I'm pretty sure EMSCRIPTEN_KEEPALIVE won't have the intended behaviour of actually exporting symbols when compiled with non-emscripten triples.
On Sat, Jan 29, 2022 at 4:08 PM Floh <[email protected]> wrote: > Thanks for the thorough explanation Sam! Regarding this PR: > https://github.com/emscripten-core/emscripten/pull/16149, as far as I > have seen, only the EM_JS() macros caused trouble (with a non-emscripten > triple), I haven't seen any linker warnings regarding EMSCRIPTEN_KEEPALIVE > functions (which I'm using too in the same code base). > > I'll try to bring the current workaround (use wasm32-emscripten just for > the C code with the EM_JS macros, and wasm32-freestanding for the Zig > code), into a better shape tomorrow and then will most likely write a Zig > ticket, I think the Zig stdlib needs a few fixes for wasm32-emscripten (if > just some empty stubs), so that a complete project can be compiled with > this triple. > > Cheers! > -Floh. > > On Saturday, 29 January 2022 at 20:47:24 UTC+1 [email protected] wrote: > >> Short term fix/wrokaround is here: >> https://github.com/emscripten-core/emscripten/pull/16149 >> >> On Sat, Jan 29, 2022 at 11:32 AM Sam Clegg <[email protected]> wrote: >> >>> The undefined symbol error you are seeing here is coming from the >>> post-linking phase. The way EM_JS works is that the function is that >>> function `foo` declared as external using >>> `__attribute__((import_name("foo")))` and the data symbol `__em_js_foo` is >>> defined in the data section along with `__attribute__((used, >>> visibility("default")))`. For more details on this see >>> https://github.com/emscripten-core/emscripten/blob/main/system/include/emscripten/em_js.h#L23-L49 >>> . >>> >>> I believe the problem you are seeing stems from the different meaning of >>> `__attribute__((used))` under emscripten compared to with triples. The >>> problem stems from the fact that we use `__attribute__((used))` to >>> implement the EMSCRIPTEN_KEEPALIVE macro, which is defined to mean "keep >>> this symbol alive *and* export it to JS under its symbol name". >>> >>> If you use wasm-objdump to look at an object file containing EM_JS >>> symbols you will see them marked as both "no_strip" and "exported". For >>> example: >>> >>> ``` >>> - 38: D <__em_js__noarg> segment=0 offset=0 size=36 [ exported >>> no_strip binding=global vis=default ] >>> - 39: D <__em_js__noarg_int> segment=0 offset=36 size=55 [ exported >>> no_strip binding=global vis=default ] >>> - 40: D <__em_js__noarg_double> segment=0 offset=91 size=61 [ exported >>> no_strip binding=global vis=default ] >>> - 41: D <__em_js__intarg> segment=0 offset=152 size=41 [ exported >>> no_strip binding=global vis=default ] >>> ``` >>> >>> If you compile the same source using a non-emscripten triple you will >>> see them only marked as `no_strip` which is a more traditional meaning of >>> the `used` attribute which simply tells the linker to keep them around in >>> the binary, not to export them. Here is where the hack/difference is: >>> https://github.com/llvm/llvm-project/blob/333f5019300c6e56782374627e64da0b62ffa3bc/llvm/lib/MC/WasmObjectWriter.cpp#L1773-L1777 >>> >>> There are two ways we can solve this issue I believe. >>> >>> 1. Long term solution: Stop abusing `__attribute__((used))`, and thus >>> remove this special handling in emscripten. We should really have a >>> separate attribute to mark a symbol as exported. I've been trying to get >>> this done for while but its stalled. See >>> https://reviews.llvm.org/D76547 >>> 2. Short term solution: Use the more explicit (but not >>> EMSCIRPTEN_KEEPALIVE-compatible), 'export-name' attribute in em_js.h. I >>> think this should "just work". >>> >>> cheers, >>> sam >>> >>> >>> On Sat, Jan 29, 2022 at 10:22 AM Floh <[email protected]> wrote: >>> >>>> Spot on Alon :) >>>> >>>> It works if I hardwire just the C library (with the EM_JS functions) to >>>> the wasm32-emscripten triple. >>>> >>>> The Zig code needs to be compiled either with wasm32-wasi or >>>> wasm32-freestanding, when using wasm32-emscripten, parts of the Zig stdlib >>>> won't compile. >>>> >>>> Also, when I tried to use wasm32-freestanding with the C code, then >>>> wasm-ld complained about some missing stack-check functions (don't have the >>>> exact symbol at hand currently). >>>> >>>> ...I think I have enough to build a little 'proof-of-concept', even >>>> though it's a bit hacky :) >>>> >>>> Thanks! >>>> -Floh. >>>> On Saturday, 29 January 2022 at 18:58:53 UTC+1 [email protected] wrote: >>>> >>>>> Sam can confirm, but I would guess perhaps the emscripten triple is >>>>> necessary. That is, clang and/or wasm-ld might do something for EM_JS code >>>>> but only in emscripten mode. >>>>> >>>>> If we can confirm that then we should definitely get a bug filed on >>>>> Zig - hopefully it would be easy to add support for the emscripten triple >>>>> there and