Add support for C11 stdatomic atomics

This fixes deprecation warnings when building for macOS >= 10.12 systems.
Additionally, using stdatomic.h (or the more modern __atomic_ builtins)
is required when targeting CHERI-enabled architectures such as
CHERI-RISC-V or Arm's Morello since the compiler rejects __sync_* atomic
for pointer types (they only work with integers).

configure.ac

@@ -663,6 +663,22 @@ dnl ===========================================================================
 # Thread-safety primitives
 #
 
+AC_CACHE_CHECK([stdatomic.h atomic primitives], fc_cv_have_stdatomic_atomic_primitives, [
+	fc_cv_have_stdatomic_atomic_primitives=false
+	AC_TRY_LINK([
+		#include <stdatomic.h>
+
+		void memory_barrier (void) { atomic_thread_fence (memory_order_acq_rel); }
+		int atomic_add (atomic_int *i) { return atomic_fetch_add_explicit (i, 1, memory_order_relaxed); }
+		int mutex_trylock (atomic_flag *m) { return atomic_flag_test_and_set_explicit (m, memory_order_acquire); }
+		void mutex_unlock (atomic_flag *m) { atomic_flag_clear_explicit (m, memory_order_release); }
+		], [], fc_cv_have_stdatomic_atomic_primitives=true
+	)
+])
+if $fc_cv_have_stdatomic_atomic_primitives; then
+	AC_DEFINE(HAVE_STDATOMIC_PRIMITIVES, 1, [Have Intel __sync_* atomic primitives])
+fi
+
 AC_CACHE_CHECK([for Intel atomic primitives], fc_cv_have_intel_atomic_primitives, [
 	fc_cv_have_intel_atomic_primitives=false
 	AC_TRY_LINK([

meson-cc-tests/stdatomic-primitives-test.c

@@ -0,0 +1,8 @@
+#include <stdatomic.h>
+
+void memory_barrier (void) { atomic_thread_fence (memory_order_acq_rel); }
+int atomic_add (atomic_int *i) { return atomic_fetch_add_explicit (i, 1, memory_order_relaxed); }
+int mutex_trylock (atomic_flag *m) { return atomic_flag_test_and_set_explicit (m, memory_order_acquire); }
+void mutex_unlock (atomic_flag *m) { atomic_flag_clear_explicit (m, memory_order_release); }
+
+int main(void) { return 0;}

meson.build

@@ -190,6 +190,10 @@ else
   conf.set('FLEXIBLE_ARRAY_MEMBER', 1)
 endif
 
+if cc.links(files('meson-cc-tests/stdatomic-primitives-test.c'), name: 'stdatomic.h atomics')
+  conf.set('HAVE_STDATOMIC_PRIMITIVES', 1)
+endif
+
 if cc.links(files('meson-cc-tests/intel-atomic-primitives-test.c'), name: 'Intel atomics')
   conf.set('HAVE_INTEL_ATOMIC_PRIMITIVES', 1)
 endif

src/fcatomic.h

@@ -51,6 +51,25 @@ typedef <type> fc_atomic_int_t;
 #define fc_atomic_ptr_cmpexch(P,O,N)	*(P) == (O) ? (*(P) = (N), FcTrue) : FcFalse // atomic release
 
 
+#elif !defined(FC_NO_MT) && defined(HAVE_STDATOMIC_PRIMITIVES)
+
+#include <stdatomic.h>
+
+typedef atomic_int fc_atomic_int_t;
+#define FC_ATOMIC_INT_FORMAT		"d"
+#define fc_atomic_int_add(AI, V)	atomic_fetch_add (&(AI), (V))
+
+#define fc_atomic_ptr_get(P)		atomic_load ((_Atomic(void *)*) (P))
+static inline FcBool _fc_atomic_ptr_cmpexch(_Atomic(void *)*P, void * O, _Atomic(void *) N) {
+  return atomic_compare_exchange_strong(P, &O, N);
+}
+#define fc_atomic_ptr_cmpexch(P,O,N)	_fc_atomic_ptr_cmpexch ((_Atomic(void *)*) (P), (O), (N))
+
+/* Casting -1 to _Atomic(int) produces a compiler error with Clang (but not GCC)
+ * so we have to override FC_REF_CONSTANT_VALUE for stdatomic.h atomics.
+ * See https://bugs.llvm.org/show_bug.cgi?id=40249. */
+#define FC_REF_CONSTANT_VALUE (-1)
+
 #elif !defined(FC_NO_MT) && defined(_MSC_VER) || defined(__MINGW32__)
 
 #include "fcwindows.h"
@@ -137,7 +156,9 @@ typedef int fc_atomic_int_t;
 #endif
 
 /* reference count */
+#ifndef FC_REF_CONSTANT_VALUE
 #define FC_REF_CONSTANT_VALUE ((fc_atomic_int_t) -1)
+#endif
 #define FC_REF_CONSTANT {FC_REF_CONSTANT_VALUE}
 typedef struct _FcRef { fc_atomic_int_t count; } FcRef;
 static inline void   FcRefInit     (FcRef *r, int v) { r->count = v; }

src/fccache.c

@@ -135,7 +135,7 @@ FcCacheIsMmapSafe (int fd)
 	    status =  use ? MMAP_USE : MMAP_DONT_USE;
 	else
 	    status = MMAP_CHECK_FS;
-	(void) fc_atomic_ptr_cmpexch (&static_status, NULL, (void *) status);
+	(void) fc_atomic_ptr_cmpexch (&static_status, NULL, (void *) (intptr_t) status);
     }
 
     if (status == MMAP_CHECK_FS)