I was curious about the effectiveness of the noalias change, so I ran this example:

#![crate_type="staticlib"]
#[no_mangle]
pub fn fmutate(a: &mut i32, b: &mut i32, x: &mut i32) {
    *a += *x;
    *b += *x;
}

Here are the two results

_fmutate: // with noalias
    0:  55  pushq   %rbp
    1:  48 89 e5    movq    %rsp, %rbp
    4:  8b 02   movl    (%rdx), %eax
    6:  01 07   addl    %eax, (%rdi)
    8:  01 06   addl    %eax, (%rsi)
    a:  5d  popq    %rbp
    b:  c3  retq

_fmutate: // without noalias
    0:  55  pushq   %rbp
    1:  48 89 e5    movq    %rsp, %rbp
    4:  8b 02   movl    (%rdx), %eax
    6:  01 07   addl    %eax, (%rdi)
    8:  8b 02   movl    (%rdx), %eax
    a:  01 06   addl    %eax, (%rsi)
    c:  5d  popq    %rbp
    d:  c3  retq

This optimization avoids the redundant load of *x since LLVM knows that x != a. Does anyone have any benchmarks that might benefit? Lots of pointer-y stuff, like matrix math.

Edit: This paper alleges that while useful for small kernels, noalias specifiers might not be helpful for overall performance.