Incorrect bindings generated for emscripten

[pepyakin]

Input C/C++ Header

adder.h:

unsigned char add(unsigned char a, unsigned char b);

Bindgen Invocation

bindgen::Builder::default()
        .header("adder.h")
        .generate()
        .expect("Unable to generate bindings");

Actual Results

Generated bindings:

/* automatically generated by rust-bindgen */

extern "C" {
    #[link_name = "_add"]
    pub fn add(a: ::std::os::raw::c_uchar, b: ::std::os::raw::c_uchar)
     -> ::std::os::raw::c_uchar;
}

rustc/emcc fails with a message:

error: unresolved symbol: _add

(full message)

Expected Results

Emscripten emits names without leading _:

$ nm libadder.a

adder_f1ebf194.bc:
-------- T add

so I expect following bindings to be generated:

/* automatically generated by rust-bindgen */

extern "C" {
    pub fn add(a: ::std::os::raw::c_uchar, b: ::std::os::raw::c_uchar)
     -> ::std::os::raw::c_uchar;
}

I created repository for this case: https://github.com/pepyakin/emcc-bindgen-repro

This can be workarounded with setting trust_clang_mangling to false.

[emilio]

Which clang version are you using?

[pepyakin]

$ clang --version                                                                                                                                                                                                          
clang version 4.0.0 (https://github.com/kripken/emscripten-fastcomp-clang/ c7c210fee24e0227f882337521b25b1ed9c36d5b) (https://github.com/kripken/emscripten-fastcomp/ 90b726ede4acf47c1bca089de6c79a0b8f2c5d9a) (emscripten 1.37.18 : 1.37.18)
Target: x86_64-apple-darwin16.6.0
Thread model: posix
InstalledDir: /Users/pepyakin/tmp/emsdk_portable/clang/fastcomp/build_incoming_64/bin

[emilio]

I need the actual libclang version bindgen is loading, sorry for the hassle.

You can get it with something like:

RUST_LOG=bindgen bindgen

Which should output something like:

INFO:bindgen: Clang Version: clang version 4.0.1 (tags/RELEASE_401/final)

[upsuper]

Looks like another case that clang is trying to add _ prefix... We should probably just extend the check at https://github.com/rust-lang-nursery/rust-bindgen/blob/28de37efc4d265ef6a15468e95b2bc3aa8a76ec3/src/ir/context.rs#L359-L367

[pepyakin]

Interesting,

$ RUST_LOG=bindgen bindgen                                                                                                                                                                                                 
INFO:bindgen: Clang Version: clang version 4.0.0 (https://github.com/kripken/emscripten-fastcomp-clang/ c7c210fee24e0227f882337521b25b1ed9c36d5b) (https://github.com/kripken/emscripten-fastcomp/ 90b726ede4acf47c1bca089de6c79a0b8f2c5d9a) (emscripten 1.37.18 : 1.37.18)
WARN:bindgen: Using clang (4, 0), expected (3, 9)

[pepyakin]

Going further things getting more interesting...

For this header:

struct Foo {
    unsigned long derp;
};

unsigned char add(unsigned char a, unsigned char b);

bindgen (invoked with this same default config as in first post) generates these bindings:

/* automatically generated by rust-bindgen */

#[repr(C)]
#[derive(Debug, Copy)]
pub struct Foo {
    pub derp: ::std::os::raw::c_ulong,
}
#[test]
fn bindgen_test_layout_Foo() {
    assert_eq!(::std::mem::size_of::<Foo>() , 8usize , concat ! (
               "Size of: " , stringify ! ( Foo ) ));
    assert_eq! (::std::mem::align_of::<Foo>() , 8usize , concat ! (
                "Alignment of " , stringify ! ( Foo ) ));
    assert_eq! (unsafe {
                & ( * ( 0 as * const Foo ) ) . derp as * const _ as usize } ,
                0usize , concat ! (
                "Alignment of field: " , stringify ! ( Foo ) , "::" ,
                stringify ! ( derp ) ));
}
impl Clone for Foo {
    fn clone(&self) -> Self { *self }
}
extern "C" {
    #[link_name = "_add"]
    pub fn add(a: ::std::os::raw::c_uchar, b: ::std::os::raw::c_uchar)
     -> ::std::os::raw::c_uchar;
}

cargo test --target=asmjs-unknown-emscripten
node /Users/pepyakin/tmp/emcc-bindgen-repro/emcc-bindgen-repro/target/asmjs-unknown-emscripten/debug/emcc_bindgen_repro-0fb8f20a7ea6eea7.js

fails with the message

---- bindgen_test_layout_Foo stdout ----
	thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `4`,
 right: `8`: Size of: Foo', /Users/pepyakin/tmp/emcc-bindgen-repro/emcc-bindgen-repro/target/asmjs-unknown-emscripten/debug/build/emcc-bindgen-repro-a4b6fa2b8f183cc1/out/bindings.rs:10:4
note: Run with `RUST_BACKTRACE=1` for a backtrace.

It seems to me different data model is used for generating these bindings. asmjs/wasm32 are both 32bit platforms, and I'm running on 64bit macOS machine.

[emilio]

Yes, I was doubting between that or #528, but given it's with clang 4.0, @upsuper is right.

@pepyakin, wanna submit a patch? :)

[pepyakin]

@emilio
Oh, nevermind my last comment, seems like just target is not passed, as in this case: #942

Yeah, sure!

[pepyakin]

@emilio

It seems that original issue is also fixed with .clang_arg(format!("--target={}", target))!

I'm wondering, if it would be better to pass --target arg to clang_args instead of adding one more exception for needs_mangling_hack? This also should fix #942

[emilio]

Yup, I don't love magic, but looks passing --target automatically if not found is a reasonable thing to do anyway...