On Tuesday 2008-04-01 14:49, Patrick McHardy wrote:
Jan Engelhardt wrote:
[NETFILTER]: xt_hashlimit: add workaround for >>32 case
Hardware surprisingly does nothing when a 32-bit right-shift is
to be done. Worse yet, compilers do not even work around it.
Thats because the C standard states that the result is undefined.
Anyways, I think this patch is slightly nicer because it
gets rid of the double negation and the %32 == 0 special-casing
for IPv6.
Do you want to add an ACKed-by?
The special casing for %32==0 in the IPv6 block was not a workaround for
'>>32', but speed. Assuming the C code is already perfect and gets
transformed 1:1 into assembly without any further optimization, the cost
of the operation is 4 simple 'mov' instructions for %32==0, whereas
calling maskl() will involve a fair number of operations (at least 27 by
the count). [With your patch, maskl() is now 28 + a branch.]
Not that it really matters to me, I submitted patch (no replies yet)
that replaces it with the pfxlen static map anyway :->
#if defined(CONFIG_IP6_NF_IPTABLES) || defined(CONFIG_IP6_NF_IPTABLES_MODULE)
static void hashlimit_ipv6_mask(__be32 *i, unsigned int p)
{
switch (p) {
- case 0:
- i[0] = i[1] = 0;
- i[2] = i[3] = 0;
- break;
- case 1 ... 31:
+ case 0 ... 31:
i[0] = maskl(i[0], p);
i[1] = i[2] = i[3] = 0;
break;
- case 32:
- i[1] = i[2] = i[3] = 0;
- break;
- case 33 ... 63:
+ case 32 ... 63:
i[1] = maskl(i[1], p - 32);
i[2] = i[3] = 0;
break;
- case 64:
- i[2] = i[3] = 0;
- break;
- case 65 ... 95:
+ case 64 ... 95:
i[2] = maskl(i[2], p - 64);
i[3] = 0;
- case 96:
- i[3] = 0;
- break;
- case 97 ... 127:
+ case 96 ... 127:
i[3] = maskl(i[3], p - 96);
break;
case 128:
--
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