[EH] Rename format_exception.cpp

[aheejin]

I think we need a separate file in libc++abi for 'our' EH, meaning both
Emscripten EH and Wasm EH. (I think they can be in the same file, as
long as we have ifdefs) This also adds a file header.

I think this method can be in that file, rather than having a separate
file for this. I named this cxa_emscripten.cpp because it's the name
#16627 used and I hope we can put all Emscripten specific stuff there,
but maybe @sbc100 had another thought; if so let me know.

[aheejin]

cc @hoodmane

[hoodmane]

By the way, I tried at some point to work out how to implement similar features with wasm exception handling but I got stuck -- in particular, exception.getArg(tag, 0) returns a number but I was unable to figure out how to interpret that number. When you do implement this for wasm exception handling I'm curious to see what the code looks like.

[sbc100]

How about cxa_exception_emscripten.cpp?

Also I noticed that the error message was change to terminating with uncaught exception.. where as before it could be used to get a description of an exception in general.

It seems like it might be useful to be able to decribe and exception without it being the an uncaught one. So we might need two different functions here.. one can call the other?

[aheejin]

I thought about this, but is there a way to access Wasm exceptions from JS side unless they are uncaught exceptions? Hmm, but yeah, you're right, someone might want to catch them from JS and not actually terminate the program.. Will fix that in another PR.

Renamed the file to cxa_exception_emscripten.cpp.

[aheejin]

@hoodmane

By the way, I tried at some point to work out how to implement similar features with wasm exception handling but I got stuck -- in particular, exception.getArg(tag, 0) returns a number but I was unable to figure out how to interpret that number. When you do implement this for wasm exception handling I'm curious to see what the code looks like.

Sorry for making you wait for so long. My next PR, which I will upload probably today or tomorrow, will address this. The reason you didn't get the right value is in libc++abi what you throw is not really the object pointer itself but __cxa_exception->unwindHeader:

emscripten/system/lib/libcxxabi/src/cxa_exception.cpp

Line 284 in 5c4a24f

_Unwind_RaiseException(&exception_header->unwindHeader);

[aheejin]

Can we land this?

[hoodmane]

Sorry for making you wait for so long

No worries, we are not using wasm-exceptions anyways -- Safari support is too recent.