gc: use single allocation for prot_array.

* gc.c (prot_array): Add self pointer; arr member
becomes flexible array.
(prot_array_mark): We now check the handle itself for
null, because the whole thing is freed.
(prot_array_free): Function removed.
(prot_array_ops): Wire cobj_destroy_free_op in place
of prot_array_free. This fixes a memory leak because
prot_array_free was not freeing the handle, only
the array.
(gc_prot_array_alloc): Fix to allocate everything
in one swoop and store the self-pointer in the
named member rather than arr[-1]. The self argument
is not required; we drop it. The size argument cannot
be anywhere near INT_PTR_MAX, because such an array
wouldn't fit into virtual memory, so it is always
safe to add a small value to the size.
(prot_array_free): Obtain the self-pointer, and
free the handle, replacing it with a null pointer.

* gc.h (gc_prot_array_alloc): Declaration updated.

* lib.c (ssort_vec): Don't pass self to gc_prot_array_alloc.

* lib.h (container): New macro.

4 files changed, 21 insertions, 27 deletions

diff --git a/gc.c b/gc.c
index 2c61c914..eafc354c 100644
--- a/gc.c
+++ b/gc.c
@@ -138,7 +138,8 @@ val break_obj;
 
 struct prot_array {
   cnum size;
-  val *arr;
+  val self;
+  val arr[FLEX_ARRAY];
 };
 
 struct cobj_class *prot_array_cls;
@@ -1260,48 +1261,37 @@ void gc_stack_overflow(void)
 static void prot_array_mark(val obj)
 {
   struct prot_array *pa = coerce(struct prot_array *, obj->co.handle);
-  cnum i;
 
-  if (pa->arr)
+  if (pa) {
+    cnum i;
     for (i = 0; i < pa->size; i++)
       gc_mark(pa->arr[i]);
-}
-
-static void prot_array_free(val obj)
-{
-  struct prot_array *pa = coerce(struct prot_array *, obj->co.handle);
-
-  if (pa->arr) {
-    free(pa->arr - 1);
-    pa->arr = 0;
   }
 }
 
 static struct cobj_ops prot_array_ops = cobj_ops_init(eq,
                                                       cobj_print_op,
-                                                      prot_array_free,
+                                                      cobj_destroy_free_op,
                                                       prot_array_mark,
                                                       cobj_eq_hash_op);
 
-val *gc_prot_array_alloc(cnum size, val self)
+val *gc_prot_array_alloc(cnum size)
 {
   struct prot_array *pa = convert(struct prot_array *,
-                                  chk_malloc(sizeof *pa));
-
-  if (size >= INT_PTR_MAX)
-    uw_throwf(error_s, lit("~s: array too large"), self, nao);
-
+                                  chk_calloc(offsetof(struct prot_array, arr) +
+                                             size * sizeof(val), 1));
   pa->size = size;
-  pa->arr = convert(val *,
-                    chk_calloc(sizeof *pa->arr, (size + 1))) + 1;
-
-  pa->arr[-1] = cobj(convert(mem_t *, pa), prot_array_cls, &prot_array_ops);
+  pa->self = cobj(convert(mem_t *, pa), prot_array_cls, &prot_array_ops);
 
   return pa->arr;
 }
 
 void gc_prot_array_free(val *arr)
 {
-  if (arr)
-    prot_array_free(arr[-1]);
+  if (arr) {
+    struct prot_array *pa = container(arr, struct prot_array, arr);
+    val obj = pa->self;
+    obj->co.handle = 0;
+    free(pa);
+  }
 }
diff --git a/gc.h b/gc.h
index 58d057db..e89e695a 100644
--- a/gc.h
+++ b/gc.h
@@ -87,5 +87,5 @@ INLINE void gc_stack_check(void)
     gc_stack_overflow();
 }
 
-val *gc_prot_array_alloc(cnum size, val self);
+val *gc_prot_array_alloc(cnum size);
 void gc_prot_array_free(val *);
diff --git a/lib.c b/lib.c
index bea09ea3..2e9de482 100644
--- a/lib.c
+++ b/lib.c
@@ -10922,7 +10922,7 @@ static void mergesort(val vec, val lessfun, val keyfun, cnum from, cnum to,
 static void ssort_vec(val vec, val lessfun, val keyfun, val self)
 {
   cnum len = c_fixnum(length(vec), self);
-  val *aux = gc_prot_array_alloc(len, self);
+  val *aux = gc_prot_array_alloc(len);
   mergesort(vec, lessfun, keyfun, 0, len, aux);
   gc_prot_array_free(aux);
 }
diff --git a/lib.h b/lib.h
index 9e71b5f9..dc90aa23 100644
--- a/lib.h
+++ b/lib.h
@@ -46,6 +46,10 @@ typedef double_uintptr_t dbl_ucnum;
 #define coerce(TYPE, EXPR) ((TYPE) (EXPR))
 #endif
 
+#define container(PTR, TYPE, MEMB)                          \
+  convert(TYPE *,                                           \
+          convert(mem_t *, (PTR)) - offsetof(TYPE, MEMB))
+
 #if __STDC_VERSION__ >= 199901L
 #define FLEX_ARRAY
 #else