Move extra size checks behind a config flag.

This will let us turn that flag into a generic "turn on runtime checks" flag
that guards other functionality we have planned.

src/tcache.c

@@ -101,7 +101,6 @@ tcache_alloc_small_hard(tsdn_t *tsdn, arena_t *arena, tcache_t *tcache,
 }
 
 /* Enabled with --enable-extra-size-check. */
-#ifdef JEMALLOC_EXTRA_SIZE_CHECK
 static void
 tbin_extents_lookup_size_check(tsdn_t *tsdn, cache_bin_t *tbin, szind_t binind,
     size_t nflush, extent_t **extents){
@@ -129,7 +128,6 @@ tbin_extents_lookup_size_check(tsdn_t *tsdn, cache_bin_t *tbin, szind_t binind,
 		abort();
 	}
 }
-#endif
 
 void
 tcache_bin_flush_small(tsd_t *tsd, tcache_t *tcache, cache_bin_t *tbin,
@@ -144,15 +142,16 @@ tcache_bin_flush_small(tsd_t *tsd, tcache_t *tcache, cache_bin_t *tbin,
 	unsigned nflush = tbin->ncached - rem;
 	VARIABLE_ARRAY(extent_t *, item_extent, nflush);
 
-#ifndef JEMALLOC_EXTRA_SIZE_CHECK
 	/* Look up extent once per item. */
-	for (unsigned i = 0 ; i < nflush; i++) {
-		item_extent[i] = iealloc(tsd_tsdn(tsd), *(tbin->avail - 1 - i));
+	if (config_opt_safety_checks) {
+		tbin_extents_lookup_size_check(tsd_tsdn(tsd), tbin, binind,
+		    nflush, item_extent);
+	} else {
+		for (unsigned i = 0 ; i < nflush; i++) {
+			item_extent[i] = iealloc(tsd_tsdn(tsd),
+			    *(tbin->avail - 1 - i));
+		}
 	}
-#else
-	tbin_extents_lookup_size_check(tsd_tsdn(tsd), tbin, binind, nflush,
-	    item_extent);
-#endif
 	while (nflush > 0) {
 		/* Lock the arena bin associated with the first object. */
 		extent_t *extent = item_extent[0];