Add macros to allocate temporary Lisp objects with alloca.

Respect MAX_ALLOCA and fall back to regular GC for large objects.
* character.h (parse_str_as_multibyte): Move prototype to ...
* lisp.h (parse_str_as_multibyte): ... here.
(struct Lisp_Cons): Add GCALIGNED attribute if supported.
(scoped_cons, scoped_list2, build_local_vector, build_local_string):
New macros.
(scoped_cons_init, pointer_valid_for_lisp_object, local_vector_init)
(local_string_init): New functions.
* alloc.c (verify_alloca) [ENABLE_CHECKING]: New function.
(init_alloc_once): Call it.

1 files changed, 25 insertions, 2 deletions

diff --git a/src/alloc.c b/src/alloc.c
index 31b0644c285..13043d6d9d7 100644
--- a/src/alloc.c
+++ b/src/alloc.c
@@ -7117,8 +7117,29 @@ die (const char *msg, const char *file, int line)
            file, line, msg);
   terminate_due_to_signal (SIGABRT, INT_MAX);
 }
-#endif
-
+
+/* Stress alloca with inconveniently sized requests and check
+   whether all allocated areas may be used for Lisp_Object.  */
+
+NO_INLINE static void
+verify_alloca (void)
+{
+  int i;
+  enum { ALLOCA_CHECK_MAX = 256 };
+  /* Start from size of the smallest Lisp object.  */
+  for (i = sizeof (struct Lisp_Cons); i <= ALLOCA_CHECK_MAX; i++)
+    {
+      char *ptr = alloca (i);
+      eassert (pointer_valid_for_lisp_object (ptr));
+    }
+}
+
+#else /* not ENABLE_CHECKING */
+
+#define verify_alloca() ((void) 0)
+
+#endif /* ENABLE_CHECKING */
+
 /* Initialization.  */
 
 void
@@ -7128,6 +7149,8 @@ init_alloc_once (void)
   purebeg = PUREBEG;
   pure_size = PURESIZE;
 
+  verify_alloca ();
+
 #if GC_MARK_STACK || defined GC_MALLOC_CHECK
   mem_init ();
   Vdead = make_pure_string ("DEAD", 4, 4, 0);