Missed optimization opportunity when trivially converting between tuple struct and field struct by field values

[recatek]

Example code:

#[repr(C)]
pub struct Triple<'a>(&'a [u32], &'a [u32], &'a [u32]);

#[repr(C)]
pub struct ThreeFields<'a> {
    a: &'a [u32],
    b: &'a [u32],
    c: &'a [u32],
}

#[inline(always)]
pub fn should_be_no_op(val: Triple) -> ThreeFields {
    ThreeFields {
        a: val.0,
        b: val.1,
        c: val.2,
    }
}

pub fn sum_slices_1(val: ThreeFields) -> u32 {
    sum(&val)
}

pub fn sum_slices_2(val: Triple) -> u32 {
    let val = should_be_no_op(val);
    sum(&val)
}

#[inline(never)]
pub fn sum(val: &ThreeFields) -> u32 {
    val.a.iter().sum::<u32>() + val.b.iter().sum::<u32>() + val.c.iter().sum::<u32>()
}

In rustc 1.84 stable this generates a number of moves that I don't think need to be there, especially when inlining:

example::sum_slices_1::hc2a0527df8a4985d:
        jmp     qword ptr [rip + example::sum::h228167780f08fdbb@GOTPCREL]

example::sum_slices_2::hc5b364eabe45dc38:
        sub     rsp, 56
        movups  xmm0, xmmword ptr [rdi]
        movups  xmm1, xmmword ptr [rdi + 16]
        movups  xmm2, xmmword ptr [rdi + 32]
        movups  xmmword ptr [rsp + 8], xmm0
        movups  xmmword ptr [rsp + 24], xmm1
        movups  xmmword ptr [rsp + 40], xmm2
        lea     rdi, [rsp + 8]
        call    qword ptr [rip + example::sum::h228167780f08fdbb@GOTPCREL]
        add     rsp, 56
        ret

See https://rust.godbolt.org/z/3sG9nz3oc

While this is a pretty pointless example, this comes up in situations where you might want to convert a tuple of slices into a struct of slices in order to assign names to the tuple members (I've been running into these issues working on gecs).

See also: #107436 and #135786