Iterating over ranges generates overflow check with opt-level = "z" and lto enabled

[manuelVo]

The following code generates an overflow check, when opt-level="z" and lto=true are set in the Cargo.toml. As the implementation of Iterator for Range states this check shouldn't actually be there: https://doc.rust-lang.org/src/core/iter/range.rs.html#215

My build target is x86_64-unknown-linux-gnu. I'm building with cargo build --release.

This is the code producing the bug:

fn main() {
    let arr1 = [1, 2, 3];
    let arr2 = [4, 5];
    println!("{} {}", add(&arr1), add(&arr2));
}

#[inline(never)]
pub extern fn add(numbers: &[i32]) -> i32 {
    let mut result = 0;
    for i in 0..numbers.len() {
        result += numbers[i];
    }
    result
}

Combined with this Cargo.toml:

[package]
name = "rust_binary"
version = "0.1.0"

[profile.release]
lto = true
opt-level = "z"

This is the resulting add function:

0000000000006a76 <rust_binary::add>:
    6a76: xor    ecx,ecx
    6a78: xor    eax,eax
    6a7a: jmp    6a8b <rust_binary::add+0x15>
    6a7c: mov    rdx,rcx
    6a7f: add    rdx,0x1
    6a83: jb     6a90 <rust_binary::add+0x1a>   # This is the overflow check
    6a85: add    eax,DWORD PTR [rdi+rcx*4]
    6a88: inc    rcx
    6a8b: cmp    rcx,rsi
    6a8e: jb     6a7c <rust_binary::add+0x6>
    6a90: ret    

Meta

rustc 1.30.0-nightly (33b923fd4 2018-08-18)
binary: rustc
commit-hash: 33b923fd44c5c5925e635815fce68bdf1f98740f
commit-date: 2018-08-18
host: x86_64-unknown-linux-gnu
release: 1.30.0-nightly
LLVM version: 7.0

[manuelVo]

Setting codegen-units=1 fixes this, so this might be a duplicate of #48371.

[workingjubilee]

Current codegen with an equivalent Godbolt:

example::add:
        xor     ecx, ecx
        xor     eax, eax
.LBB0_1:
        cmp     rsi, rcx
        je      .LBB0_2
        add     eax, dword ptr [rdi + 4*rcx]
        inc     rcx
        jmp     .LBB0_1
.LBB0_2:
        ret

which matches with the objdump of a cargo project I did just to check:

0000000000005ae7 <add>:
    5ae7:	31 c9                	xor    ecx,ecx
    5ae9:	31 c0                	xor    eax,eax
    5aeb:	48 39 ce             	cmp    rsi,rcx
    5aee:	74 08                	je     5af8 <add+0x11>
    5af0:	03 04 8f             	add    eax,DWORD PTR [rdi+rcx*4]
    5af3:	48 ff c1             	inc    rcx
    5af6:	eb f3                	jmp    5aeb <add+0x4>
    5af8:	c3                   	ret    

Appears to be resolved.