> From: Tyler Retzlaff [mailto:roret...@linux.microsoft.com] > Sent: Tuesday, 18 April 2023 17.15 > > On Tue, Apr 18, 2023 at 12:06:42PM +0100, Bruce Richardson wrote: > > On Tue, Apr 11, 2023 at 08:48:45AM +0200, Morten Brørup wrote: > > > When getting objects from the mempool, the number of objects to get > is > > > often constant at build time. > > > > > > This patch adds another code path for this case, so the compiler can > > > optimize more, e.g. unroll the copy loop when the entire request is > > > satisfied from the cache. > > > > > > On an Intel(R) Xeon(R) E5-2620 v4 CPU, and compiled with gcc 9.4.0, > > > mempool_perf_test with constant n shows an increase in rate_persec > by an > > > average of 17 %, minimum 9.5 %, maximum 24 %. > > > > > > The code path where the number of objects to get is unknown at build > time > > > remains essentially unchanged. > > > > > > Signed-off-by: Morten Brørup <m...@smartsharesystems.com> > > > > Change looks a good idea. Some suggestions inline below, which you may > want to > > take on board for any future version. I'd strongly suggest adding some > > extra clarifying code comments, as I suggest below. > > With those exta code comments: > > > > Acked-by: Bruce Richardson <bruce.richard...@intel.com> > > > > > --- > > > lib/mempool/rte_mempool.h | 24 +++++++++++++++++++++--- > > > 1 file changed, 21 insertions(+), 3 deletions(-) > > > > > > diff --git a/lib/mempool/rte_mempool.h b/lib/mempool/rte_mempool.h > > > index 9f530db24b..ade0100ec7 100644 > > > --- a/lib/mempool/rte_mempool.h > > > +++ b/lib/mempool/rte_mempool.h > > > @@ -1500,15 +1500,33 @@ rte_mempool_do_generic_get(struct > rte_mempool *mp, void **obj_table, > > > if (unlikely(cache == NULL)) > > > goto driver_dequeue; > > > > > > - /* Use the cache as much as we have to return hot objects first */ > > > - len = RTE_MIN(remaining, cache->len); > > > cache_objs = &cache->objs[cache->len]; > > > + > > > + if (__extension__(__builtin_constant_p(n)) && n <= cache->len) { > > don't take direct dependency on compiler builtins. define a macro so we > don't have to play shotgun surgery later. > > also what is the purpose of using __extension__ here? are you annotating > the use of __builtin_constant_p() or is there more? because if that's > the only reason i see no need to use __extension__ when already using a > compiler specific builtin like this, that it is not standard is implied > and enforced by a compile break.
ARM 32-bit memcpy() [1] does it this way, so I did the same. [1]: https://elixir.bootlin.com/dpdk/v23.03/source/lib/eal/arm/include/rte_memcpy_32.h#L122 While I agree that a macro for __builtin_constant_p() would be good, it belongs in a patch to fix portability, not in this patch.
