[gcc r12-5691] libstdc++: Optimize ref-count updates in COW std::string

[gcc r12-5691] libstdc++: Optimize ref-count updates in COW std::string

[Jonathan Wakely]

https://gcc.gnu.org/g:2b83bc6097fd138b2dc81f3c8f1786c9079dcd67

commit r12-5691-g2b83bc6097fd138b2dc81f3c8f1786c9079dcd67
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue Nov 30 13:21:55 2021 +0000

    libstdc++: Optimize ref-count updates in COW std::string
    
    Most ref-count updates in the COW string are done via the functions in
    <ext/atomicity.h>, which will use non-atomic ops when the program is
    known to be single-threaded. The _M_is_leaked() and _M_is_shared()
    functions use __atomic_load_n directly, because <ext/atomicity.h>
    doesn't provide a load operation. Those functions can check the
    __is_single_threaded() predicate to avoid using __atomic_load_n when not
    needed.
    
    The move constructor for the fully-dynamic-string increments the
    ref-count by either 2 or 1, for leaked or non-leaked strings
    respectively. That can be changed to use a non-atomic store of 1 for all
    non-shared strings. It can be non-atomic because even if the program is
    multi-threaded, conflicting access to the rvalue object while it's being
    moved from would be data race anyway. It can store 1 directly for all
    non-shared strings because it doesn't matter whether the initial
    refcount was -1 or 0, it should be 1 after the move constructor creates
    a second owner.
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/cow_string.h (basic_string::_M_is_leaked): Use
            non-atomic load when __is_single_threaded() is true.
            (basic_string::_M_is_shared): Likewise.
            (basic_string::(basic_string&&)) [_GLIBCXX_FULLY_DYNAMIC_STRING]:
            Use non-atomic store when rvalue is not shared.

Diff:
---
 libstdc++-v3/include/bits/cow_string.h | 24 ++++++++++++------------
 1 file changed, 12 insertions(+), 12 deletions(-)

diff --git a/libstdc++-v3/include/bits/cow_string.h b/libstdc++-v3/include/bits/cow_string.h
index ced395b80b8..4fae1d02981 100644
--- a/libstdc++-v3/include/bits/cow_string.h
+++ b/libstdc++-v3/include/bits/cow_string.h
@@ -105,7 +105,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    *  destroy the empty-string _Rep object.
    *
    *  All but the last paragraph is considered pretty conventional
-   *  for a C++ string implementation.
+   *  for a Copy-On-Write C++ string implementation.
   */
   // 21.3  Template class basic_string
   template<typename _CharT, typename _Traits, typename _Alloc>
@@ -207,10 +207,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	  // so we need to use an atomic load. However, _M_is_leaked
 	  // predicate does not change concurrently (i.e. the string is either
 	  // leaked or not), so a relaxed load is enough.
-	  return __atomic_load_n(&this->_M_refcount, __ATOMIC_RELAXED) < 0;
-#else
-	  return this->_M_refcount < 0;
+	  if (!__gnu_cxx::__is_single_threaded())
+	    return __atomic_load_n(&this->_M_refcount, __ATOMIC_RELAXED) < 0;
 #endif
+	  return this->_M_refcount < 0;
 	}
 
 	bool
@@ -222,10 +222,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	  // but one reference concurrently with this check, so we need this
 	  // load to be acquire to synchronize with release fetch_and_add in
 	  // _M_dispose.
-	  return __atomic_load_n(&this->_M_refcount, __ATOMIC_ACQUIRE) > 0;
-#else
-	  return this->_M_refcount > 0;
+	  if (!__gnu_cxx::__is_single_threaded())
+	    return __atomic_load_n(&this->_M_refcount, __ATOMIC_ACQUIRE) > 0;
 #endif
+	  return this->_M_refcount > 0;
 	}
 
 	void
@@ -629,12 +629,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 #else
 	// Rather than allocate an empty string for the rvalue string,
 	// just share ownership with it by incrementing the reference count.
-	// If the rvalue string was "leaked" then it was the unique owner,
-	// so need an extra increment to indicate shared ownership.
-	if (_M_rep()->_M_is_leaked())
-	  __gnu_cxx::__atomic_add_dispatch(&_M_rep()->_M_refcount, 2);
-	else
+	// If the rvalue string was the unique owner then there are exactly
+	// two owners now.
+	if (_M_rep()->_M_is_shared())
 	  __gnu_cxx::__atomic_add_dispatch(&_M_rep()->_M_refcount, 1);
+	else
+	  _M_rep()->_M_refcount = 1;
 #endif
       }