Use a bitmap in extents_t to speed up search.

Rather than iteratively checking all sufficiently large heaps during
search, maintain and use a bitmap in order to skip empty heaps.

include/jemalloc/internal/bitmap_types.h

@@ -2,7 +2,13 @@
 #define JEMALLOC_INTERNAL_BITMAP_TYPES_H
 
 /* Maximum bitmap bit count is 2^LG_BITMAP_MAXBITS. */
-#define LG_BITMAP_MAXBITS	LG_SLAB_MAXREGS
+#if LG_SLAB_MAXREGS > LG_CEIL_NSIZES
+/* Maximum bitmap bit count is determined by maximum regions per slab. */
+#  define LG_BITMAP_MAXBITS	LG_SLAB_MAXREGS
+#else
+/* Maximum bitmap bit count is determined by number of extent size classes. */
+#  define LG_BITMAP_MAXBITS	LG_CEIL_NSIZES
+#endif
 #define BITMAP_MAXBITS		(ZU(1) << LG_BITMAP_MAXBITS)
 
 typedef struct bitmap_level_s bitmap_level_t;

include/jemalloc/internal/extent_structs.h

@@ -102,6 +102,13 @@ struct extents_s {
 	 */
 	extent_heap_t		heaps[NPSIZES+1];
 
+	/*
+	 * Bitmap for which set bits correspond to non-empty heaps.
+	 *
+	 * Synchronization: mtx.
+	 */
+	bitmap_t		bitmap[BITMAP_GROUPS(NPSIZES+1)];
+
 	/*
 	 * LRU of all extents in heaps.
 	 *