> It doesn't make sense to me that an embedded VM/interpreter could ever outperform direct code. You're adding a layer of abstraction and indirection, so how is it possible that a more indirect solution can have better performance?

It is funny, but (like I’ve already mentioned[1] a few months ago) for serialization(-adjacent) formats in particular the preferential position of bytecode interpreters is apparently rediscovered again and again.

The earliest example I know about is Microsoft’s MIDL, which started off generating C code for NDR un/marshalling but very soon (ca. 1995) switched to bytecode (which Microsoft for some reason called “format strings”; these days there’s also typelib marshalling and WinRT metadata-driven marshalling, the latter completely undocumented, but both data-driven). Bellard’s nonfree ffasn1 also (seemingly) uses bytecode, unlike the main FOSS implementation. Protocol Buffers started off with codegen (burying Google user in de/serialization code) but UPB uses “table-driven”, i.e. bytecode, parsing[2].

The most interesting chapter in this long history is in my opinion Swift’s bytecode-based value witnesses[3,4]. Swift (uniquely) has support for ABI compatibility with polymorphic value types, so e.g. you can have a field in the middle of your struct whose size and alignment only become known at dynamic linking time. It does this in pretty much the way you expect[5]: each type has a vtable (“value witness”) full of compiler-generated methods like size, alignment, copy, move, etc., which for polymorphic type instances will can call the argument’s witness methods and compute on the results. Anyways, here too the story is that they started with native codegen, got buried under the generated code, and switched to bytecode instead. (I wonder–are they going to PGO and JIT next, like hyperpb[6] for Protobuf?)

[1] https://news.ycombinator.com/item?id=44665671, I’m too lazy to copy over the links so refer there for the missing references.

[2] https://news.ycombinator.com/item?id=44664592 and parent’s second link.

[3] https://forums.swift.org/t/sr-14273-byte-code-based-value-wi...

[4] Rexin, “Compact value witnesses in Swift”, 2023 LLVM Dev. Mtg., https://www.youtube.com/watch?v=hjgDwdGJIhI

[5] Pestov, McCall, “Implementing Swift generics”, 2017 LLVM Dev. Mtg., https://www.youtube.com/watch?v=ctS8FzqcRug

[6] https://mcyoung.xyz/2025/07/16/hyperpb/

A tail-call interpreter in (nightly) Rust