Revert "[cleanup] Cleanup handling of padding field in Map"

This reverts commit 6f763280.

Reason for revert: speculative revert for MSAN failure:
https://ci.chromium.org/p/v8/builders/luci.v8.ci/V8%20Linux%20-%20arm64%20-%20sim%20-%20MSAN/25398

Original change's description:
> [cleanup] Cleanup handling of padding field in Map
> 
> Bug: v8:8834
> Change-Id: Iae9cbcccb8747298c5680ac1e865efebee85acec
> Reviewed-on: https://chromium-review.googlesource.com/c/1480385
> Reviewed-by: Toon Verwaest <verwaest@chromium.org>
> Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
> Commit-Queue: Igor Sheludko <ishell@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#59774}

TBR=ulan@chromium.org,ishell@chromium.org,verwaest@chromium.org

Change-Id: Icf42e4566382c8539c8d7fb709a91bf0c6353858
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: v8:8834
Reviewed-on: https://chromium-review.googlesource.com/c/1481025Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Commit-Queue: Sathya Gunasekaran <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59776}

src/heap/factory.cc

@@ -2040,7 +2040,6 @@ Map Factory::InitializeMap(Map map, InstanceType type, int instance_size,
                    Map::OwnsDescriptorsBit::encode(true) |
                    Map::ConstructionCounterBits::encode(Map::kNoSlackTracking);
   map->set_bit_field3(bit_field3);
-  map->clear_padding();
   map->set_elements_kind(elements_kind);
   map->set_new_target_is_base(true);
   isolate()->counters()->maps_created()->Increment();

src/objects/map-inl.h

@@ -618,6 +618,9 @@ void Map::InitializeDescriptors(Isolate* isolate, DescriptorArray descriptors,
 }
 
 void Map::set_bit_field3(uint32_t bits) {
+  if (kInt32Size != kTaggedSize) {
+    RELAXED_WRITE_UINT32_FIELD(*this, kBitField3Offset + kInt32Size, 0);
+  }
   RELAXED_WRITE_UINT32_FIELD(*this, kBitField3Offset, bits);
 }
 
@@ -625,13 +628,6 @@ uint32_t Map::bit_field3() const {
   return RELAXED_READ_UINT32_FIELD(*this, kBitField3Offset);
 }
 
-void Map::clear_padding() {
-  if (FIELD_SIZE(kOptionalPaddingOffset) == 0) return;
-  DCHECK_EQ(4, FIELD_SIZE(kOptionalPaddingOffset));
-  memset(reinterpret_cast<void*>(address() + kOptionalPaddingOffset), 0,
-         FIELD_SIZE(kOptionalPaddingOffset));
-}
-
 LayoutDescriptor Map::GetLayoutDescriptor() const {
   return FLAG_unbox_double_fields ? layout_descriptor()
                                   : LayoutDescriptor::FastPointerLayout();

src/objects/map.h

@@ -275,10 +275,6 @@ class Map : public HeapObject {
   //
   DECL_PRIMITIVE_ACCESSORS(bit_field3, uint32_t)
 
-  // Clear uninitialized padding space. This ensures that the snapshot content
-  // is deterministic. Depending on the V8 build mode there could be no padding.
-  V8_INLINE void clear_padding();
-
 // Bit positions for |bit_field3|.
 #define MAP_BIT_FIELD3_FIELDS(V, _)                               \
   V(EnumLengthBits, int, kDescriptorIndexBitCount, _)             \
@@ -837,8 +833,8 @@ class Map : public HeapObject {
   V(kBitFieldOffset, kUInt8Size)                                          \
   V(kBitField2Offset, kUInt8Size)                                         \
   V(kBitField3Offset, kUInt32Size)                                        \
-  /* Adds padding to make tagged fields kTaggedSize-aligned. */             \
-  V(kOptionalPaddingOffset, OBJECT_POINTER_PADDING(kOptionalPaddingOffset)) \
+  V(k64BitArchPaddingOffset,                                              \
+    kSystemPointerSize == kUInt32Size ? 0 : kUInt32Size)                  \
   /* Pointer fields. */                                                   \
   V(kPointerFieldsBeginOffset, 0)                                         \
   V(kPrototypeOffset, kTaggedSize)                                        \