LibJS: Expose minimum possible cell size of JS::Heap

Use this to avoid creating a 16 byte cell allocator on x86_64, where the
size of FreelistEntry is 24 bytes. Every JS::Cell must be at least the
size of the FreelistEntry or things start crashing, so the 16 byte
allocator was wasted on that platform.

2 files changed, 7 insertions, 1 deletions

diff --git a/Userland/Libraries/LibJS/Heap/Heap.cpp b/Userland/Libraries/LibJS/Heap/Heap.cpp
index 11042ea5e9..0f9501ee44 100644
--- a/Userland/Libraries/LibJS/Heap/Heap.cpp
+++ b/Userland/Libraries/LibJS/Heap/Heap.cpp
@@ -23,7 +23,10 @@ namespace JS {
 Heap::Heap(VM& vm)
     : m_vm(vm)
 {
-    m_allocators.append(make<CellAllocator>(16));
+    if constexpr (HeapBlock::min_possible_cell_size <= 16) {
+        m_allocators.append(make<CellAllocator>(16));
+    }
+    static_assert(HeapBlock::min_possible_cell_size <= 24, "Heap Cell tracking uses too much data!");
     m_allocators.append(make<CellAllocator>(32));
     m_allocators.append(make<CellAllocator>(64));
     m_allocators.append(make<CellAllocator>(128));
diff --git a/Userland/Libraries/LibJS/Heap/HeapBlock.h b/Userland/Libraries/LibJS/Heap/HeapBlock.h
index 85a6307ae3..6b2c1eff12 100644
--- a/Userland/Libraries/LibJS/Heap/HeapBlock.h
+++ b/Userland/Libraries/LibJS/Heap/HeapBlock.h
@@ -101,6 +101,9 @@ private:
     size_t m_next_lazy_freelist_index { 0 };
     FreelistEntry* m_freelist { nullptr };
     alignas(Cell) u8 m_storage[];
+
+public:
+    static constexpr size_t min_possible_cell_size = sizeof(FreelistEntry);
 };
 
 }