demo · v141
Object Size
Without custom descriptors, every Wasm object that wants to surface type info to JS has to carry a pointer to a separate type-metadata object. With custom descriptors, the engine attaches that information once at the type level — every instance shrinks. For runtimes that allocate millions of small objects (Kotlin/Wasm, Dart/Wasm, AssemblyScript) the memory savings add up fast.
where the bytes go
// Pre-descriptors object layout (32-bit, 4-byte words)
struct Player {
void* type_metadata_ptr; // 4 bytes — explicit pointer to type info
string name; // 4 bytes (ref)
i32 hp; // 4 bytes
i32 score; // 4 bytes
} // total: 16 bytes per Player
// With custom descriptors
struct Player {
// type metadata attached at TYPE level, shared across all instances
string name; // 4 bytes
i32 hp; // 4 bytes
i32 score; // 4 bytes
} // total: 12 bytes per Player (25% saving on this shape)why this angle
The chromestatus motivation calls out object overhead as one of the two big wins. The slider above lets a reader plug in their own scale — a runtime allocating 1M small structs at 16 bytes vs 12 bytes is 4MB saved, real money on mobile devices. Compilers that target Wasm GC (Kotlin/Wasm, Dart, OCaml, Scala.js eventually) all benefit transparently — the source language doesn't change, the emitted Wasm just gets denser.