Implement read/write of DW_OP_WASM_location

[philipc]

Does not support evaluation though.

Sample dwarfdump output:

DW_AT_frame_base            len 0x0007: ed03000000009f: DW_OP_WASM_location 0x3 0x0 DW_OP_stack_value

Fixes #544

[fitzgen]

Thanks @philipc! Code itself looks good to me.

However, I have a question: is the intention never to support evaluating these Wasm operations?

If so, I disagree with that decision. I think this support will be necessary to implement for Wasm hosts that want to expose sandboxed, user-level debugging of Wasm code without also exposing the native host code's machine state. That is, in situations where the "Wasm DWARF" isn't translated to "native DWARF" alongside the Wasm's translation into native code, for example in web browsers.

If the intention is to support evaluating these Wasm ops eventually, then it may make sense to just implement that support now (I can do it in a follow up PR, unless you want to do it) because the UnsupportedEvaluation error will become unused/go away as soon as this support is implemented, and it would be better not to have multiple breaking changes to Error in back-to-back releases.

[fitzgen]

Will this always be DW_OP_WASM_location 0x01, or can it also represent DW_OP_WASM_location 0x03? Fine if the former, just curious.

[philipc]

Currently always 0x01. I don't know the history on why 0x03 was added. My guess is that it is for cases where you don't know the index yet, so you need to be sure there is enough space to write the index later. If so, that will need to use a different variant that somehow records the location of the index so that you know where to write it later.

[philipc]

However, I have a question: is the intention never to support evaluating these Wasm operations?

No. My primary reason is that I don't understand how these operations should be evaluated (see comments in #544).

I can do it in a follow up PR

That's be great!