[PATCH v4] mempool: optimize get objects with constant n

["Morten Brørup" <mb@smartsharesystems.com>]

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 <mb@smartsharesystems.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>

---
v4:
* Rebased to main.
v3:
* Tried to rebase to main, but failed to include all my changes.
v2:
* Added comments describing why some code is omitted when 'n' is known
  at buuild time.
* Improved source code readability by explicitly setting 'remaining' where
  relevant, instead of at initialization.
---
 lib/mempool/rte_mempool.h | 41 +++++++++++++++++++++++++++++++++------
 1 file changed, 35 insertions(+), 6 deletions(-)

diff --git a/lib/mempool/rte_mempool.h b/lib/mempool/rte_mempool.h
index 9f530db24b..7d14dea5c4 100644
--- a/lib/mempool/rte_mempool.h
+++ b/lib/mempool/rte_mempool.h
@@ -1492,23 +1492,52 @@ rte_mempool_do_generic_get(struct rte_mempool *mp, void **obj_table,
 			   unsigned int n, struct rte_mempool_cache *cache)
 {
 	int ret;
-	unsigned int remaining = n;
+	unsigned int remaining;
 	uint32_t index, len;
 	void **cache_objs;
 
 	/* No cache provided */
-	if (unlikely(cache == NULL))
+	if (unlikely(cache == NULL)) {
+		remaining = n;
 		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) {
+		/*
+		 * The request size is known at build time, and
+		 * the entire request can be satisfied from the cache,
+		 * so let the compiler unroll the fixed length copy loop.
+		 */
+		cache->len -= n;
+		for (index = 0; index < n; index++)
+			*obj_table++ = *--cache_objs;
+
+		RTE_MEMPOOL_CACHE_STAT_ADD(cache, get_success_bulk, 1);
+		RTE_MEMPOOL_CACHE_STAT_ADD(cache, get_success_objs, n);
+
+		return 0;
+	}
+
+	/*
+	 * Use the cache as much as we have to return hot objects first.
+	 * If the request size 'n' is known at build time, the above comparison
+	 * ensures that n > cache->len here, so omit RTE_MIN().
+	 */
+	len = __extension__(__builtin_constant_p(n)) ? cache->len :
+			RTE_MIN(n, cache->len);
 	cache->len -= len;
-	remaining -= len;
+	remaining = n - len;
 	for (index = 0; index < len; index++)
 		*obj_table++ = *--cache_objs;
 
-	if (remaining == 0) {
+	/*
+	 * If the request size 'n' is known at build time, the case
+	 * where the entire request can be satisfied from the cache
+	 * has already been handled above, so omit handling it here.
+	 */
+	if (!__extension__(__builtin_constant_p(n)) && remaining == 0) {
 		/* The entire request is satisfied from the cache. */
 
 		RTE_MEMPOOL_CACHE_STAT_ADD(cache, get_success_bulk, 1);
-- 
2.17.1