Runtime: define get_return_address for WebAssembly

[MaxDesiatov]

WebAssembly/WASI doesn't support stack traces and return addresses. To make it possible to compile exclusivity code for this platform, it seems to make the most sense for the get_return_address() to return a zero value. Since it's only used for diagnostics, this shouldn't cause any issues when targeting WASI.

Related to SR-9307.

(cc @compnerd)

[compnerd]

Should this not be __wasi__, because this is not related to the object file format?

[compnerd]

CC: @gottesmm @mikeash

[compnerd]

Id like to get @gottesmm's opinion on this as well

[compnerd]

@swift-ci please test

[compnerd]

@swift-ci please test Windows platform

[atrick]

What does exclusivity checking have to do with threads? Other than the implementation needing to be thread-safe, which is trivial if there are no threads.

[MaxDesiatov]

Great point, sorry I missed that. I've re-enabled it, now the only thing left in this PR is defining get_return_address.

[MaxDesiatov]

@compnerd could you have another look please?

[compnerd]

Does this not break other things as you cannot get the return address? This may let it compile but can cause correctness issues.

[karwa]

@compnerd @MaxDesiatov

Just looking in to this. It seems that WASI does not support getting the return address right now (see also: WebAssemblyTargetLowering::LowerRETURNADDR). AFAICT this is only used for diagnostics, so it shouldn’t be a big deal to leave it out.

The other option (besides returning a null pointer), would be to omit the “PC” field from the “Access” type on WASI. Would you prefer that approach?

[MaxDesiatov]

@compnerd get_return_address() is only used in SwiftTLSContext::get().accessSet.insert call, which as far as I understand exists for diagnostics. In my understanding the worst that can happen is that AccessSet will have a zero value in the pc field. That isn't dereferenced, and is only used for diagnostic output.

[MaxDesiatov]

@swift-ci please test

[MaxDesiatov]

@swift-ci please smoke test

[compnerd]

I think that it may be better to disable the larger portions of Swift for WASM until the get return address can be implemented. Note that the else path would still prevent you from progressing, which this is undoing.

[kateinoigakukun]

Close due to the merge of #42097