- are only invoked for big memory sizes. */
-
-#if HAVE_ALLOCA
-
-/* Store the mmalloca() results in a hash table. This is needed to reliably
- distinguish a mmalloca() result and an alloca() result.
-
- Although it is possible that the same pointer is returned by alloca() and
- by mmalloca() at different times in the same application, it does not lead
- to a bug in freea(), because:
- - Before a pointer returned by alloca() can point into malloc()ed memory,
- the function must return, and once this has happened the programmer must
- not call freea() on it anyway.
- - Before a pointer returned by mmalloca() can point into the stack, it
- must be freed. The only function that can free it is freea(), and
- when freea() frees it, it also removes it from the hash table. */
-
-#define MAGIC_NUMBER 0x1415fb4a
-#define MAGIC_SIZE sizeof (int)
-/* This is how the header info would look like without any alignment
- considerations. */
-struct preliminary_header { void *next; int magic; };
-/* But the header's size must be a multiple of sa_alignment_max. */
-#define HEADER_SIZE \
- (((sizeof (struct preliminary_header) + sa_alignment_max - 1) / sa_alignment_max) * sa_alignment_max)
-union header {
- void *next;
- struct {
- char room[HEADER_SIZE - MAGIC_SIZE];
- int word;
- } magic;
-};
-verify (HEADER_SIZE == sizeof (union header));
-/* We make the hash table quite big, so that during lookups the probability
- of empty hash buckets is quite high. There is no need to make the hash
- table resizable, because when the hash table gets filled so much that the
- lookup becomes slow, it means that the application has memory leaks. */
-#define HASH_TABLE_SIZE 257
-static void * mmalloca_results[HASH_TABLE_SIZE];
-
-#endif
+ are only invoked for big memory sizes.
+ Here we use a bit in the address as an indicator, an idea by Ondřej Bílka.
+ malloca() can return three types of pointers:
+ - Pointers ≡ 0 mod 2*sa_alignment_max come from stack allocation.
+ - Pointers ≡ sa_alignment_max mod 2*sa_alignment_max come from heap
+ allocation.
+ - NULL comes from a failed heap allocation. */
+
+/* Type for holding very small pointer differences. */
+typedef unsigned char small_t;
+/* Verify that it is wide enough. */
+verify (2 * sa_alignment_max - 1 <= (small_t) -1);