Unnecessary imports for MINIMAL_RUNTIME

[allsey87]

I am building the following code with em++ and despite not using canvas or webgl and specifying MINIMAL_RUNTIME, for some reason the resulting wasm file wants to import the functions: emscripten_set_canvas_element_size and emscripten_webgl_create_context. Why are these functions being requested in code that makes no use of these features (especially when requesting a minimal runtime)?

Code

#include <thread>
#include <future>
#include <mutex>
#include <iostream>
#include <vector>

int main() {
    int a = 3; int b = 5;
    std::mutex m;
    int total = 0;
    std::vector<std::future<void> > vec_futures;
    /* add a */
    vec_futures.emplace_back(std::async(std::launch::async, [&m, &total, a] {
        std::this_thread::sleep_for(std::chrono::milliseconds(1));
        m.lock();
        total += a;
        m.unlock();
    }));
    /* add b */
    vec_futures.emplace_back(std::async(std::launch::async, [&m, &total, b] {
        std::this_thread::sleep_for(std::chrono::milliseconds(1));
        m.lock();
        total += b;
        m.unlock();
    }));
    for(auto& future: vec_futures) {
        future.wait();
    }
    std::cout << total << std::endl;
    return 0;
}

Command line

em++ -pthread -fwasm-exceptions -s PTHREAD_POOL_SIZE=4 -s MINIMAL_RUNTIME=1 --minify 0 node.cpp -o node.html

Version of emscripten/emsdk:

emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 3.0.1 (5ebe1d421445fa9a4a02c1efc6d42bde3ac90da2)
clang version 14.0.0 (https://github.com/llvm/llvm-project d98dfb2baabb647a6ac4c4f4399999cfc44433db)
Target: wasm32-unknown-emscripten
Thread model: posix
InstalledDir: path/to/emsdk/upstream/bin

[kripken]

I don't see an optimization level there. Try to link with -O3.

(I'm not sure why those imports exist in the first place - can disassemble the wasm to see - but the optimizer will remove them if they truly are not needed.)

[allsey87]

The problem with -O3 is that I am writing my own runtime and -O3 reduces all the import and export names to single characters, e.g., fd_write just becomes a, etc.

[kripken]

Oh, if you are writing your own runtime, I don't think you need MINIMAL_RUNTIME? That mostly affects the JS, which you don't want. Instead, try STANDLONE_WASM

[allsey87]

STANDALONE_WASM doesn't support threads (yet) unfortunately

[sbc100]

I think the reason is that our core threading library depends on these functions:

See:

emscripten/system/lib/pthread/library_pthread.c

Line 199 in 351f5f7

q->returnValue.i = emscripten_webgl_create_context(q->args[0].cp, q->args[1].vp);

emscripten/system/lib/pthread/library_pthread.c

Line 203 in 351f5f7

emscripten_set_canvas_element_size(q->args[0].cp, q->args[1].i, q->args[2].i);

I was working on PR to break these dependencies but it got stalled: #14912

[sbc100]

If you don't use GL or canvas then simply supplying stubs (for now) should be fine.. they should never be called.

[allsey87]

Thanks for the suggestion, @sbc100! This is indeed what I am doing, I just thought I would report it since it didn't seem correct.