From: Tariq Toukan
> Sent: 15 June 2017 12:36
> Define LOG_TXBB_SIZE, log of TXBB_SIZE, and use it with a shift
> operation instead of a multiplication with TXBB_SIZE.
> Operations are equivalent as TXBB_SIZE is a power of two.
>
> Performance tests:
> Tested on ConnectX3Pro, Intel(R) Xeon(R) CPU E5-2680 v3 @ 2.50GHz
>
> Gain is too small to be measurable, no degradation sensed.
> Results are similar for IPv4 and IPv6.
I can't imagine there is any difference at all.
The compiler will use a shift for a 'multiply by a constant power of 2'.
...
If you want to save a few cycles I think the loop:
> - for (i = 0; i < tx_info->nr_txbb * TXBB_SIZE;
> + for (i = 0; i < tx_info->nr_txbb << LOG_TXBB_SIZE;
requires the compiler generate code to read nr_txbb every
iteration.
Caching the values might help (unless it causes a different
register spill).
David
