Make arena stats collection go through cache bins.

This eliminates the need for the arena stats code to "know" about tcaches; all
that it needs is a cache_bin_array_descriptor_t to tell it where to find
cache_bins whose stats it should aggregate.

include/jemalloc/internal/tcache_structs.h

@@ -7,21 +7,46 @@
 #include "jemalloc/internal/ticker.h"
 
 struct tcache_s {
-	/* Data accessed frequently first: prof, ticker and small bins. */
-	uint64_t	prof_accumbytes;/* Cleared after arena_prof_accum(). */
-	ticker_t	gc_ticker;	/* Drives incremental GC. */
+	/*
+	 * To minimize our cache-footprint, we put the frequently accessed data
+	 * together at the start of this struct.
+	 */
+
+	/* Cleared after arena_prof_accum(). */
+	uint64_t	prof_accumbytes;
+	/* Drives incremental GC. */
+	ticker_t	gc_ticker;
 	/*
 	 * The pointer stacks associated with bins follow as a contiguous array.
 	 * During tcache initialization, the avail pointer in each element of
 	 * tbins is initialized to point to the proper offset within this array.
 	 */
 	cache_bin_t	bins_small[NBINS];
-	/* Data accessed less often below. */
-	ql_elm(tcache_t) link;		/* Used for aggregating stats. */
-	arena_t		*arena;		/* Associated arena. */
-	szind_t		next_gc_bin;	/* Next bin to GC. */
+
+	/*
+	 * This data is less hot; we can be a little less careful with our
+	 * footprint here.
+	 */
+	/* Lets us track all the tcaches in an arena. */
+	ql_elm(tcache_t) link;
+	/*
+	 * The descriptor lets the arena find our cache bins without seeing the
+	 * tcache definition.  This enables arenas to aggregate stats across
+	 * tcaches without having a tcache dependency.
+	 */
+	cache_bin_array_descriptor_t cache_bin_array_descriptor;
+
+	/* The arena this tcache is associated with. */
+	arena_t		*arena;
+	/* Next bin to GC. */
+	szind_t		next_gc_bin;
 	/* For small bins, fill (ncached_max >> lg_fill_div). */
 	uint8_t		lg_fill_div[NBINS];
+	/*
+	 * We put the cache bins for large size classes at the end of the
+	 * struct, since some of them might not get used.  This might end up
+	 * letting us avoid touching an extra page if we don't have to.
+	 */
 	cache_bin_t	bins_large[NSIZES-NBINS];
 };