Re: [PATCH net-next] ipv6: Implement optimized IPv6 masked address comparison for ARM64

quoted

That's clearly not right - I'm not sure quite what undefined behaviour
assumption convinces GCC to optimise the whole thing away>

While the pointer casting is a bit ghastly, I don't actually think that
GCC is taking advantage of undefined behaviour here, rather it looks 
like
you have a simple typo on line 3:

quoted

	const __uint128_t *ul1 = (const __uint128_t *)a1;
	const __uint128_t *ulm = (const __uint128_t *)m;
	const __uint128_t *ul2 = (const __uint128_t *)a1;

ul2 = a2, surely?

As it is (stripping casts) you have a1 ^ a1, which will get you to 0
pretty quickly. Fixing that up for you;

  bool
  ipv6_masked_addr_cmp_new(const struct in6_addr *a1, const struct
  in6_addr *m,
  		     const struct in6_addr *a2)
  {
  	const __uint128_t *ul1 = (const __uint128_t *)a1;
  	const __uint128_t *ulm = (const __uint128_t *)m;
  	const __uint128_t *ul2 = (const __uint128_t *)a2;

  	return !!((*ul1 ^ *ul2) & *ulm);
  }

$ gcc -O2

  ipv6_masked_addr_cmp_new:
	ldp	x4, x3, [x0]
	ldp	x5, x2, [x2]
	ldp	x0, x1, [x1]
	eor	x4, x4, x5
	eor	x2, x3, x2
	and	x0, x0, x4
	and	x1, x1, x2
	orr	x0, x0, x1
	cmp	x0, 0
	cset	w0, ne
	ret

Which at least looks like it might calculate something useful :-)

Hi Robin / James

Thanks for checking and sorry for the confusion. I'll retest this.