Interesting investigation!

I had an experiment with getting the Rust compiler to vectorise things itself, and it seems LLVM does a pretty good job automatically, e.g. on my computer (x86-64), running `rustc -O bytesum.rs` optimises the core of the addition:

  fn inner(x: &[u8]) -> u8 {
      let mut s = 0;
      for b in x.iter() {
          s += *b;
      }
      s
  }

to

  .LBB0_6:
  	movdqa	%xmm1, %xmm2
  	movdqa	%xmm0, %xmm3
  	movdqu	-16(%rsi), %xmm0
  	movdqu	(%rsi), %xmm1
  	paddb	%xmm3, %xmm0
  	paddb	%xmm2, %xmm1
  	addq	$32, %rsi
  	addq	$-32, %rdi
  	jne	.LBB0_6

I can convince clang to automatically vectorize the inner loop in [1] to equivalent code (by passing -O3), but I can't seem to get GCC to do anything but a byte-by-byte tranversal.

[1]: https://github.com/jvns/howcomputer/blob/master/bytesum.c