Confusion about EM_JS functions and dead code elimination

[hoodmane]

How do I prevent dead code elimination from removing an EM_JS function I define in a different compilation unit? I have mostly been using -sMAIN_MODULE=1 which prevents these errors but now I am trying to use -sMAIN_MODULE=2 or no dynamic linking builds and I am running into trouble with missing EM_JS symbols. I assume I am messing up something simple. How do I prevent these missing symbol errors? (See full example below).

@antocuni

Version of emscripten/emsdk:

emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 3.1.32 (eab98adf462c39f3c31d348331c4830bcaa36949)

Failing command line in full:

main.c

void say_hello(void);

int
main(void) {
    say_hello();
}

hello.c

#include <emscripten.h>

EM_JS(void, say_hello, (void), {
    console.log("Hello from JavaScript");
})

Compilation

$ emcc -c hello.c -o hello.o
$ emcc -c main.c -o main.o

Full link command and output with -v appended

$ emcc main.o hello.o -o main.js -v
 "/home/ubuntu/pyodide/emsdk/emsdk/upstream/bin/clang" --version
 "/home/ubuntu/pyodide/emsdk/emsdk/upstream/bin/wasm-ld" -o main.wasm main.o hello.o -L/home/ubuntu/pyodide/emsdk/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten -lGL -lal -lhtml5 -lstubs-debug -lnoexit -lc-debug -ldlmalloc -lcompiler_rt -lc++-noexcept -lc++abi-debug-noexcept -lsockets -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr --allow-undefined-file=/tmp/tmp6cceqbst.undefined --strip-debug --export-if-defined=main --export-if-defined=__start_em_asm --export-if-defined=__stop_em_asm --export-if-defined=__start_em_lib_deps --export-if-defined=__stop_em_lib_deps --export-if-defined=__start_em_js --export-if-defined=__stop_em_js --export-if-defined=__main_argc_argv --export-if-defined=fflush --export=emscripten_stack_get_end --export=emscripten_stack_get_free --export=emscripten_stack_get_base --export=emscripten_stack_get_current --export=emscripten_stack_init --export=stackSave --export=stackRestore --export=stackAlloc --export=__wasm_call_ctors --export=__errno_location --export=__get_temp_ret --export=__set_temp_ret --export-table -z stack-size=65536 --initial-memory=16777216 --no-entry --max-memory=16777216 --global-base=1024
wasm-ld: error: main.o: undefined symbol: say_hello
emcc: error: '/home/ubuntu/pyodide/emsdk/emsdk/upstream/bin/wasm-ld -o main.wasm main.o hello.o -L/home/ubuntu/pyodide/emsdk/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten -lGL -lal -lhtml5 -lstubs-debug -lnoexit -lc-debug -ldlmalloc -lcompiler_rt -lc++-noexcept -lc++abi-debug-noexcept -lsockets -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr --allow-undefined-file=/tmp/tmp6cceqbst.undefined --strip-debug --export-if-defined=main --export-if-defined=__start_em_asm --export-if-defined=__stop_em_asm --export-if-defined=__start_em_lib_deps --export-if-defined=__stop_em_lib_deps --export-if-defined=__start_em_js --export-if-defined=__stop_em_js --export-if-defined=__main_argc_argv --export-if-defined=fflush --export=emscripten_stack_get_end --export=emscripten_stack_get_free --export=emscripten_stack_get_base --export=emscripten_stack_get_current --export=emscripten_stack_init --export=stackSave --export=stackRestore --export=stackAlloc --export=__wasm_call_ctors --export=__errno_location --export=__get_temp_ret --export=__set_temp_ret --export-table -z stack-size=65536 --initial-memory=16777216 --no-entry --max-memory=16777216 --global-base=1024' failed (returned 1)

[sbc100]

I believe this is a duplicate of #18927 and was fixed in #18928.

Since this is effect multiple folks I guess we should push for a release ASAP to get the fix out.

[sbc100]

Hopefully we will release 3.1.34 today with #18928 included.

Closing this as a duplicate.