Arithmetic not folded

[jrmuizel]

The following program:

#![crate_type = "lib"]
use std::ptr;
use std::slice;
pub struct RawSlice<'a>
{
        s: &'a mut [u8]
}
impl<'a> RawSlice<'a> {
    fn write(&mut self, buf: &[u8])  {
        let dest_data = self.s.as_mut_ptr();
        let src_data = buf.as_ptr();
        unsafe {
            ptr::copy_nonoverlapping(src_data, dest_data, buf.len());
            self.s = slice::from_raw_parts_mut(dest_data.offset(buf.len() as isize), self.s.len() - buf.len());
        }
    }
}
pub fn do_write(r: &mut RawSlice, k: [u8; 4]) {
    r.write(&[1, 2, 3, 4]);
    r.write(&[1, 2, 3, 4]);
    r.write(&[1, 2, 3, 4]);
    r.write(&[1, 2, 3, 4]);
}

Compiles to:

        pushq   %rbp
        movq    %rsp, %rbp
        movq    (%rdi), %rax
        movl    $67305985, (%rax)
        addq    $-4, 8(%rdi)
        movl    $67305985, 4(%rax)
        addq    $-4, 8(%rdi)
        movl    $67305985, 8(%rax)
        addq    $-4, 8(%rdi)
        movl    $67305985, 12(%rax)
        addq    $16, %rax
        movq    8(%rdi), %rcx
        addq    $-4, %rcx
        movq    %rax, (%rdi)
        movq    %rcx, 8(%rdi)
        popq    %rbp
        retq

Ideally all of the addq -4 would be folded together. I suspect that either rust is emitting the wrong aliasing info or llvm is not properly dealing with the information.

[jrmuizel]

I guess this might be #31681

[jrmuizel]

I confirmed that reverting #31545 fixes this.

[steveklabnik]

Triage:

1. the code still does not optimize properly today
2. @jrmuizel confirmed that it's because we removed the noalias stuff
3. since this has been filed, we've re-enabled and then re-disabled noalias again.
4. this means that this is an effective dup of Enable noalias annotations #54878; the new 're-enable noalias' bug.

Closing! Thanks for the report.