[ER] A missed removal of division by zero test

[leonardo-m]

This function foo:

type T = i16;

pub fn foo(start: T) -> T {
    if start <= 0 { return 0; }
    let mut count = 0;
    for i in start .. 10_000 {
        if 752 % i != 0 {
            count += 1;
        }
    }
    count
}

fn main() {
    println!("{}", (T::MIN ..= T::MAX)
                   .map(foo)
                   .sum::<T>());
}

Compiled with optimizations by rustc (1.54.0-nightly 4de7572 2021-05-01) gives:

foo:
        lea     eax, [rdi - 1]
        movzx   eax, ax
        cmp     eax, 9998
        ja      .LBB0_1
        mov     r8d, 9999
        sub     r8d, edi
        mov     esi, edi
        neg     esi
        xor     ecx, ecx
.LBB0_4:
        cmp     r8w, si
        jae     .LBB0_6
        mov     ax, 752
        xor     edx, edx
        idiv    di
        lea     eax, [rdi + 1]
        cmp     dx, 1
        sbb     cx, -1
        movzx   edx, ax
        mov     edi, eax
        cmp     edx, 10000
        jne     .LBB0_4
        mov     eax, ecx
        ret
.LBB0_1:
        xor     ecx, ecx
        mov     eax, ecx
        ret
.LBB0_6:
        push    rax
        lea     rdi, [rip + str.0]
        lea     rdx, [rip + .L__unnamed_1]
        mov     esi, 57
        call    qword ptr [rip + core::panicking::panic@GOTPCREL]
        ud2

str.0:
        .ascii  "attempt to calculate the remainder with a divisor of zero"

Running the code you confirm that the divisor can't be zero.
I think Rustc should be able to remove this test.
(The same code with T=u16 contains no division by zero test).

[inquisitivecrystal]

@rustbot label A-codegen C-enhancement I-slow +T-compiler

[klensy]

Still an issue with 1.60.0-nightly (08df8b8 2022-01-30).

I think this is similar to the same problem with (missed) optimizations, when we match on some variable and expect compiler to match on values of that variable, not on it's type.

For provided example we can cast i16 to u16 after check, so panic go away:

pub fn foo(start: T) -> T {
    if start <= 0 { return 0; }
    let start = start as u16; // add this to change type
    let mut count = 0;
    ...

[leonardo-m]

Note that with rustc 1.60.0-nightly (08df8b8 2022-01-30) replacing in my original foo code the "for i in start .. 10_000 {" with an equivalent while loop is enough to remove the dividend test.

[klensy]

Note that with rustc 1.60.0-nightly (08df8b8 2022-01-30) replacing in my original foo code the "for i in start .. 10_000 {" with an equivalent while loop is enough to remove the dividend test.

That works for 1.54 too.

[jieyouxu]

Triage: for the example in the issue description, division by zero was present in 1.69 but disappeared in 1.70.

[leonardo-m]

Right, closed.

[jieyouxu]

Sorry, I was reviewing the PR that's adding a regression test for this, we should close this issue after the regression test lands