Using if-else to match cmp::Ordering is better than match

[fee1-dead]

Godbolt

Consider the following snippet:

#[inline(never)]
fn if_else(cmp: Ordering) -> u32 {
    let mut x = 0;
    if cmp == Less {
        x += black_box(1);
    } else if cmp == Greater {
        x += black_box(2);
    } else {
        x += black_box(3);
    }
    x
}

#[inline(never)]
fn matching(cmp: Ordering) -> u32 {
    let mut x = 0;
    match cmp {
        Less => {
            x += black_box(1);
        }
        Greater => {
            x += black_box(2);
        }
        _ => {
            x += black_box(3);
        }
    }
    x
}

This is part of if_else:

example::if_else:
        sub     rsp, 12
        cmp     dil, 1
        je      .LBB0_5
        cmp     dil, -1
        jne     .LBB0_4

...and this is part of matching:

example::matching:
        sub     rsp, 12
        cmp     dil, -1
        je      .LBB1_3
        movsx   rax, dil
        cmp     rax, 1
        jne     .LBB1_2

I think these two should generate the same code, but currently using match gives one extra instruction.

@rustbot label A-codegen C-enhancement

[leonardo-m]

See also Issue #86354

[dotdash]

This seems to have been fixed in the meantime, the Godbolt link shows that two comparisons are switched, but the extra instruction is gone,