Adding support for WebAssembly's call_indirect operator directly in your C code by compiling LLVM IR with your C code through emscripten

README.md

Making call_indirect operator accessible in C

• emcc supports compiling .ll and .bc files
• Based on LLVM's tests, we can have LLVM IR generate a call_indirect function (with several types of function signatures of parameters)

call-indirect.ll

target datalayout = "e-p:32:32-i64:64-v128:32:128-n32-S128"
target triple = "asmjs-unknown-emscripten"

; https://github.com/llvm-mirror/llvm/blob/45ed11f7a623932117ee4a3deb540e3869c38fe4/test/CodeGen/WebAssembly/call.ll#L84-L92

define void @call_indirect(void ()* %callee) {
  call void %callee()
  ret void
}

index.html

<script src='loader.js'></script>
<h1>Check the console</h1>

loader.js

let imports = {
    env: {
        memory: WebAssembly.Memory({ initial: 256 }),
        memoryBase: 0,
        table: WebAssembly.Table({ initial: 1, element: 'anyfunc' }),
        tableBase: 0,
        abort: () => {
            debugger;
        },
        _call_js: () => {
            console.log('main.c called call_js');
            debugger;
        }
    }
};

fetch('main.wasm').then((resp) => {
    return resp.arrayBuffer();
}).then((buffer) => {
    return WebAssembly.instantiate(
        buffer,
        imports
    );
}).then(({instance, module}) => {
    imports.env.table.set(0, instance.exports._call_js_log);
    instance.exports._main();
});

main.c

void call_indirect(int index);
void call_js();

// WebAssembly#Table's can only have Exported Function Exotic Objects
// https://github.com/WebAssembly/design/blob/master/JS.md#webassemblytable-objects
// We'll put call_js_log in the Table then, and call_indirect(0) should call it
void call_js_log()
{
    call_js();
}

int main()
{
    // Assumes the environment sets the default Table up with an entry at 0
    call_indirect(0);
    return 0;
}

Makefile

main.wasm: main.c call-indirect.ll
	emcc $^ -o $@ \
		-O3 \
		-s LEGALIZE_JS_FFI=0 \
		-s WASM=1 -s SIDE_MODULE=1 \
		-g4

.PHONY: serve
serve: main.wasm loader.js index.html
	python3 -m http.server 8888
	echo "Visit http://localhost:8888/"

.PHONE: clean
clean:
	rm main.wasm