included range in for loop gives more assembly code than excluded range

[melonges]

Given the following code where source will be excluded

pub fn store(source: &i32, dest: &mut i32) -> i32 {
    let mut accum = 0;
    for _ in 0..*source {  // source will be excluded
        accum += *dest;
    }
    accum
}

The assembly output is:

example::store:
        mov     ecx, dword ptr [rdi]
        mov     edx, dword ptr [rsi]
        imul    edx, ecx
        xor     eax, eax
        test    ecx, ecx
        cmovg   eax, edx
        ret

if source will be included the assembly code will increase more than

pub fn store(source: &i32, dest: &mut i32) -> i32 {
    let mut accum = 0;
    for _ in 0..=*source { //  source will be included
        accum += *dest;
    }
    accum
}

The assembly output is:

example::store:
        mov     ecx, dword ptr [rdi]
        test    ecx, ecx
        js      .LBB0_1
        mov     r8d, dword ptr [rsi]
        xor     eax, eax
        xor     esi, esi
.LBB0_3:
        xor     edi, edi
        cmp     esi, ecx
        setl    dl
        add     eax, r8d
        cmp     esi, ecx
        jge     .LBB0_5
        mov     dil, dl
        add     esi, edi
        cmp     esi, ecx
        jle     .LBB0_3
.LBB0_5:
        ret
.LBB0_1:
        xor     eax, eax
        ret

I tested on rustc 1.64.0 with -C opt-level=3

[Rageking8]

@rustbot label -A-diagnostics +A-codegen +I-heavy

[lukas-code]

This regressed between 1.42 and 1.43: Godbolt link

related: #45222

[leonardo-m]

(If you have two nested loop, the code is bad even in older compilers).

[jdahlstrom]

This regressed between 1.42 and 1.43: Godbolt link

The 1.42 codegen is tricky, as it seems at first that it doesn't account for the case where *source == i32::MAX and just goes add eax, 1. But it ends up giving the expected result, which is 0 (mod 2^31) no matter the value of *dest. But in many other cases inclusive ranges necessarily produce suboptimal code because the T::MAX case requires special handling.

[nikic]

Simpler example without the unnecessary memory indirections: https://rust.godbolt.org/z/GPvrbvbYY

External iteration over inclusive ranges is well known to optimize badly. Optimization may be viable in this particular case though.

[nikic]

This is the fold we'd need: https://alive2.llvm.org/ce/z/JsWRvT

[ilyvion]

Came across this weirdness today; it's still a problem in December 2023:

pub fn inclusive() -> usize {
    (2..=100).step_by(2).sum::<usize>()
}

pub fn exclusive() -> usize {
    (2..101).step_by(2).sum::<usize>()
}

produces assembly with 46 lines and 3 lines, respectively. Was going to create a new bug for it, but I'm pretty sure it's just another version of this one.

[veera-sivarajan]

@rustbot claim