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. 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