[heap] Fix disabling of map space with --no-use-map-space flag

HeapAllocator didn't fall back to old space allocation when the
heap had no map space.

Bug: v8:12578, chromium:1313119
Change-Id: Ic02334f42f9fb80a8a9dcf99a94a7ac16da24053
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3570423Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79776}

src/heap/heap-allocator-inl.h

@@ -30,9 +30,7 @@ OldLargeObjectSpace* HeapAllocator::lo_space() const {
   return static_cast<OldLargeObjectSpace*>(spaces_[LO_SPACE]);
 }
 
-PagedSpace* HeapAllocator::map_space() const {
-  return static_cast<PagedSpace*>(spaces_[MAP_SPACE]);
-}
+PagedSpace* HeapAllocator::space_for_maps() const { return space_for_maps_; }
 
 NewSpace* HeapAllocator::new_space() const {
   return static_cast<NewSpace*>(spaces_[NEW_SPACE]);
@@ -111,7 +109,7 @@ V8_WARN_UNUSED_RESULT V8_INLINE AllocationResult HeapAllocator::AllocateRaw(
           break;
         case AllocationType::kMap:
           DCHECK_EQ(alignment, AllocationAlignment::kTaggedAligned);
-          allocation = map_space()->AllocateRawUnaligned(size_in_bytes);
+          allocation = space_for_maps()->AllocateRawUnaligned(size_in_bytes);
           break;
         case AllocationType::kReadOnly:
           DCHECK(read_only_space()->writable());

src/heap/heap-allocator.cc

@@ -22,8 +22,15 @@ void HeapAllocator::Setup() {
   for (int i = FIRST_SPACE; i <= LAST_SPACE; ++i) {
     spaces_[i] = heap_->space(i);
   }
+
+  space_for_maps_ = spaces_[MAP_SPACE]
+                        ? static_cast<PagedSpace*>(spaces_[MAP_SPACE])
+                        : static_cast<PagedSpace*>(spaces_[OLD_SPACE]);
+
   shared_old_allocator_ = heap_->shared_old_allocator_.get();
-  shared_map_allocator_ = heap_->shared_map_allocator_.get();
+  shared_map_allocator_ = heap_->shared_map_allocator_
+                              ? heap_->shared_map_allocator_.get()
+                              : shared_old_allocator_;
 }
 
 void HeapAllocator::SetReadOnlySpace(ReadOnlySpace* read_only_space) {

src/heap/heap-allocator.h

@@ -75,7 +75,7 @@ class V8_EXPORT_PRIVATE HeapAllocator final {
  private:
   V8_INLINE PagedSpace* code_space() const;
   V8_INLINE CodeLargeObjectSpace* code_lo_space() const;
-  V8_INLINE PagedSpace* map_space() const;
+  V8_INLINE PagedSpace* space_for_maps() const;
   V8_INLINE NewSpace* new_space() const;
   V8_INLINE NewLargeObjectSpace* new_lo_space() const;
   V8_INLINE OldLargeObjectSpace* lo_space() const;
@@ -100,6 +100,7 @@ class V8_EXPORT_PRIVATE HeapAllocator final {
 
   Heap* const heap_;
   Space* spaces_[LAST_SPACE + 1];
+  PagedSpace* space_for_maps_;
   ReadOnlySpace* read_only_space_;
 
   ConcurrentAllocator* shared_old_allocator_;

test/mjsunit/shared-memory/shared-struct-without-map-space.js

+// Copyright 2022 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+//
+// Flags: --no-use-map-space --harmony-struct
+
+"use strict";
+
+// Test ensures that deserialization works without map space and
+// that we can allocate maps in the shared heap.
+
+let SomeStruct = new SharedStructType(['field1', 'field2']);