From: Alexey Dobriyan
> Sent: 23 March 2017 23:33
> Current addr4_match() code has special test for /0 prefixes because of
> standard required undefined behaviour. However, it is possible to omit
> it on 64-bit because shifting can be done in a 64-bit register and then
> truncated to the expected value (which is 0 mask).
>
> Implicit truncation by htonl() fits nicely into R32-within-R64 model
> on x86-64.
...
> static inline bool addr4_match(__be32 a1, __be32 a2, u8 prefixlen)
> {
> +#ifdef CONFIG_64BIT
> + /* L in UL is not a typo. */
> + return !((a1 ^ a2) & htonl(~0UL << (32 - prefixlen)));
> +#else
> /* C99 6.5.7 (3): u32 << 32 is undefined behaviour */
> if (prefixlen == 0)
> return true;
> return !((a1 ^ a2) & htonl(0xFFFFFFFFu << (32 - prefixlen)));
> +#endif
Can't this just be written:
if (sizeof (long) == 4 && prefixlen == 0)
return true;
return !((a1 ^ a2) & htonl(0xFFFFFFFFUL << (32 - prefixlen)));
David
