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.
 	 *