JSVal and ABI

js.rs

// See
// https://github.com/mozilla-servo/rust-mozjs/blob/master/jsapi.rs
// https://github.com/mozilla-servo/rust-mozjs/blob/master/glue.rs

use core::libc::*;

type JSVal = u64;

#[link_args = "-L. -ljsval"]
#[link_name = "jsval"]
extern {
    fn JSVAL_TO_INT32(++v: JSVal) -> int32_t;
    fn INT32_TO_JSVAL(++i: int32_t) -> JSVal;
}

fn main() {
    let i1 = 1;
    let v = unsafe { INT32_TO_JSVAL(i1) };
    let i2 = unsafe { JSVAL_TO_INT32(v) };
    fail_unless!(i1 == i2);
}

jsval.c

// See
// https://github.com/mozilla-servo/mozjs/blob/master/js/src/jsval.h
// https://github.com/mozilla-servo/mozjs/blob/master/js/src/jsapi.h
// https://github.com/mozilla-servo/rust-mozjs/blob/master/jsglue.cpp

#include <stdint.h>

typedef union jsval {
    uint64_t asBits;
    struct {
        int32_t payload;
        uint32_t tag;
    } s;
} jsval;

int32_t JSVAL_TO_INT32(jsval v) {
    return v.s.payload;
}

jsval INT32_TO_JSVAL(int32_t i) {
    jsval v;
    v.s.tag = 0;
    v.s.payload = i;
    return v;
}

Makefile

ifndef ARM
run: libjsval.so js.x64
	LD_LIBRARY_PATH=. ./js.x64
else
run: libjsval.so js.arm
endif

clean:
	rm -f *.ll *.s *.so *.x64 *.arm

ifndef ARM
%.o: %.c
	gcc -fPIC -c $< -o $@
lib%.so: %.o
	gcc -shared $< -o $@
else
%.o: %.c
	arm-linux-androideabi-gcc -fPIC -c $< -o $@
lib%.so: %.o
	arm-linux-androideabi-gcc -shared $< -o $@
endif

%.x64: %.rs
	rustc --target x86_64-unknown-linux-gnu $< -o $@
%.arm: %.rs
	rustc --target arm-linux-androideabi --android-cross-path /opt/ndk $< -o $@