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Comment by miloignis | Hacker News Reader

I haven't messed with .stack, but on first impression I agree with you that it seems like a mistake.

Even if it wasn't observable though, I think guaranteed tail-call elimination via either a new opcode or having it be a required property of regular calls in the spec (we already missed that ship, of course) is important. Without it, proper compilation and execution of some languages on wasm would depend on an otherwise invisible property of the engine - that is, tail call elimination isn't just an optimization, it's a critical aspect of language semantics that needs to be guaranteed to be relied on.

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If it is a mistake depends on the point of view. What seems to be more important: exceptions of tail-call elimination? Most modern languages use exceptions in one way or another, conversely very few use the later feature. From the point of view of existing software it is way more important to optimize VMs for exception handling than for tail call elimination.

Well wasm doesn't have exceptions right now either! The exceptions proposal is also in phase 3, like tail-calls. Right now wasm just supports traps, which can't be caught or inspected from within wasm code, and don't actually need to record stack frames (but the JavaScript host does in web browsers). I'm not sure what benefit exposing the wasm part of the stack for traps gives the JavaScript host side except perhaps easing debugging? (though that is important, I think there are other valid solutions for that)

To be clear, I do want wasm to support the major language use cases, and I think implementing exception support is a good (though tricky, see the exceptions proposal) idea.

After reading the discussion, I think I'm much more in favour of adding tail calls, than adding exceptions.

Actually I wonder if one couldn't adapt the tail-call mechanism slightly to also serve as an exception mechanism. I think you could do both by allowing `br` to jump to an arbitrary label previously established. An exception then simply being a "tail-call" that unwinds more than one stack frame, and calls into the "catch".

Not sure how well JITable this would be, but I think an arbitrary backjump and call should be mostly fine in terms of control flow?

Would be interested in what other solutions you envision. In production error reporting to services like Sentry relies on getting access to a stack and unlike a native runtime there is nothing one could do to create a stack without the engine’s support.

If you control the compiler, you don't need the engine's support for exceptions (though you might want it for performance reasons) - emitting function prologues and epilogues that maintain debugging information like a call stack (or call trace in a ring buffer like some Schemes) would be one way.

I actually did this in my purely functional language, but had it off by default for performance reasons - if you hit an error at runtime, the debugging code would re-execute from the last checkpoint with debugging information turned on to construct the missing debug data, something I could only do because the language is side-effect free.

Well WTF is this doing https://docs.rs/wasm-bindgen/0.2.11/wasm_bindgen/fn.throw.ht... then ?

Calling into an external JavaScript function, which will then throw a JavaScript exception (as the linked documentation states).

Wasm cannot throw the exception itself, nor can it catch the exception. How exceptions are handled when raised in code called via FFI in another language are a property of the embedding and wasm engine, not of wasm itself, which (as of yet, barring that exceptions proposal) has no concept of them besides unrecoverable traps.

Great thank you for the info

Maybe I'm naive and I surely don't know anything about the WASM spec, but aren't CPUs implementing both exceptions and tail call elimination as gotos (some jump machine code instruction) ? Exceptions might have to pop some frame, TCE doesn't.

To avoid some classes of security issue, and also because it started much closer to asm.js in features, WASM uses structured programming control flow like blocks, if..then, and break statements. Functions can only be called by the call instruction right now, so this proposal is needed to jump to an function body.

Are you saying there is no goto in wasm ? Genuine question I don't have any knoweldge of WASM.

Yes, that is correct.

It's more important for exception handling to be implementable than for TCE to be implementable, yes. But it's more important for TCE to be provided by the virtual machine (or, alternatively, for call and return to be manipulations of user-level state, as on traditional CPUs, rather than magic privileged operations as in Wasm) because you can implement exception handling with TCE but you can't implement TCE with exception handling.

Plenty of C/C++ code de facto assumes it’s present even though it’s not mandated. Anybody trying a list algo in recursive style is relying on it implicitly.

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