Drop support for clang atomic and gcc __sync builtins

The requirements for BIND 9.17+ now requires C11 support from the
compiler, so we can safely drop most of the stdatomic.h shims from
lib/isc/unix/include/stdatomic.h.

This commit removes support for clang atomic builtins (clang >= 3.6.0
includes stdatomic.h header) and for Gcc __sync builtins.

The only compatibility shim that remains is support for __atomic
builtins for Gcc >= 4.7.0 since CentOS 7 still includes only Gcc 4.8.1
and the proper stdatomic.h header was only introduced in Gcc >= 4.9.

PLATFORMS.md

@@ -17,11 +17,11 @@ the OpenSSL cryptography library, and the `nghttp2` HTTP/2 library.
 
 The following C11 features are used in BIND 9:
 
-* Atomic operations support from the compiler is needed, either in the form of
-  builtin operations.
+* Atomic operations support, either in the form of C11 atomics or
+  `__atomic` builtin operations.
 
-* Thread Local Storage support from the compiler is needed, either in the form
-  of C11 `_Thread_local`/`thread_local`, or the `__thread` GCC extension.
+* Thread Local Storage support, either in the form of C11
+  `_Thread_local`/`thread_local`, or the `__thread` GCC extension.
 
 The C11 variants are preferred.
 

configure.ac

@@ -1176,7 +1176,7 @@ AC_CHECK_HEADERS(
 	 AX_RESTORE_FLAGS([atomic])
 	])
      ],
-     [AC_MSG_RESULT([__sync builtins])
+     [AC_MSG_FAILURE([not found])
      ])
   ])
 LIBS="$LIBS $ISC_ATOMIC_LIBS"

lib/isc/include/isc/atomic.h

@@ -11,11 +11,11 @@
 
 #pragma once
 
-#if HAVE_STDATOMIC_H
+#if __STDC_VERSION__ >= 201112L && !defined(__STDC_NO_ATOMICS__)
 #include <stdatomic.h>
-#else /* if HAVE_STDATOMIC_H */
+#else
 #include <isc/stdatomic.h>
-#endif /* if HAVE_STDATOMIC_H */
+#endif
 
 /*
  * We define a few additional macros to make things easier

lib/isc/include/isc/stdatomic.h

@@ -18,64 +18,25 @@
 #include <uchar.h>
 #endif /* HAVE_UCHAR_H */
 
-#if !defined(__has_feature)
-#define __has_feature(x) 0
-#endif /* if !defined(__has_feature) */
-
-#if !defined(__has_extension)
-#define __has_extension(x) __has_feature(x)
-#endif /* if !defined(__has_extension) */
-
-#if !defined(__GNUC_PREREQ__)
-#if defined(__GNUC__) && defined(__GNUC_MINOR__)
-#define __GNUC_PREREQ__(maj, min) \
-	((__GNUC__ << 16) + __GNUC_MINOR__ >= ((maj) << 16) + (min))
-#else /* if defined(__GNUC__) && defined(__GNUC_MINOR__) */
-#define __GNUC_PREREQ__(maj, min) 0
-#endif /* if defined(__GNUC__) && defined(__GNUC_MINOR__) */
-#endif /* if !defined(__GNUC_PREREQ__) */
-
-#if !defined(__CLANG_ATOMICS) && !defined(__GNUC_ATOMICS)
-#if __has_extension(c_atomic) || __has_extension(cxx_atomic)
-#define __CLANG_ATOMICS
-#elif __GNUC_PREREQ__(4, 7)
+/* GCC 4.7.0 introduced __atomic builtins, but not the __GNUC_ATOMICS define */
+#if !defined(__GNUC_ATOMICS) && __GNUC__ == 4 && __GNUC_MINOR__ >= 7
 #define __GNUC_ATOMICS
-#elif !defined(__GNUC__)
+#endif
+
+#if !defined(__GNUC_ATOMICS)
 #error "isc/stdatomic.h does not support your compiler"
-#endif /* if __has_extension(c_atomic) || __has_extension(cxx_atomic) */
-#endif /* if !defined(__CLANG_ATOMICS) && !defined(__GNUC_ATOMICS) */
+#endif /* if !defined(__GNUC_ATOMICS) */
 
 #define ATOMIC_VAR_INIT(x) x
 
-#ifndef __ATOMIC_RELAXED
-#define __ATOMIC_RELAXED 0
-#endif /* ifndef __ATOMIC_RELAXED */
-#ifndef __ATOMIC_CONSUME
-#define __ATOMIC_CONSUME 1
-#endif /* ifndef __ATOMIC_CONSUME */
-#ifndef __ATOMIC_ACQUIRE
-#define __ATOMIC_ACQUIRE 2
-#endif /* ifndef __ATOMIC_ACQUIRE */
-#ifndef __ATOMIC_RELEASE
-#define __ATOMIC_RELEASE 3
-#endif /* ifndef __ATOMIC_RELEASE */
-#ifndef __ATOMIC_ACQ_REL
-#define __ATOMIC_ACQ_REL 4
-#endif /* ifndef __ATOMIC_ACQ_REL */
-#ifndef __ATOMIC_SEQ_CST
-#define __ATOMIC_SEQ_CST 5
-#endif /* ifndef __ATOMIC_SEQ_CST */
-
-enum memory_order {
+typedef enum memory_order {
 	memory_order_relaxed = __ATOMIC_RELAXED,
 	memory_order_consume = __ATOMIC_CONSUME,
 	memory_order_acquire = __ATOMIC_ACQUIRE,
 	memory_order_release = __ATOMIC_RELEASE,
 	memory_order_acq_rel = __ATOMIC_ACQ_REL,
 	memory_order_seq_cst = __ATOMIC_SEQ_CST
-};
-
-typedef enum memory_order memory_order;
+} memory_order;
 
