddress(capacity);
   memset(reinterpret_cast<byte*>(hashtable_start), kNotFound,

src/objects/ordered-hash-table.h

@@ -527,8 +527,16 @@ class SmallOrderedHashTable : public HeapObject {
     return NumberOfDeletedElementsOffset() + kOneByteSize;
   }
 
+  static constexpr Offset PaddingOffset() {
+    return NumberOfBucketsOffset() + kOneByteSize;
+  }
+
+  static constexpr size_t PaddingSize() {
+    return RoundUp<kTaggedSize>(PaddingOffset()) - PaddingOffset();
+  }
+
   static constexpr Offset DataTableStartOffset() {
-    return RoundUp<kTaggedSize>(NumberOfBucketsOffset());
+    return PaddingOffset() + PaddingSize();
   }
 
   static constexpr int DataTableSizeFor(int capacity) {