Hide bin slab-locality query behind arena_locality_hint

bin_t is an arena implementation detail; tcache should not reach into
it. Extract the slab-address lookup into bin.c as bin_current_slab_addr,
and expose it to tcache only through arena_locality_hint(tsdn, arena,
szind), which composes bin_choose + bin_current_slab_addr.

include/jemalloc/internal/arena_externs.h

@@ -111,6 +111,7 @@ arena_t *arena_choose_huge(tsd_t *tsd);
 size_t arena_fill_small_fresh(tsdn_t *tsdn, arena_t *arena, szind_t binind,
     void **ptrs, size_t nfill, bool zero);
 bool   arena_boot(sc_data_t *sc_data, base_t *base, bool hpa);
+void  *arena_locality_hint(tsdn_t *tsdn, arena_t *arena, szind_t szind);
 void   arena_prefork0(tsdn_t *tsdn, arena_t *arena);
 void   arena_prefork1(tsdn_t *tsdn, arena_t *arena);
 void   arena_prefork2(tsdn_t *tsdn, arena_t *arena);

include/jemalloc/internal/bin.h

@@ -95,6 +95,9 @@ bool  bin_refill_slabcur_no_fresh_slab(tsdn_t *tsdn, bool is_auto,
 void *bin_malloc_no_fresh_slab(tsdn_t *tsdn, bool is_auto, bin_t *bin,
     szind_t binind);
 
+/* Slab queries. */
+void *bin_current_slab_addr(tsdn_t *tsdn, bin_t *bin);
+
 /* Bin selection. */
 bin_t *bin_choose(tsdn_t *tsdn, arena_t *arena, szind_t binind,
     unsigned *binshard_p);

src/arena.c

@@ -211,6 +211,14 @@ arena_stats_merge(tsdn_t *tsdn, arena_t *arena, unsigned *nthreads,
 	}
 }
 
+void *
+arena_locality_hint(tsdn_t *tsdn, arena_t *arena, szind_t szind) {
+	assert(szind < SC_NBINS);
+	bin_t *bin = bin_choose(tsdn, arena, szind, NULL);
+	assert(bin != NULL);
+	return bin_current_slab_addr(tsdn, bin);
+}
+
 static void
 arena_background_thread_inactivity_check(
     tsdn_t *tsdn, arena_t *arena, bool is_background_thread) {

src/bin.c

@@ -330,3 +330,16 @@ bin_choose(tsdn_t *tsdn, arena_t *arena, szind_t binind,
 	}
 	return arena_get_bin(arena, binind, binshard);
 }
+
+void *
+bin_current_slab_addr(tsdn_t *tsdn, bin_t *bin) {
+	malloc_mutex_lock(tsdn, &bin->lock);
+	edata_t *slab = (bin->slabcur != NULL)
+	    ? bin->slabcur
+	    : edata_heap_first(&bin->slabs_nonfull);
+	assert(slab != NULL || edata_heap_empty(&bin->slabs_nonfull));
+	void *ret = (slab != NULL) ? edata_addr_get(slab) : NULL;
+	assert(ret != NULL || slab == NULL);
+	malloc_mutex_unlock(tsdn, &bin->lock);
+	return ret;
+}

src/tcache.c

@@ -203,26 +203,6 @@ tcache_gc_item_delay_compute(szind_t szind) {
 	return (uint8_t)item_delay;
 }
 
-static inline void *
-tcache_gc_small_heuristic_addr_get(
-    tsd_t *tsd, tcache_slow_t *tcache_slow, szind_t szind) {
-	assert(szind < SC_NBINS);
-	tsdn_t *tsdn = tsd_tsdn(tsd);
-	bin_t  *bin = bin_choose(tsdn, tcache_slow->arena, szind, NULL);
-	assert(bin != NULL);
-
-	malloc_mutex_lock(tsdn, &bin->lock);
-	edata_t *slab = (bin->slabcur == NULL)
-	    ? edata_heap_first(&bin->slabs_nonfull)
-	    : bin->slabcur;
-	assert(slab != NULL || edata_heap_empty(&bin->slabs_nonfull));
-	void *ret = (slab != NULL) ? edata_addr_get(slab) : NULL;
-	assert(ret != NULL || slab == NULL);
-	malloc_mutex_unlock(tsdn, &bin->lock);
-
-	return ret;
-}
-
 static inline bool
 tcache_gc_is_addr_remote(void *addr, uintptr_t min, uintptr_t max) {
 	assert(addr != NULL);
@@ -417,9 +397,9 @@ tcache_gc_small(
 		goto label_flush;
 	}
 
-	/* Query arena binshard to get heuristic locality info. */
-	void *addr = tcache_gc_small_heuristic_addr_get(
-	    tsd, tcache_slow, szind);
+	/* Query arena for a locality anchor (small-bin slab address). */
+	void *addr = arena_locality_hint(tsd_tsdn(tsd), tcache_slow->arena,
+	    szind);
 	if (addr == NULL) {
 		goto label_flush;
 	}

test/unit/bin.c

@@ -683,6 +683,48 @@ TEST_BEGIN(test_bin_stats_nrequests_add) {
 }
 TEST_END
 
+/*
+ * Test bin_current_slab_addr returns slabcur first, then falls back to the
+ * first nonfull slab, and NULL when both are empty.
+ */
+TEST_BEGIN(test_bin_current_slab_addr) {
+	tsdn_t *tsdn = tsdn_fetch();
+	bin_t bin;
+	szind_t binind = 0;
+	edata_t slab1, slab2;
+
+	bin_init(&bin);
+
+	/* Empty bin: returns NULL. */
+	expect_ptr_null(bin_current_slab_addr(tsdn, &bin),
+	    "Empty bin should return NULL");
+
+	create_mock_slab(&slab1, binind, 0);
+	create_mock_slab(&slab2, binind, 1);
+
+	/* Only nonfull set: returns first-of-nonfull addr. */
+	bin_slabs_nonfull_insert(&bin, &slab1);
+	expect_ptr_eq(bin_current_slab_addr(tsdn, &bin),
+	    edata_addr_get(&slab1),
+	    "Should return nonfull-first addr when slabcur is NULL");
+
+	/* slabcur takes precedence over nonfull. */
+	bin.slabcur = &slab2;
+	expect_ptr_eq(bin_current_slab_addr(tsdn, &bin),
+	    edata_addr_get(&slab2),
+	    "Should return slabcur addr when set");
+
+	/* Only slabcur set, no nonfull. */
+	bin_slabs_nonfull_remove(&bin, &slab1);
+	expect_ptr_eq(bin_current_slab_addr(tsdn, &bin),
+	    edata_addr_get(&slab2),
+	    "Should still return slabcur addr after nonfull cleared");
+
+	free(edata_addr_get(&slab1));
+	free(edata_addr_get(&slab2));
+}
+TEST_END
+
 /*
  * Test bin_shard_sizes_boot and bin_update_shard_size.
  */
@@ -856,6 +898,7 @@ main(void) {
 	    test_bin_lower_slab_inserts_nonfull,
 	    test_bin_dalloc_slab_prepare,
 	    test_bin_stats_nrequests_add,
+	    test_bin_current_slab_addr,
 	    test_bin_shard_sizes,
 	    test_bin_alloc_free_cycle,
 	    test_bin_multi_size_class);