Use __c11 atomic macros when they are available (and fix the LLVM sup…

…port)

Co-authored-by: Pierre Alain <pierre.alain@tuta.io>

nolibc/include/stdatomic.h

@@ -1,6 +1,11 @@
 #ifndef _STDATOMIC_H
 #define _STDATOMIC_H
 
+// Compatibility with non-clang compilers
+#ifndef __has_builtin
+#  define __has_builtin(x) 0
+#endif
+
 #define atomic_load_explicit(x, mode) ( *x )
 #define atomic_load(x) ( *x )
 
@@ -9,13 +14,23 @@ extern int memory_order_acquire;
 extern int memory_order_relaxed;
 extern int memory_order_seq_cst;
 
+#if __has_builtin(__c11_atomic_fetch_add)
+#define atomic_fetch_add(X, Y) __c11_atomic_fetch_add(X, Y, __ATOMIC_SEQ_CST)
+#else
 #define atomic_fetch_add(X, Y) ({ __auto_type tmp = *X; *X = tmp + Y; tmp; })
+#endif
+
 #define atomic_fetch_add_explicit(X, Y, MOD) atomic_fetch_add(X, Y)
 
 #define atomic_thread_fence(MO) do {} while (0)
 
 typedef unsigned long long atomic_uint_fast64_t;
 
+#if __has_builtin(__c11_atomic_compare_exchange_strong)
+#define atomic_compare_exchange_strong(OBJ, EXPECTED, DESIRED) \
+	__c11_atomic_compare_exchange_strong(OBJ, EXPECTED, DESIRED, \
+			__ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST)
+#else
 #define atomic_compare_exchange_strong(OBJ, EXPECTED, DESIRED) \
   ({ int ret = 0; \
      if (*OBJ == *EXPECTED) { \
@@ -24,20 +39,31 @@ typedef unsigned long long atomic_uint_fast64_t;
      } \
      ret; \
   })
+#endif
 
+#if __has_builtin(__c11_atomic_exchange)
+#define atomic_exchange(OBJ, DESIRED) \
+	__c11_atomic_exchange(OBJ, DESIRED, __ATOMIC_SEQ_CST)
+#else
 #define atomic_exchange(OBJ, DESIRED) \
   ({ __auto_type tmp = *OBJ; \
      *OBJ = DESIRED; \
      tmp; \
   })
+#endif
 
 #define atomic_store(OBJ, DESIRED) do { *OBJ = DESIRED; } while(0)
 #define atomic_store_explicit(OBJ, DESIRED, ORDER) atomic_store(OBJ, DESIRED)
 
+#if __has_builtin(__c11_atomic_fetch_or)
+#define atomic_fetch_or(OBJ, ARG) \
+	__c11_atomic_fetch_or(OBJ, ARG, __ATOMIC_SEQ_CST)
+#else
 #define atomic_fetch_or(OBJ, ARG) \
   ({ __auto_type tmp = *OBJ; \
      *OBJ = *OBJ | ARG; \
      tmp; \
   })
+#endif
 
 #endif