Cranelift: iconst.i128 stores the wrong value for i128 values

[afonso360]

When implmenting i128 support for aarch64 I found that iconst is not correctly storing i128 values on x86_64.

It looks like the issue is that we store constants as u64's in machineinst/lower.rs, which then gets wrongly lowered to i128's.

.clif Test Case

test run
target x86_64

function %i128_const_neg_1() -> i64, i64 {
block0:
    v1 = iconst.i128 -1
    v2, v3 = isplit v1
    return v2, v3
}
; run: %i128_const_neg_1() == [0xffffffff_ffffffff, 0xffffffff_ffffffff]

Steps to Reproduce

clif-util test ./the_above.clif

Expected Results

Returns [-1, -1]

Actual Results

Returns [-1, 0]

Versions and Environment

Cranelift version or commit: e676589 (main as of writing this)

Operating system: Windows 10 19042.928

Architecture: x86_64

[bjorn3]

I am suprised that the new backend supports iconst.i128 at all. I use iconst.i64+iconcat for constructing i128 values in cg_clif.

[abrown]

I was trying to refactor the handling of immediates over in #2468, mainly from the vector perspective, but I think this could be fixed there as well. Unfortunately, I let that PR languish for a few months so I will have to revive it to fix this, unless someone here wants to pick it up.

[afonso360]

Let me try to pick that up. Lets see how it goes

[afonso360]

Fixed in #5075