Missed optimization: bounds check not elided for `i * s < n` when `0 <= i < n / s`

[SuperSamus]

I tried this code:

#[inline(never)]
pub fn step_sum(arr: &[u32]) -> u32
{
    const STEP_SIZE: usize = 8;

    let mut result = 0;

    for step in 0..(arr.len() / STEP_SIZE) {
        result += arr[step * STEP_SIZE];
    }

    result
}

I expected to see this happen: the index is in the bounds, so the check is elided.

Instead, this happened:

.LBB0_4:
        cmp     rcx, rsi
        jae     .LBB0_6
        add     eax, dword ptr [rdi + 4*rcx]
        add     rcx, 8
        dec     rdx
        jne     .LBB0_4
        ret
.LBB0_6:
        push    rax
        lea     rdx, [rip + .L__unnamed_1]
        mov     rdi, rcx
        call    qword ptr [rip + core::panicking::panic_bounds_check::h300eea3d2ac1c8da@GOTPCREL]

Godbolt

Also, that isn't even the strictest scenario where the compiler should optimize this.
Stricter example:

#[inline(never)]
pub fn step_sum(arr: &[u32]) -> u32
{
    const STEP_SIZE: usize = 8;

    let mut result = 0;

    for step in 0..(arr.len().div_ceil(STEP_SIZE)) {
        let offset = (arr.len() - 1) % STEP_SIZE;
        result += arr[step * STEP_SIZE + offset];
    }

    result
}

[veera-sivarajan]

@rustbot label +A-llvm