Feature Request: MINIMAL_RUNTIME + MODULARIZE selective export of symbols

[DavidPeicho]

Version of emscripten/emsdk: 3.1.31

We had a big discussion in #17911 and I think it would be great to talk about that again. The current solution to export symbols on Module with MINIMAL_RUNTIME+MODULARIZE is to use EXPORT_ALL (added in the linked PR). However, this is also going to export internal library symbols, which is most likely not wanted.

Some of the ideas that were added in the PR:

• Create a new EMSCRIPTEN_EXPORT macro
• Create a new flag EXPORT_KEEPALIVE

From what I remember, EMSCRIPTEN_EXPORT required a patch to LLVM, so I would vote for the second solution. If I am correct, the solution would be to take all kept alive symbols and to append it to EXPORTED_FUNCTIONS. AFAIK, this should just work because EXPORTED_FUNCTIONS is used here:

emscripten/src/jsifier.js

Line 424 in fbc5d5c

contentText += `\nModule["${mangled}"] = ${mangled};`;

Am I correct? If you are happy with the idea, I can start implementing it, maybe with some guidelines.

[DavidPeicho]

I am also wondering what should we do about other stuff like HEAP8? Is that supposed to be manually exported inside a library?

[sbc100]

I kind of like that idea of a new flag EXPORT_KEEPALIVE which could default to 1 for the normal runtime (so as not to break existing code) but default to 0 for minimal runtime. Then you could set that flag to have just the keepalive subset of symbols exported?

Obviously being able to choose on a per-symbol basis would be better but also a lot more work to make it happen.

Regarding HEAPX symbols, I think we they should probably be treated as if they were JS library symbols (i.e. they should be exportable via -sEXPORTED_FUNCTIONS.

[DavidPeicho]

Good idea! Made a PR here: #18819

For the HEAP symbols, this should maybe be another PR.