Fix 'comparison of non-null parameter is always false' warning (Clang)

* alloc.c (GC_set_stop_func, GC_try_to_collect): Use
NONNULL_ARG_NOT_NULL to check argument marked as non-null (in
function declaration) is actually non-null.
* finalize.c (GC_general_register_disappearing_link,
GC_register_long_link, GC_move_disappearing_link, GC_move_long_link):
Likewise.
* misc.c (GC_set_warn_proc, GC_set_abort_func, GC_set_oom_fn): Likewise.
* include/private/gc_priv.h (NONNULL_ARG_NOT_NULL): New macro.

4 files changed, 19 insertions, 9 deletions

diff --git a/alloc.c b/alloc.c
index 7194802a..220ce866 100644
--- a/alloc.c
+++ b/alloc.c
@@ -152,7 +152,7 @@ STATIC GC_stop_func GC_default_stop_func = GC_never_stop_func;
 GC_API void GC_CALL GC_set_stop_func(GC_stop_func stop_func)
 {
   DCL_LOCK_STATE;
-  GC_ASSERT(stop_func != 0);
+  GC_ASSERT(NONNULL_ARG_NOT_NULL(stop_func));
   LOCK();
   GC_default_stop_func = stop_func;
   UNLOCK();
@@ -1079,7 +1079,7 @@ STATIC GC_bool GC_try_to_collect_general(GC_stop_func stop_func,
 /* Externally callable routines to invoke full, stop-the-world collection. */
 GC_API int GC_CALL GC_try_to_collect(GC_stop_func stop_func)
 {
-    GC_ASSERT(stop_func != 0);
+    GC_ASSERT(NONNULL_ARG_NOT_NULL(stop_func));
     return (int)GC_try_to_collect_general(stop_func, FALSE);
 }
 
diff --git a/finalize.c b/finalize.c
index 429f37ee..02b3d257 100644
--- a/finalize.c
+++ b/finalize.c
@@ -207,7 +207,7 @@ STATIC int GC_register_disappearing_link_inner(
 GC_API int GC_CALL GC_general_register_disappearing_link(void * * link,
                                                          const void * obj)
 {
-    if (((word)link & (ALIGNMENT-1)) != 0 || NULL == link)
+    if (((word)link & (ALIGNMENT-1)) != 0 || !NONNULL_ARG_NOT_NULL(link))
         ABORT("Bad arg to GC_general_register_disappearing_link");
     return GC_register_disappearing_link_inner(&GC_dl_hashtbl, link, obj,
                                                "dl");
@@ -456,7 +456,7 @@ GC_API GC_await_finalize_proc GC_CALL GC_get_await_finalize_proc(void)
 #ifndef GC_LONG_REFS_NOT_NEEDED
   GC_API int GC_CALL GC_register_long_link(void * * link, const void * obj)
   {
-    if (((word)link & (ALIGNMENT-1)) != 0 || NULL == link)
+    if (((word)link & (ALIGNMENT-1)) != 0 || !NONNULL_ARG_NOT_NULL(link))
         ABORT("Bad arg to GC_register_long_link");
     return GC_register_disappearing_link_inner(&GC_ll_hashtbl, link, obj,
                                                "long dl");
@@ -536,7 +536,8 @@ GC_API GC_await_finalize_proc GC_CALL GC_get_await_finalize_proc(void)
     int result;
     DCL_LOCK_STATE;
 
-    if (((word)new_link & (ALIGNMENT-1)) != 0 || new_link == NULL)
+    if (((word)new_link & (ALIGNMENT-1)) != 0
+        || !NONNULL_ARG_NOT_NULL(new_link))
       ABORT("Bad new_link arg to GC_move_disappearing_link");
     if (((word)link & (ALIGNMENT-1)) != 0)
       return GC_NOT_FOUND; /* Nothing to do. */
@@ -553,7 +554,8 @@ GC_API GC_await_finalize_proc GC_CALL GC_get_await_finalize_proc(void)
       int result;
       DCL_LOCK_STATE;
 
-      if (((word)new_link & (ALIGNMENT-1)) != 0 || new_link == NULL)
+      if (((word)new_link & (ALIGNMENT-1)) != 0
+          || !NONNULL_ARG_NOT_NULL(new_link))
         ABORT("Bad new_link arg to GC_move_disappearing_link");
       if (((word)link & (ALIGNMENT-1)) != 0)
         return GC_NOT_FOUND; /* Nothing to do. */
diff --git a/include/private/gc_priv.h b/include/private/gc_priv.h
index 2102d627..9b060775 100644
--- a/include/private/gc_priv.h
+++ b/include/private/gc_priv.h
@@ -2357,6 +2357,14 @@ GC_INNER ptr_t GC_store_debug_info(ptr_t p, word sz, const char *str,
 # define GC_STATIC_ASSERT(expr) (void)sizeof(char[(expr)? 1 : -1])
 #endif
 
+/* Runtime check for an argument declared as non-null is actually not null. */
+#if defined(__GNUC__) && __GNUC__ >= 4
+  /* Workaround tautological-pointer-compare Clang warning.     */
+# define NONNULL_ARG_NOT_NULL(arg) (*(volatile void **)&(arg) != NULL)
+#else
+# define NONNULL_ARG_NOT_NULL(arg) (NULL != (arg))
+#endif
+
 #define COND_DUMP_CHECKS \
           do { \
             GC_ASSERT(GC_compute_large_free_bytes() == GC_large_free_bytes); \
diff --git a/misc.c b/misc.c
index 8b871966..cbe77edb 100644
--- a/misc.c
+++ b/misc.c
@@ -1675,7 +1675,7 @@ GC_API void GC_CALLBACK GC_ignore_warn_proc(char *msg, GC_word arg)
 GC_API void GC_CALL GC_set_warn_proc(GC_warn_proc p)
 {
     DCL_LOCK_STATE;
-    GC_ASSERT(p != 0);
+    GC_ASSERT(NONNULL_ARG_NOT_NULL(p));
 #   ifdef GC_WIN32_THREADS
 #     ifdef CYGWIN32
         /* Need explicit GC_INIT call */
@@ -1748,7 +1748,7 @@ GC_API GC_warn_proc GC_CALL GC_get_warn_proc(void)
   GC_API void GC_CALL GC_set_abort_func(GC_abort_func fn)
   {
       DCL_LOCK_STATE;
-      GC_ASSERT(fn != 0);
+      GC_ASSERT(NONNULL_ARG_NOT_NULL(fn));
       LOCK();
       GC_on_abort = fn;
       UNLOCK();
@@ -2077,7 +2077,7 @@ GC_API GC_word GC_CALL GC_get_gc_no(void)
 
 GC_API void GC_CALL GC_set_oom_fn(GC_oom_func fn)
 {
-    GC_ASSERT(fn != 0);
+    GC_ASSERT(NONNULL_ARG_NOT_NULL(fn));
     DCL_LOCK_STATE;
     LOCK();
     GC_oom_fn = fn;