open up a bunch of use cases... >>>>> >>>>> On Sat, Jan 29, 2022 at 9:12 AM Floh <[email protected]> wrote: >>>>> >>>>>> I'm currently tinkering with bringing one of my toy Zig projects to >>>>>> the web via >>>>>> Alon's nice gist here which uses emcc only for the linker step: >>>>>> >>>>>> https://gist.github.com/kripken/58c0e640227fe5bac9e7b30100a2a1d3 >>>>>> >>>>>> ...and it *nearly* works except for code that uses EM_JS() macros. >>>>>> >>>>>> The project (https://github.com/floooh/pacman.zig) consists of some >>>>>> C code (my cross-platform 'sokol headers') which uses EM_JS() quite >>>>>> extensively (very handy for STB-style single-file libraries), and at the >>>>>> top, the "game code" is written in Zig. >>>>>> >>>>>> I'm compiling all code with Zig with the wasm32-wasi target >>>>>> (wasm32-emscripten exists, but currently doesn't seem to be supported by >>>>>> the Zig compiler), and then use emcc for linking. >>>>>> >>>>>> Long story short, it works except for the one problem that emcc >>>>>> cannot resolve any functions which have been defined with EM_JS(). If I >>>>>> compile the same library with emcc instead of Zig it works. >>>>>> >>>>>> So my question is: does emcc also do some "EM_JS() magic" when >>>>>> compiling the source code which contains EM_JS macros? Maybe I'm missing >>>>>> some Clang command line options which emcc inserts? >>>>>> >>>>>> The errors look like this: >>>>>> >>>>>> error: undefined symbol: sapp_js_add_clipboard_listener (referenced >>>>>> by top-level compiled C/C++ code) >>>>>> >>>>>> Followed by: >>>>>> >>>>>> warning: _sapp_js_add_clipboard_listener may need to be added to >>>>>> EXPORTED_FUNCTIONS if it arrives from a system library >>>>>> ...there's also a single warning about malloc: >>>>>> >>>>>> ...if I compile with "-s ERROR_ON_UNDEFINED_SYMBOLS=0", then the code >>>>>> breaks at runtime failing to resolve those EM_JS() functions, e.g.: >>>>>> >>>>>> "missing function: sapp_js_pointer_init" >>>>>> >>>>>> Compiling the same static link library with emcc, it magically works. >>>>>> >>>>>> If I look at both libraries with nm I don't see much of a difference, >>>>>> e.g. here's the relevant parts from the emcc-compiled library, every >>>>>> EM_JS >>>>>> symbol has an "D __em_js..." entry, and a matching "U sapp_js..." entry, >>>>>> e.g.: >>>>>> >>>>>> 0000185f D __em_js__sapp_js_add_beforeunload_listener >>>>>> ... >>>>>> U sapp_js_add_beforeunload_listener >>>>>> ... >>>>>> >>>>>> The Zig-compiled library has the same entries: >>>>>> >>>>>> 00001841 D __em_js__sapp_js_add_beforeunload_listener >>>>>> ... >>>>>> U sapp_js_add_beforeunload_listener >>>>>> ... >>>>>> >>>>>> ...yet one library (the zig-compiled) produces linker errors for >>>>>> those symbols, and the other (emcc-compiled) works. >>>>>> >>>>>> Clearly I'm missing something. I was expecting that all the EM_JS() >>>>>> magic is in the linker (by extracting the __em_js_* Javascript source >>>>>> code >>>>>> strings, and then "somehow" providing the C function import). Any ideas >>>>>> what I'm missing? >>>>>> >>>>>> Thanks! >>>>>> -Floh. >>>>>> >>>>>> >>>>>> >>>>>> >>>>>> >>>>>> -- >>>>>> You received this message because you are subscribed to the Google >>>>>> Groups "emscripten-discuss" group. >>>>>> To unsubscribe from this group and stop receiving emails from it, >>>>>> send an email to [email protected]. >>>>>> To view this discussion on the web visit >>>>>> https://groups.google.com/d/msgid/emscripten-discuss/15129292-2f07-44d9-99a9-a27ac4721a0cn%40googlegroups.com >>>>>> <https://groups.google.com/d/msgid/emscripten-discuss/15129292-2f07-44d9-99a9-a27ac4721a0cn%40googlegroups.com?utm_medium=email&utm_source=footer> >>>>>> . >>>>>> >>>>> -- >>>> You received this message because you are subscribed to the Google >>>> Groups "emscripten-discuss" group. >>>> To unsubscribe from this group and stop receiving emails from it, send >>>> an email to [email protected]. >>>> To view this discussion on the web visit >>>> https://groups.google.com/d/msgid/emscripten-discuss/ad13a6a8-0248-406d-ba91-591bcec62e54n%40googlegroups.com >>>> <https://groups.google.com/d/msgid/emscripten-discuss/ad13a6a8-0248-406d-ba91-591bcec62e54n%40googlegroups.com?utm_medium=email&utm_source=footer> >>>> . >>>> >>> -- > You received this message because you are subscribed to the Google Groups > "emscripten-discuss" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected]. > To view this discussion on the web visit > https://groups.google.com/d/msgid/emscripten-discuss/75b6eb8b-f6a3-4ce8-a075-462f1902e087n%40googlegroups.com > <https://groups.google.com/d/msgid/emscripten-discuss/75b6eb8b-f6a3-4ce8-a075-462f1902e087n%40googlegroups.com?utm_medium=email&utm_source=footer> > . > -- You received this message because you are subscribed to the Google Groups "emscripten-discuss" group. 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