[PATCH 5/5] barrier: optimise by not having the hasher in a loop

[Thiago Macieira via Gcc-patches]

Our thread's ID does not change so we don't have to get it every time
and hash it every time.
---
 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 ae058bd3dc3..eb31a89b175 100644
--- a/libstdc++-v3/include/std/barrier
+++ b/libstdc++-v3/include/std/barrier
@@ -101,12 +101,10 @@ It looks different from literature pseudocode for two 
main reasons:
       }
 
       bool
-      _M_arrive(__barrier_phase_t __old_phase)
+      _M_arrive(__barrier_phase_t __old_phase, size_t __current)
       {
        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);
 
        const auto __half_step = _S_add_to_phase(__old_phase, 1);
        const auto __full_step = _S_add_to_phase(__old_phase, 2);
@@ -167,11 +165,13 @@ 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);
        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