Make some parallel mode algorithms usable in constexpr contexts

This makes the __parallel::equal and __parallel:lexicographical_compare
algorithms usable in constant expressions, by dispatching to the
sequential algorithm when calling during constant evaluation.

	* include/parallel/algobase.h (equal, lexicographical_compare): Add
	_GLIBCXX20_CONSTEXPR and dispatch to sequential algorithm when being
	constant evaluated.
	* include/parallel/algorithmfwd.h (equal, lexicographical_compare):
	Add _GLIBCXX20_CONSTEXPR.

From-SVN: r276431

3 files changed, 51 insertions, 1 deletions

diff --git a/libstdc++-v3/ChangeLog b/libstdc++-v3/ChangeLog
index 2a95767..45ad851 100644
--- a/libstdc++-v3/ChangeLog
+++ b/libstdc++-v3/ChangeLog
@@ -1,5 +1,11 @@
 2019-10-01  Jonathan Wakely  <jwakely@redhat.com>
 
+	* include/parallel/algobase.h (equal, lexicographical_compare): Add
+	_GLIBCXX20_CONSTEXPR and dispatch to sequential algorithm when being
+	constant evaluated.
+	* include/parallel/algorithmfwd.h (equal, lexicographical_compare):
+	Add _GLIBCXX20_CONSTEXPR.
+
 	* testsuite/17_intro/using_namespace_std_tr1_neg.cc: Skip test for
 	parallel mode.
 	* testsuite/20_util/hash/84998.cc: Likewise.
diff --git a/libstdc++-v3/include/parallel/algobase.h b/libstdc++-v3/include/parallel/algobase.h
index 829eb11..d78bdc9 100644
--- a/libstdc++-v3/include/parallel/algobase.h
+++ b/libstdc++-v3/include/parallel/algobase.h
@@ -214,19 +214,31 @@ namespace __parallel
 
   // Public interface
   template<typename _IIter1, typename _IIter2>
+    _GLIBCXX20_CONSTEXPR
     inline bool
     equal(_IIter1 __begin1, _IIter1 __end1, _IIter2 __begin2)
     {
+#if __cplusplus > 201703L
+      if (std::is_constant_evaluated())
+	return _GLIBCXX_STD_A::equal(__begin1, __end1, __begin2);
+#endif
+
       return __gnu_parallel::mismatch(__begin1, __end1, __begin2).first
               == __end1;
     }
 
   // Public interface
   template<typename _IIter1, typename _IIter2, typename _Predicate>
+    _GLIBCXX20_CONSTEXPR
     inline bool
     equal(_IIter1 __begin1, _IIter1 __end1, _IIter2 __begin2, 
           _Predicate __pred)
     {
+#if __cplusplus > 201703L
+      if (std::is_constant_evaluated())
+	return _GLIBCXX_STD_A::equal(__begin1, __end1, __begin2, __pred);
+#endif
+
       return __gnu_parallel::mismatch(__begin1, __end1, __begin2, __pred).first
               == __end1;
     }
@@ -286,9 +298,15 @@ namespace __parallel
     }
 
   template<typename _IIter1, typename _IIter2>
+    _GLIBCXX20_CONSTEXPR
     inline bool
     equal(_IIter1 __begin1, _IIter1 __end1, _IIter2 __begin2, _IIter2 __end2)
     {
+#if __cplusplus > 201703L
+      if (std::is_constant_evaluated())
+	return _GLIBCXX_STD_A::equal(__begin1, __end1, __begin2, __end2);
+#endif
+
       typedef __gnu_parallel::_EqualTo<
 	typename std::iterator_traits<_IIter1>::value_type,
 	typename std::iterator_traits<_IIter2>::value_type> _EqualTo;
@@ -299,15 +317,22 @@ namespace __parallel
     }
 
   template<typename _IIter1, typename _IIter2, typename _BinaryPredicate>
+    _GLIBCXX20_CONSTEXPR
     inline bool
     equal(_IIter1 __begin1, _IIter1 __end1,
 	  _IIter2 __begin2, _IIter2 __end2, _BinaryPredicate __binary_pred)
     {
+#if __cplusplus > 201703L
+      if (std::is_constant_evaluated())
+	return _GLIBCXX_STD_A::equal(__begin1, __end1, __begin2, __end2,
+				     __binary_pred);
+#endif
+
       return __equal_switch(__begin1, __end1, __begin2, __end2, __binary_pred,
 			    std::__iterator_category(__begin1),
 			    std::__iterator_category(__begin2));
     }
-#endif
+#endif // C++14
 
   // Sequential fallback
   template<typename _IIter1, typename _IIter2>
@@ -391,10 +416,17 @@ namespace __parallel
 
   // Public interface
   template<typename _IIter1, typename _IIter2>
+    _GLIBCXX20_CONSTEXPR
     inline bool
     lexicographical_compare(_IIter1 __begin1, _IIter1 __end1,
                             _IIter2 __begin2, _IIter2 __end2)
     {
+#if __cplusplus > 201703L
+      if (std::is_constant_evaluated())
+	return _GLIBCXX_STD_A::lexicographical_compare(__begin1, __end1,
+						       __begin2, __end2);
+#endif
+
       typedef iterator_traits<_IIter1> _TraitsType1;
       typedef typename _TraitsType1::value_type _ValueType1;
       typedef typename _TraitsType1::iterator_category _IteratorCategory1;
@@ -411,11 +443,19 @@ namespace __parallel
 
   // Public interface
   template<typename _IIter1, typename _IIter2, typename _Predicate>
+    _GLIBCXX20_CONSTEXPR
     inline bool
     lexicographical_compare(_IIter1 __begin1, _IIter1 __end1,
                             _IIter2 __begin2, _IIter2 __end2,
                             _Predicate __pred)
     {
+#if __cplusplus > 201703L
+      if (std::is_constant_evaluated())
+	return _GLIBCXX_STD_A::lexicographical_compare(__begin1, __end1,
+						       __begin2, __end2,
+						       __pred);
+#endif
+
       typedef iterator_traits<_IIter1> _TraitsType1;
       typedef typename _TraitsType1::iterator_category _IteratorCategory1;
 
diff --git a/libstdc++-v3/include/parallel/algorithmfwd.h b/libstdc++-v3/include/parallel/algorithmfwd.h
index a6d03a5..a227eba 100644
--- a/libstdc++-v3/include/parallel/algorithmfwd.h
+++ b/libstdc++-v3/include/parallel/algorithmfwd.h
@@ -130,10 +130,12 @@ namespace __parallel
           __gnu_parallel::sequential_tag);
 
   template<typename _IIter1, typename _IIter2>
+    _GLIBCXX20_CONSTEXPR
     bool
     equal(_IIter1, _IIter1, _IIter2);
 
   template<typename _IIter1, typename _IIter2, typename _Predicate>
+    _GLIBCXX20_CONSTEXPR
     bool
     equal(_IIter1, _IIter1, _IIter2, _Predicate);
 
@@ -285,10 +287,12 @@ namespace __parallel
                             __gnu_parallel::sequential_tag);
 
   template<typename _IIter1, typename _IIter2>
+    _GLIBCXX20_CONSTEXPR
     bool
     lexicographical_compare(_IIter1, _IIter1, _IIter2, _IIter2);
 
   template<typename _IIter1, typename _IIter2, typename _Predicate>
+    _GLIBCXX20_CONSTEXPR
     bool
     lexicographical_compare(_IIter1, _IIter1, _IIter2, _IIter2, _Predicate);