[committed] libstdc++: Use helper type for checking thread ID

[Jonathan Wakely via Gcc-patches]

This encapsulates the storing and checking of the thread ID into a class
type, so that the macro _GLIBCXX_HAS_GTHREADS is only checked in one
place. The code doing the checks just calls member functions of the new
type, without caring whether that really does any work or not.

libstdc++-v3/ChangeLog:

        * include/std/stop_token (_Stop_state_t::_M_requester): Define
        new struct with members to store and check the thread ID.
        (_Stop_state_t::_M_request_stop()): Use _M_requester._M_set().
        (_Stop_state_t::_M_remove_callback(_Stop_cb*)): Use
        _M_requester._M_is_current_thread().

Tested powerpc64le-linux. Committed to trunk.


commit 43f9e5aff06f1ca2296fdbd3141fe90ec0be1912
Author: Jonathan Wakely <jwak...@redhat.com>
Date:   Wed Nov 11 09:28:50 2020

    libstdc++: Use helper type for checking thread ID
    
    This encapsulates the storing and checking of the thread ID into a class
    type, so that the macro _GLIBCXX_HAS_GTHREADS is only checked in one
    place. The code doing the checks just calls member functions of the new
    type, without caring whether that really does any work or not.
    
    libstdc++-v3/ChangeLog:
    
            * include/std/stop_token (_Stop_state_t::_M_requester): Define
            new struct with members to store and check the thread ID.
            (_Stop_state_t::_M_request_stop()): Use _M_requester._M_set().
            (_Stop_state_t::_M_remove_callback(_Stop_cb*)): Use
            _M_requester._M_is_current_thread().

diff --git a/libstdc++-v3/include/std/stop_token 
b/libstdc++-v3/include/std/stop_token
index ccec6fab15cf..7cd01c9713ee 100644
--- a/libstdc++-v3/include/std/stop_token
+++ b/libstdc++-v3/include/std/stop_token
@@ -162,9 +162,18 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       std::atomic<value_type> _M_owners{1};
       std::atomic<value_type> _M_value{_S_ssrc_counter_inc};
       _Stop_cb* _M_head = nullptr;
+      struct
+      {
 #ifdef _GLIBCXX_HAS_GTHREADS
-      __gthread_t _M_requester;
+       __gthread_t _M_id;
+       void _M_set() { _M_id = __gthread_self(); }
+       bool _M_is_current_thread() const
+       { return __gthread_equal(_M_id, __gthread_self()); }
+#else
+       void _M_set() { }
+       constexpr bool _M_is_current_thread() const { return true; }
 #endif
+      } _M_requester;
 
       _Stop_state_t() = default;
 
@@ -237,9 +246,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
          }
        while (!_M_try_lock_and_stop(__old));
 
-#ifdef _GLIBCXX_HAS_GTHREADS
-       _M_requester = __gthread_self();
-#endif
+       _M_requester._M_set();
 
        while (_M_head)
          {
@@ -343,18 +350,17 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
        // Callback is not in the list, so must have been removed by a call to
        // _M_request_stop.
 
-#ifdef _GLIBCXX_HAS_GTHREADS
        // Despite appearances there is no data race on _M_requester. The only
        // write to it happens before the callback is removed from the list,
        // and removing it from the list happens before this read.
-       if (!__gthread_equal(_M_requester, __gthread_self()))
+       if (!_M_requester._M_is_current_thread())
          {
            // Synchronize with completion of callback.
            __cb->_M_done.acquire();
            // Safe for ~stop_callback to destroy *__cb now.
            return;
          }
-#endif
+
        if (__cb->_M_destroyed)
          *__cb->_M_destroyed = true;
       }