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
