Macro for allowing exception catching similar to EMSCRIPTEN_KEEPALIVE

[ravisumit33]

In my code base, I have few functions which are in the EXCEPTION_CATCHING_ALLOWED list. Now, whenever I change something about these functions, their mangled name changes and I need to find those names and update them in the allow list. One option is to put them inside extern 'C' block but I don't want to do that as the code base is used for other platforms as well. I was wondering if there can be a macro similar to EMSCRIPTEN_KEEPALIVE that allows exception catching in the function?

[sbc100]

Unfortunately there is no way to do this today. It would require some kind of custom attribute to be placed on the symbol, which is possible but would require upstream clang/llvm changes.

In the mean time are you using a separate file for maintaining your EXCEPTION_CATCHING_ALLOWED list? You can specify it using -sEXCEPTION_CATCHING_ALLOWED=@filename and then list your symbols in a simple one-line-per-symbol format.

[ravisumit33]

@sbc100 Yes i am using a separate file to list symbols and use this file as -sEXCEPTION_CATCHING_ALLOWED=@filename. Changing mangled name of symbols whenever that symbol is updated is an issue.

[sbc100]

I think this would be great feature, but it does require some llvm hacking. Perhaps we could piggy back on function annotations: https://github.com/WebAssembly/annotations since I imagine they will be adding the ability to add custom llvm attributes to symbols at the source level.

[ravisumit33]

Yes. The issue get much harder when the function you want to enable exception catching on is a template function. So there will be many symbols for that function and it is very hard to find all of those and list them in the exception catching allow list.

[sbc100]

I'm curious what the cost is for you of enabling exceptions globally instead?

The good news is that we will have native exception handling soon which should avoid the need such labeling (although it will be while before all browser support it).

[ravisumit33]

Enabling exception catching globally has a big performance impact for us. For example, size of the wasm in which we have enabled exception catching selectively is 4.3 MB while other one having global allowance is of 7 MB. Its not only the size we want to optimize but also the runtime performance that why we use O3 instead of Oz or Os.

[ravisumit33]

We are also looking for zero-cost exception handling in native wasm but until then this is what we have. One thing which I faced recently is that with O3 functions get inlined and EXCEPTION_CATCHING_ALLOWED list doesn't work for them as expected.