x86 backend: 128-bit integers returned in xmm0 registers by windows 64 function cause compiler crash

[ominitay]

This bug specifically affects the x86 backend.

A minimal reproduction is given below:

const assert = @import("std").debug.assert;

fn foo() callconv(.Win64) u128 {
    return 0;
}

pub fn main() void {
    assert(foo() == 0);
}

The Microsoft x64 calling convention specifies that types which don't fit in general purpose registers, such as u128, are to be returned in XMM0. This code example causes the compiler to hit unreachable code in airCmp, as the backend expects that u128 is either in memory or a pair of general purpose registers, not an SSE register.

The solution to this I think would be, in genCall, to move values of this type out of the SSE register into memory. As far as I can tell, this is what LLVM does too.

This issue is currently blocking #18680, from which it was discovered and reduced.