[PATCH] D76547: [WebAssembly] Add `wasm-exported` function attribute

[Aaron Ballman via Phabricator via cfe-commits]

aaron.ballman added a comment.

In D76547#4231501 <https://reviews.llvm.org/D76547#4231501>, @sbc100 wrote:

> In D76547#4231476 <https://reviews.llvm.org/D76547#4231476>, @aaron.ballman wrote:
>
>> In D76547#4231422 <https://reviews.llvm.org/D76547#4231422>, @sbc100 wrote:
>>
>>> The reason `__attribute__((export_name("foo")))` doesn't work in all use cases is that we have a lot of existing code that uses the `EMSCRIPTEN_KEEPALIVE` macro.     We also have run into other folks who want to include this is some kind of `FOO_API`, or `EXPORT_API` type macros.  Its not possible to have such a macro map to the existing export_name since they don't include the symbol name: e.g:
>>>
>>>   EMSCRIPTEN_KEEPALIVE int foo();`
>>>   
>>>   JNI_EXPORT int myfunct();
>>>
>>> In these cases we need something that uses the llvm symbol name for the export.
>>
>> I think there's two ways we could address this without adding a new attribute (maybe you've thought of this and have reasons for this to be a bad suggestion):
>>
>> - It seems that `export_name` doesn't care if you put in an empty string for the argument, so we could treat that case as meaning "export with the name of the symbol this attribute is attached to"
>> - We could allow `export_name` to take zero or one argument. The one-argument form is the same as it is today, but the zero argument form exports with the name of the symbol the attribute is attached to.
>>
>> Do you think either of those could work?
>
> Yes, I think the second one would be ideal.  The first one is slightly less idea because it prevents something being exported with the empty string as its name (wasm allows such things).

Ah, yeah, that's a good reason to avoid the first one.

> Can an attribute take an optional argument?  That would be great solution.   For my initial version of this change I did look into making `export_name(DEFAULT)` work (note the lack of quotes around the word DEFAULT here), but I could not find way to make a single attribute take both a string *or* a constant.

Yup, it takes only a little bit of extra work to do right. Have the attribute's argument list take a `VariadicStringArgument` instead of `StringArgument` so you can pass zero or more arguments, then have the attribute handler in SemaDeclAttr.cpp diagnose if the attribute is given > 1 argument. The rest should be things like documentation or fall out somewhat naturally (there will be accessors added to the `WebAssemblyExportNameAttr` class that let you access the arguments with iterators, and a size accessor as well, so you can tell if the semantic attribute does/does not have an argument).


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