64-bit: Returning a multi-field structure by value does not work

[benob]

When returning a structure from a function, only the first element is returned.

#include <stdio.h>

typedef struct {
    int x; int y;
} data;

data f(int x, int y) {
    data tmp;
    tmp.x = x;
    tmp.y = y;
    return tmp;
}

int main() {
    data z = f(5,6);
    printf("%d %d\n", z.x, z.y);
}

This program prints "5 0" instead of "5 6". If adding a third field to the structure, d8 dies with the following error:

ReferenceError: $ is not defined
    HEAP[$14]=$;; HEAP[$14+4]=1;; HEAP[$14+8]=5;

(llvm v2.9, v8 3.2.10.2, emscripten cca5278)

-- edit:
This only affects 64-bit compilation. Please compile with -m32 until 64-bit is properly supported.

[kripken]

That source code works for me with both llvm-gcc and clang with various emscripten settings.

Can you paste the generated .ll and .js files?

[benob]

Here is the .ll file: http://pastebin.com/gtfAshzZ
and the .js file: http://pastebin.com/KrU6QYve

[kripken]

Looks like that is compiled as 64-bit? (target triple = "x86_64-unknown-linux-gnu" and many appearances of i64)

Emscripten doesn't support 64-bit code well, because it is a poor fit for JavaScript (JS integers are 32-bit, and while floats are 64-bit, the actual integers they can contain are only up to 53 bits or so).

Compiling the source with -m32 should make it work properly (you may need to install some 32-bit headers on your system).

Long-term, the solution for emscripten is to bundle our own headers and build only against them. We have an issue open for that.

[benob]

Thanks for the tip, it does work fine.

I changed the issue to reflect that it only affects 64-bit compliation.

[kripken]

I added a FAQ entry for this. Marking as closed since I don't think there is a way to properly fix this. 32-bit compilation is the way to go for now.