[PATCH 3/3] barrier: optimise by not having the hasher in a loop

[Thiago Macieira]

Our thread's ID does not change so we don't have to get it every time
and hash it every time.

libstdc++-v3/ChangeLog:

        * include/std/barrier(arrive): move hasher one level up in the
          stack.
---
 libstdc++-v3/include/std/barrier | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/libstdc++-v3/include/std/barrier b/libstdc++-v3/include/std/barrier
index e09212dfcb9..4bb5642c164 100644
--- a/libstdc++-v3/include/std/barrier
+++ b/libstdc++-v3/include/std/barrier
@@ -94,7 +94,7 @@ It looks different from literature pseudocode for two main reasons:
       alignas(__phase_alignment) __barrier_phase_t  _M_phase;
 
       bool
-      _M_arrive(__barrier_phase_t __old_phase)
+      _M_arrive(__barrier_phase_t __old_phase, size_t __current)
       {
 	const auto __old_phase_val = static_cast<unsigned char>(__old_phase);
 	const auto __half_step =
@@ -103,9 +103,7 @@ It looks different from literature pseudocode for two main reasons:
 			   static_cast<__barrier_phase_t>(__old_phase_val + 2);
 
 	size_t __current_expected = _M_expected;
-	std::hash<std::thread::id> __hasher;
-	size_t __current = __hasher(std::this_thread::get_id())
-					  % ((_M_expected + 1) >> 1);
+        __current %= ((__current_expected + 1) >> 1);
 
 	for (int __round = 0; ; ++__round)
 	  {
@@ -163,12 +161,14 @@ It looks different from literature pseudocode for two main reasons:
       [[nodiscard]] arrival_token
       arrive(ptrdiff_t __update)
       {
+	std::hash<std::thread::id> __hasher;
+	size_t __current = __hasher(std::this_thread::get_id());
 	__atomic_phase_ref_t __phase(_M_phase);
 	const auto __old_phase = __phase.load(memory_order_relaxed);
 	const auto __cur = static_cast<unsigned char>(__old_phase);
 	for(; __update; --__update)
 	  {
-	    if(_M_arrive(__old_phase))
+	    if(_M_arrive(__old_phase, __current))
 	      {
 		_M_completion();
 		_M_expected += _M_expected_adjustment.load(memory_order_relaxed);
-- 
2.30.1