if and match produce different assembly

[In-line]

https://rust.godbolt.org/z/zKEfaPKdn

inspired from https://stackoverflow.com/questions/67412863/match-compiles-into-suboptimal-asm-compared-to-if-else

[shampoofactory]

Hi. Unless I'm mistaken, the two methods produce different outputs as the b < key results have been flipped.

If we account for this anomaly and introduce a third method that more directly compares the if and match equivalents, both using cmp and Ordering, we see the output is identical.

The difference appears to be due to the use of cmp and Ordering rather than if and match. In the final assembly, if_else makes use of a conditional move which is likely going to be faster.

https://rust.godbolt.org/z/vnfM4cY45

use std::cmp::Ordering;

pub fn if_else(out: &mut i64, key: i64, b: i64, c: i64) -> bool {
    if key == b {
        return true;
    }
    if key < b {
        *out = b;
    } else {
        *out = c;
    }
    false
}

pub fn contains_match_(out: &mut i64, key: i64, b: i64, c: i64) -> bool {
    match key.cmp(&b) {
        Ordering::Equal => return true,
        Ordering::Less => {
            *out = b;
        },
        Ordering::Greater => {
            *out = c;
        },
    }
    false
}

pub fn if_else_cmp(out: &mut i64, key: i64, b: i64, c: i64) -> bool {
    let ordering = key.cmp(&b);
    if ordering == Ordering::Equal {
        return true;
    }
    if ordering == Ordering::Less {
        *out = b;
    } else {
        *out = c;
    }
    false
}

example::if_else:
        cmp     rsi, rdx
        je      .LBB0_2
        cmovl   rcx, rdx
        mov     qword ptr [rdi], rcx
.LBB0_2:
        cmp     rsi, rdx
        sete    al
        ret

example::contains_match_:
        cmp     rsi, rdx
        jl      .LBB1_2
        mov     al, 1
        mov     rdx, rcx
        je      .LBB1_3
.LBB1_2:
        mov     qword ptr [rdi], rdx
        xor     eax, eax
.LBB1_3:
        ret

example::if_else_cmp:
        cmp     rsi, rdx
        jl      .LBB2_2
        mov     al, 1
        mov     rdx, rcx
        je      .LBB2_3
.LBB2_2:
        mov     qword ptr [rdi], rdx
        xor     eax, eax
.LBB2_3:
        ret