Rust unable to optimize away assert vs. clang.

[jrmuizel]

pub fn f(arr: &mut [u32]) {
    for i in 0..arr.len() {
        for j in 0..i {
            assert!(j < arr.len());
        }
    }
}

Rustc does not optimize away the the assert. Adding -C passes=constraint-elimination doesn't help.

However, clang does optimize away this assert when compiled with -O2 -mllvm -enable-constraint-elimination

void f(int *ptr, size_t len) {
    for (size_t i = 0; i < len; i++) {
            for (size_t j = 0; j < i; j++) {
                assert(j < len);
            }
    }
}

GCC is also able to remove the assert with GCC trunk.

[dalcde]

More generally, rust/llvm does not understand that inequality is transitive. The following code generates three inequality asserts in the assembly:

pub fn cmp(a: u32, b: u32, c: u32) {
    assert!(a < b);
    assert!(b < c);
    assert!(a < c);
}

Rust playground: https://play.rust-lang.org/?version=stable&mode=release&edition=2021&gist=8cfdb982df6b7e781b398b9408b3cd10

[jrmuizel]

More generally, rust/llvm does not understand that inequality is transitive. The following code generates three inequality asserts in the assembly:

Adding -C passes=constraint-elimination fixes that.

[jrmuizel]

but not if the types are changed to i32 for which I filed: llvm/llvm-project#53273

[the8472]

Assuming you need this for slicing then creating a temporary works.

pub fn f(arr: &mut [u32]) {
    for i in 0..arr.len() {
        let subslice = &arr[0..i];
        for j in 0..subslice.len() {
            assert!(j < subslice.len());
        }
    }
}

https://rust.godbolt.org/z/n6sa6Gf34

[thvdveld]

Since llvm/llvm-project#53273 is now fixed, when is this going to land into nightly Rust (I have no idea when Rust updates LLVM)? And should it fix the origin issue?

[nikic]

This still requires ConstraintElimination to be enabled on the LLVM side. If it is, then this will land with the LLVM 15 upgrade in ~August.

[andjo403]

this is fixed in latest stable rustc 1.73 see https://rust.godbolt.org/z/G1E35e375