Arena: share bin offsets in a global.

This saves us a cache miss when lookup up the arena bin offset in a remote
arena during tcache flush.  All arenas share the base offset, and so we don't
need to look it up repeatedly for each arena.  Secondarily, it shaves 288 bytes
off the arena on, e.g., x86-64.

include/jemalloc/internal/arena_externs.h

@@ -23,6 +23,12 @@ extern emap_t arena_emap_global;
 extern size_t opt_oversize_threshold;
 extern size_t oversize_threshold;
 
+/*
+ * arena_bin_offsets[binind] is the offset of the first bin shard for size class
+ * binind.
+ */
+extern uint32_t arena_bin_offsets[SC_NBINS];
+
 void arena_basic_stats_merge(tsdn_t *tsdn, arena_t *arena,
     unsigned *nthreads, const char **dss, ssize_t *dirty_decay_ms,
     ssize_t *muzzy_decay_ms, size_t *nactive, size_t *ndirty, size_t *nmuzzy);

include/jemalloc/internal/arena_inlines_b.h

@@ -534,4 +534,10 @@ arena_dalloc_bin_locked_finish(tsdn_t *tsdn, arena_t *arena, bin_t *bin,
 	}
 }
 
+static inline bin_t *
+arena_get_bin(arena_t *arena, szind_t binind, unsigned binshard) {
+	bin_t *shard0 = (bin_t *)((uintptr_t)arena + arena_bin_offsets[binind]);
+	return shard0 + binshard;
+}
+
 #endif /* JEMALLOC_INTERNAL_ARENA_INLINES_B_H */

include/jemalloc/internal/arena_structs.h

@@ -76,13 +76,6 @@ struct arena_s {
 	/* The page-level allocator shard this arena uses. */
 	pa_shard_t		pa_shard;
 
-	/*
-	 * bins is used to store heaps of free regions.
-	 *
-	 * Synchronization: internal.
-	 */
-	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.
@@ -97,6 +90,12 @@ struct arena_s {
 	base_t			*base;
 	/* Used to determine uptime.  Read-only after initialization. */
 	nstime_t		create_time;
+
+	/*
+	 * The arena is allocated alongside its bins; really this is a
+	 * dynamically sized array determined by the binshard settings.
+	 */
+	bin_t			bins[0];
 };
 
 /* Used in conjunction with tsd for fast arena-related context lookup. */