Unnecessary comparison and conditional branch

[tspiteri]

This is a reopening of #37114 which was fixed in 1.28.0 but seems to have regressed again since 1.30.0.

pub fn foo(v: &mut Vec<i32>) {
    let len = v.len();
    assert!(len > 1);
    v[len - 2] += 1;
    v.pop();
}

The asm for 1.38 is

example::foo:
        push    rax
        mov     rax, qword ptr [rdi + 16]
        cmp     rax, 2
        jb      .LBB7_4
        mov     rcx, qword ptr [rdi]
        add     dword ptr [rcx + 4*rax - 8], 1
; 1.28 does not generate these 3 instructions:
        mov     rax, qword ptr [rdi + 16]
        test    rax, rax
        je      .LBB7_3
; end of unnecessary code
        add     rax, -1
        mov     qword ptr [rdi + 16], rax
.LBB7_3:
        pop     rax
        ret
.LBB7_4:
        call    std::panicking::begin_panic
        ud2

https://godbolt.org/z/SLPlgo

[tspiteri]

Hmm, this actually regressed in 1.29.2, so it's probably due to #54639.

[bluss]

Yes, -Z mutable-noalias makes the difference here. Will be fixed by #54878 again the same way :)

[nikic]

Fixed in nightly now that mutable-noalias is enabled again.