[PATCH 3/3] 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/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