[gcc r16-4102] libstdc++: Reuse predicates in std::search and std::is_permutation

[Jonathan Wakely]

https://gcc.gnu.org/g:1cf6cda0554406f3d3ca641de7c4e360953aeb22

commit r16-4102-g1cf6cda0554406f3d3ca641de7c4e360953aeb22
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Sep 10 10:10:07 2025 +0100

    libstdc++: Reuse predicates in std::search and std::is_permutation
    
    Hoist construction of the call wrappers out of the loop when we're
    repeatedly creating a call wrapper with the same bound arguments.
    
    We need to be careful about iterators that return proxy references,
    because bind1st(pred, *first) could bind a reference to a prvalue proxy
    reference returned by *first. That would then be an invalid reference by
    the time we invoked the call wrapper.
    
    If we dereference the iterator first and store the result of that on the
    stack, then we don't have a prvalue proxy reference, and can bind it (or
    the value it refers to) into the call wrapper:
    
      auto&& val = *first; // lifetime extension
      auto wrapper = bind1st(pred, val);
      for (;;)
        /* use wrapper */;
    
    This ensures that the reference returned from *first outlives the call
    wrapper, whether it's a proxy reference or not.
    
    For C++98 compatibility in __search we can use __decltype(expr) instead
    of auto&&.
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/stl_algobase.h (__search, __is_permutation):
            Reuse predicate instead of creating a new one each time.
            * include/bits/stl_algo.h (__is_permutation): Likewise.
    
    Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>

Diff:
---
 libstdc++-v3/include/bits/stl_algo.h     | 14 +++++---------
 libstdc++-v3/include/bits/stl_algobase.h | 29 +++++++++++++----------------
 2 files changed, 18 insertions(+), 25 deletions(-)

diff --git a/libstdc++-v3/include/bits/stl_algo.h b/libstdc++-v3/include/bits/stl_algo.h
index 7ff5dd84a97a..bbd1800af779 100644
--- a/libstdc++-v3/include/bits/stl_algo.h
+++ b/libstdc++-v3/include/bits/stl_algo.h
@@ -3531,18 +3531,14 @@ _GLIBCXX_END_INLINE_ABI_NAMESPACE(_V2)
 
       for (_ForwardIterator1 __scan = __first1; __scan != __last1; ++__scan)
 	{
-	  if (__scan != std::__find_if(__first1, __scan,
-				       __gnu_cxx::__ops::bind1st(__pred,
-								 *__scan)))
+	  auto&& __scan_val = *__scan;
+	  auto __scaneq = __gnu_cxx::__ops::bind1st(__pred, __scan_val);
+	  if (__scan != std::__find_if(__first1, __scan, __scaneq))
 	    continue; // We've seen this one before.
 
-	  auto __matches = std::__count_if(__first2, __last2,
-					   __gnu_cxx::__ops::bind1st(__pred,
-								     *__scan));
+	  auto __matches = std::__count_if(__first2, __last2, __scaneq);
 	  if (0 == __matches
-	      || std::__count_if(__scan, __last1,
-				 __gnu_cxx::__ops::bind1st(__pred, *__scan))
-	      != __matches)
+		|| std::__count_if(__scan, __last1, __scaneq) != __matches)
 	    return false;
 	}
       return true;
diff --git a/libstdc++-v3/include/bits/stl_algobase.h b/libstdc++-v3/include/bits/stl_algobase.h
index 03f64fb3e6c3..443cbef76dee 100644
--- a/libstdc++-v3/include/bits/stl_algobase.h
+++ b/libstdc++-v3/include/bits/stl_algobase.h
@@ -2149,21 +2149,21 @@ _GLIBCXX_END_NAMESPACE_ALGO
       if (__first1 == __last1 || __first2 == __last2)
 	return __first1;
 
+      __decltype(*__first2) __first2_val(*__first2);
+      __decltype(__gnu_cxx::__ops::bind2nd(__predicate, __first2_val))
+	__match_first = __gnu_cxx::__ops::bind2nd(__predicate, __first2_val);
+
       // Test for a pattern of length 1.
       _ForwardIterator2 __p1(__first2);
       if (++__p1 == __last2)
-	return std::__find_if(__first1, __last1,
-			      __gnu_cxx::__ops::bind2nd(__predicate,
-							*__first2));
+	return std::__find_if(__first1, __last1, __match_first);
 
       // General case.
       _ForwardIterator1 __current = __first1;
 
       for (;;)
 	{
-	  __first1 =
-	    std::__find_if(__first1, __last1,
-			   __gnu_cxx::__ops::bind2nd(__predicate, *__first2));
+	  __first1 = std::__find_if(__first1, __last1, __match_first);
 
 	  if (__first1 == __last1)
 	    return __last1;
@@ -2184,6 +2184,7 @@ _GLIBCXX_END_NAMESPACE_ALGO
 	}
       return __first1;
     }
+#undef __match_first
 
 #if __cplusplus >= 201103L
   template<typename _ForwardIterator1, typename _ForwardIterator2,
@@ -2208,18 +2209,14 @@ _GLIBCXX_END_NAMESPACE_ALGO
       std::advance(__last2, std::distance(__first1, __last1));
       for (_ForwardIterator1 __scan = __first1; __scan != __last1; ++__scan)
 	{
-	  if (__scan != std::__find_if(__first1, __scan,
-				       __gnu_cxx::__ops::bind1st(__pred,
-								 *__scan)))
+	  auto&& __scan_val = *__scan;
+	  auto __scaneq = __gnu_cxx::__ops::bind1st(__pred, __scan_val);
+	  if (__scan != std::__find_if(__first1, __scan, __scaneq))
 	    continue; // We've seen this one before.
 
-	  auto __matches
-	    = std::__count_if(__first2, __last2,
-			      __gnu_cxx::__ops::bind1st(__pred, *__scan));
-	  if (0 == __matches ||
-	      std::__count_if(__scan, __last1,
-			      __gnu_cxx::__ops::bind1st(__pred, *__scan))
-	      != __matches)
+	  auto __matches = std::__count_if(__first2, __last2, __scaneq);
+	  if (0 == __matches
+		|| std::__count_if(__scan, __last1, __scaneq) != __matches)
 	    return false;
 	}
       return true;