Le jeudi 25 novembre 2010 Ã 15:41 +0100, Jozsef Kadlecsik a Ãcrit :
...
> +/* __jhash_mix -- mix 3 32-bit values reversibly. */
> +#define __jhash_mix(a, b, c) \
> +{ \
> + a -= c; a ^= rol32(c, 4); c += b; \
> + b -= a; b ^= rol32(a, 6); a += c; \
> + c -= b; c ^= rol32(b, 8); b += a; \
> + a -= c; a ^= rol32(c, 16); c += b; \
> + b -= a; b ^= rol32(a, 19); a += c; \
> + c -= b; c ^= rol32(b, 4); b += a; \
> +}
> +
> +/* __jhash_final - final mixing of 3 32-bit values (a,b,c) into c */
> +#define __jhash_final(a, b, c) \
> +{ \
> + c ^= b; c -= rol32(b, 14); \
> + a ^= c; a -= rol32(c, 11); \
> + b ^= a; b -= rol32(a, 25); \
> + c ^= b; c -= rol32(b, 16); \
> + a ^= c; a -= rol32(c, 4); \
> + b ^= a; b -= rol32(a, 14); \
> + c ^= b; c -= rol32(b, 24); \
> +}
> +
So we now have a special __jhash_final(a, b, c) thing for the last
values.
> +/* jhash_3words - hash exactly 3, 2 or 1 word(s) */
> +u32 jhash_3words(u32 a, u32 b, u32 c, u32 initval)
> +{
> + a += JHASH_INITVAL;
> + b += JHASH_INITVAL;
> + c += initval;
> +
> + __jhash_mix(a, b, c);
> +
> + return c;
> +}
> +EXPORT_SYMBOL(jhash_3words);
But you dont use it in jhash_3words().
I do think jhash_3words() should stay inlined, unless maybe
CONFIG_CC_OPTIMIZE_FOR_SIZE=y
We hit it several time per packet in network stack in RX path.
Once in skb_get_rxhash() (unless device fills skb->rxhash)
Once at least in conntrack (if used).
Once in UDP or TCP stack
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