Issues with extern C functions receiving/returning structs by value with floating point members on x86_64

[SiegeLord]

Note that I only tested this with a quad of f32's, I imagine it fails for several other numbers/sizes of floats too. Tested with rust at dc4560d.

You need two files, test.c and test.rs:

test.c:

#include <stdio.h>

typedef struct
{
    float a, b, c, d;
} A;

A get()
{
    A a = {1, 2, 3, 4};
    return a;
}

void test(A a)
{
    printf("%f %f %f %f\n", a.a, a.b, a.c, a.d);
}

test.rs:

struct A
{
    a : f32,
    b : f32,
    c : f32,
    d : f32,
}

#[link_args = "-ltest"]
extern "C"
{
    fn get() -> A;
    fn test(a : A);
}

fn main()
{
    unsafe
    {
        let a = get();
        println(fmt!("%?", a));
        let b = A{a: 0.1, b: 0.2, c: 0.3, d: 0.4};
        test(b);
    }
}

Compile using this code:

gcc -c test.c
ar -cvq libtest.a test.o
rustc test.rs -L.

The output when running test on my system is:

./test
{a: 0, b: 1, c: 0, d: 0}
0.100000 0.200000 0.200000 0.000000

The expected output is:

./test
{a: 1, b: 2, c: 3, d: 4}
0.100000 0.200000 0.300000 0.400000

[brson]

Related to #5744 #5745