Re: [PATCH net-next V2 3/3] net: Add GRO support for vxlan traffic

On Tue, Jan 7, 2014 at 12:12 PM, Or Gerlitz [off-list ref] wrote:

On Tue, Jan 7, 2014 at 10:02 PM, Eric Dumazet [off-list ref] wrote:

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On Tue, 2014-01-07 at 21:43 +0200, Or Gerlitz wrote:

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On Tue, Jan 7, 2014 at 8:08 PM, Tom Herbert [off-list ref] wrote:

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Why ^ instead of != ?

The XOR approach is very popular in the GRO stack, e.g see the IPv4 chain
of inet_gro_receive() && tcp_gro_receive(), I guess this might relates
to more efficient assembly code for ^ vs. != and/or the fast/elegant
transitive nature of that operator

This trick is only needed/used when many compares are folded into a
single conditional :

if (a->f1 != b->f1 || a->f2 != b->f2)

->

if (((a->f1 ^ b->f1) | (a->f2 ^ b->f2)) != 0)

Please do not use XOR for a single compare.

OK, but just out of curiosity -- what's the reasoning? clarity or
efficiency or both?

Both. Compiling a simple program and comparing alternatives: gcc
produced the identical code for the single conditional (^ vs !=)
using the cmp instruction. Testing the two conditional case like Eric
provided; the second method (using ^) resulted in 4 more instructions,
but only one branch as opposed to two in the first method (!=). Method
#1 has the advantage of short circuiting when the first condition is
true, so organizing the conditionals to maximize the probability of
short circuit could be beneficial.

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