[PATCH v6 20/39] rcu: use C11 alignas

[Tyler Retzlaff <roretzla@linux.microsoft.com>]

The current location used for __rte_aligned(a) for alignment of types
and variables is not compatible with MSVC. There is only a single
location accepted by both toolchains.

For variables standard C11 offers alignas(a) supported by conformant
compilers i.e. both MSVC and GCC.

For types the standard offers no alignment facility that compatibly
interoperates with C and C++ but may be achieved by relocating the
placement of __rte_aligned(a) to the aforementioned location accepted
by all currently supported toolchains.

To allow alignment for both compilers do the following:

* Move __rte_aligned from the end of {struct,union} definitions to
  be between {struct,union} and tag.

  The placement between {struct,union} and the tag allows the desired
  alignment to be imparted on the type regardless of the toolchain being
  used for all of GCC, LLVM, MSVC compilers building both C and C++.

* Replace use of __rte_aligned(a) on variables/fields with alignas(a).

Signed-off-by: Tyler Retzlaff <roretzla@linux.microsoft.com>
Acked-by: Morten Brørup <mb@smartsharesystems.com>
---
 lib/rcu/rte_rcu_qsbr.h | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/lib/rcu/rte_rcu_qsbr.h b/lib/rcu/rte_rcu_qsbr.h
index e7ef788..d8ecf11 100644
--- a/lib/rcu/rte_rcu_qsbr.h
+++ b/lib/rcu/rte_rcu_qsbr.h
@@ -21,6 +21,8 @@
  * entered quiescent state.
  */
 
+#include <stdalign.h>
+
 #ifdef __cplusplus
 extern "C" {
 #endif
@@ -69,7 +71,7 @@
 #define RTE_QSBR_THRID_INVALID 0xffffffff
 
 /* Worker thread counter */
-struct rte_rcu_qsbr_cnt {
+struct __rte_cache_aligned rte_rcu_qsbr_cnt {
 	RTE_ATOMIC(uint64_t) cnt;
 	/**< Quiescent state counter. Value 0 indicates the thread is offline
 	 *   64b counter is used to avoid adding more code to address
@@ -78,7 +80,7 @@ struct rte_rcu_qsbr_cnt {
 	 */
 	RTE_ATOMIC(uint32_t) lock_cnt;
 	/**< Lock counter. Used when RTE_LIBRTE_RCU_DEBUG is enabled */
-} __rte_cache_aligned;
+};
 
 #define __RTE_QSBR_CNT_THR_OFFLINE 0
 #define __RTE_QSBR_CNT_INIT 1
@@ -91,28 +93,28 @@ struct rte_rcu_qsbr_cnt {
  * 1) Quiescent state counter array
  * 2) Register thread ID array
  */
-struct rte_rcu_qsbr {
-	RTE_ATOMIC(uint64_t) token __rte_cache_aligned;
+struct __rte_cache_aligned rte_rcu_qsbr {
+	alignas(RTE_CACHE_LINE_SIZE) RTE_ATOMIC(uint64_t) token;
 	/**< Counter to allow for multiple concurrent quiescent state queries */
 	RTE_ATOMIC(uint64_t) acked_token;
 	/**< Least token acked by all the threads in the last call to
 	 *   rte_rcu_qsbr_check API.
 	 */
 
-	uint32_t num_elems __rte_cache_aligned;
+	alignas(RTE_CACHE_LINE_SIZE) uint32_t num_elems;
 	/**< Number of elements in the thread ID array */
 	RTE_ATOMIC(uint32_t) num_threads;
 	/**< Number of threads currently using this QS variable */
 	uint32_t max_threads;
 	/**< Maximum number of threads using this QS variable */
 
-	struct rte_rcu_qsbr_cnt qsbr_cnt[0] __rte_cache_aligned;
+	alignas(RTE_CACHE_LINE_SIZE) struct rte_rcu_qsbr_cnt qsbr_cnt[0];
 	/**< Quiescent state counter array of 'max_threads' elements */
 
 	/**< Registered thread IDs are stored in a bitmap array,
 	 *   after the quiescent state counter array.
 	 */
-} __rte_cache_aligned;
+};
 
 /**
  * Call back function called to free the resources.
-- 
1.8.3.1