Cache an arena's index in the arena.

This saves us a pointer hop down some perf-sensitive paths.

include/jemalloc/internal/arena_inlines_a.h

@@ -3,7 +3,7 @@
 
 static inline unsigned
 arena_ind_get(const arena_t *arena) {
-	return base_ind_get(arena->base);
+	return arena->ind;
 }
 
 static inline void

include/jemalloc/internal/arena_structs.h

@@ -83,6 +83,12 @@ struct arena_s {
 	 */
 	bins_t			bins[SC_NBINS];
 
+	/*
+	 * A cached copy of base->ind.  This can get accessed on hot paths;
+	 * looking it up in base requires an extra pointer hop / cache miss.
+	 */
+	unsigned ind;
+
 	/*
 	 * Base allocator, from which arena metadata are allocated.
 	 *

src/arena.c

@@ -1475,6 +1475,7 @@ arena_new(tsdn_t *tsdn, unsigned ind, extent_hooks_t *extent_hooks) {
 	arena->base = base;
 	/* Set arena before creating background threads. */
 	arena_set(ind, arena);
+	arena->ind = ind;
 
 	nstime_init_update(&arena->create_time);