 #ifndef HAVE_UCHAR_H
 typedef uint_least16_t char16_t;
@@ -120,30 +81,6 @@ typedef ptrdiff_t	   atomic_ptrdiff_t;
 typedef intmax_t	   atomic_intmax_t;
 typedef uintmax_t	   atomic_uintmax_t;
 
-#if defined(__CLANG_ATOMICS) /* __c11_atomic builtins */
-#define atomic_init(obj, desired)	 __c11_atomic_init(obj, desired)
-#define atomic_load_explicit(obj, order) __c11_atomic_load(obj, order)
-#define atomic_store_explicit(obj, desired, order) \
-	__c11_atomic_store(obj, desired, order)
-#define atomic_fetch_add_explicit(obj, arg, order) \
-	__c11_atomic_fetch_add(obj, arg, order)
-#define atomic_fetch_sub_explicit(obj, arg, order) \
-	__c11_atomic_fetch_sub(obj, arg, order)
-#define atomic_fetch_and_explicit(obj, arg, order) \
-	__c11_atomic_fetch_and(obj, arg, order)
-#define atomic_fetch_or_explicit(obj, arg, order) \
-	__c11_atomic_fetch_or(obj, arg, order)
-#define atomic_compare_exchange_strong_explicit(obj, expected, desired, succ, \
-						fail)                         \
-	__c11_atomic_compare_exchange_strong_explicit(obj, expected, desired, \
-						      succ, fail)
-#define atomic_compare_exchange_weak_explicit(obj, expected, desired, succ, \
-					      fail)                         \
-	__c11_atomic_compare_exchange_weak_explicit(obj, expected, desired, \
-						    succ, fail)
-#define atomic_exchange_explicit(obj, desired, order) \
-	__c11_atomic_exchange_explicit(obj, expected, order)
-#elif defined(__GNUC_ATOMICS) /* __atomic builtins */
 #define atomic_init(obj, desired)	 (*obj = desired)
 #define atomic_load_explicit(obj, order) __atomic_load_n(obj, order)
 #define atomic_store_explicit(obj, desired, order) \
@@ -164,42 +101,6 @@ typedef uintmax_t	   atomic_uintmax_t;
 	__atomic_compare_exchange_n(obj, expected, desired, 1, succ, fail)
 #define atomic_exchange_explicit(obj, desired, order) \
 	__atomic_exchange_n(obj, desired, order)
-#else /* __sync builtins */
-#define atomic_init(obj, desired)	 (*obj = desired)
-#define atomic_load_explicit(obj, order) __sync_fetch_and_add(obj, 0)
-#define atomic_store_explicit(obj, desired, order) \
-	do {                                       \
-		__sync_synchronize();              \
-		*obj = desired;                    \
-		__sync_synchronize();              \
-	} while (0);
-#define atomic_fetch_add_explicit(obj, arg, order) \
-	__sync_fetch_and_add(obj, arg)
-#define atomic_fetch_sub_explicit(obj, arg, order) \
-	__sync_fetch_and_sub(obj, arg, order)
-#define atomic_fetch_and_explicit(obj, arg, order) \
-	__sync_fetch_and_and(obj, arg, order)
-#define atomic_fetch_or_explicit(obj, arg, order) \
-	__sync_fetch_and_or(obj, arg, order)
-#define atomic_compare_exchange_strong_explicit(obj, expected, desired, succ, \
-						fail)                         \
-	({                                                                    \
-		__typeof__(obj) __v;                                          \
-		_Bool		__r;                                          \
-		__v = (__typeof__(obj))__sync_val_compare_and_swap(           \
-			obj, *(expected), desired);                           \
-		__r = ((__typeof__(obj))*(expected) == __v);                  \
-		*(expected) = __v;                                            \
-		__r;                                                          \
-	})
-#define atomic_compare_exchange_weak_explicit(obj, expected, desired, succ,   \
-					      fail)                           \
-	atomic_compare_exchange_strong_explicit(obj, expected, desired, succ, \
-						fail)
-#define atomic_exchange_explicit(obj, desired, order) \
-	__sync_lock_test_and_set(obj, desired)
-
-#endif /* if defined(__CLANG_ATOMICS) */
 
 #define atomic_load(obj) atomic_load_explicit(obj, memory_order_seq_cst)
 #define atomic_store(obj, arg) \