netdev - RE: [PATCH v2 net] net: solve a NAPI race

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Date:   Wed, 1 Mar 2017 10:41:37 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Alexander Duyck' <alexander.duyck@...il.com>,
        Eric Dumazet <eric.dumazet@...il.com>
CC:     David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Tariq Toukan <tariqt@...lanox.com>,
        Saeed Mahameed <saeedm@...lanox.com>
Subject: RE: [PATCH v2 net] net: solve a NAPI race

From: Alexander Duyck
> Sent: 28 February 2017 17:20
...
> You might want to consider just using a combination AND, divide,
> multiply, and OR to avoid having to have any conditional branches
> being added due to this code path.  Basically the logic would look
> like:
>     new = val | NAPIF_STATE_SCHED;
>     new |= (val & NAPIF_STATE_SCHED) / NAPIF_STATE_SCHED * NAPIF_STATE_MISSED;
> 
> In assembler that all ends up getting translated out to AND, SHL, OR.
> You avoid the branching, or MOV/OR/TEST/CMOV type code you would end
> up with otherwise.

It is a shame gcc doesn't contain that optimisation.
It also doesn't even make a good job of (a & b)/b * c since it
always does a shr and a sal (gcc 4.7.3 and 5.4).

Worthy of a #define or static inline.
Something like:
#define BIT_IF(v, a, b) ((b & (b-1) ? (v & a)/a * b : a > b ? (v & a) / (a/b) : (v & a) * (b/a))

	David