[gcc r15-6247] libstdc++: Avoid unnecessary copies in ranges::min/max [PR112349]

[Patrick Palka]

https://gcc.gnu.org/g:b8314ebff2495ee22f9e2203093bdada9843a0f5

commit r15-6247-gb8314ebff2495ee22f9e2203093bdada9843a0f5
Author: Patrick Palka <ppalka@redhat.com>
Date:   Fri Dec 13 13:17:29 2024 -0500

    libstdc++: Avoid unnecessary copies in ranges::min/max [PR112349]
    
    Use a local reference for the (now possibly lifetime extended) result of
    *__first so that we copy it only when necessary.
    
            PR libstdc++/112349
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/ranges_algo.h (__min_fn::operator()): Turn local
            object __tmp into a reference.
            * include/bits/ranges_util.h (__max_fn::operator()): Likewise.
            * testsuite/25_algorithms/max/constrained.cc (test04): New test.
            * testsuite/25_algorithms/min/constrained.cc (test04): New test.
    
    Reviewed-by: Jonathan Wakely <jwakely@redhat.com>

Diff:
---
 libstdc++-v3/include/bits/ranges_algo.h            |  4 ++--
 libstdc++-v3/include/bits/ranges_util.h            |  4 ++--
 .../testsuite/25_algorithms/max/constrained.cc     | 25 ++++++++++++++++++++++
 .../testsuite/25_algorithms/min/constrained.cc     | 25 ++++++++++++++++++++++
 4 files changed, 54 insertions(+), 4 deletions(-)

diff --git a/libstdc++-v3/include/bits/ranges_algo.h b/libstdc++-v3/include/bits/ranges_algo.h
index 80d4f5a0d574..772bf4dd997b 100644
--- a/libstdc++-v3/include/bits/ranges_algo.h
+++ b/libstdc++-v3/include/bits/ranges_algo.h
@@ -2952,11 +2952,11 @@ namespace ranges
 	auto __result = *__first;
 	while (++__first != __last)
 	  {
-	    auto __tmp = *__first;
+	    auto&& __tmp = *__first;
 	    if (std::__invoke(__comp,
 			      std::__invoke(__proj, __result),
 			      std::__invoke(__proj, __tmp)))
-	      __result = std::move(__tmp);
+	      __result = std::forward<decltype(__tmp)>(__tmp);
 	  }
 	return __result;
       }
diff --git a/libstdc++-v3/include/bits/ranges_util.h b/libstdc++-v3/include/bits/ranges_util.h
index 7be76e07899b..b7a3c7a03ad3 100644
--- a/libstdc++-v3/include/bits/ranges_util.h
+++ b/libstdc++-v3/include/bits/ranges_util.h
@@ -757,11 +757,11 @@ namespace ranges
 	auto __result = *__first;
 	while (++__first != __last)
 	  {
-	    auto __tmp = *__first;
+	    auto&& __tmp = *__first;
 	    if (std::__invoke(__comp,
 			      std::__invoke(__proj, __tmp),
 			      std::__invoke(__proj, __result)))
-	      __result = std::move(__tmp);
+	      __result = std::forward<decltype(__tmp)>(__tmp);
 	  }
 	return __result;
       }
diff --git a/libstdc++-v3/testsuite/25_algorithms/max/constrained.cc b/libstdc++-v3/testsuite/25_algorithms/max/constrained.cc
index e7269e1b734a..ad2d47f2f101 100644
--- a/libstdc++-v3/testsuite/25_algorithms/max/constrained.cc
+++ b/libstdc++-v3/testsuite/25_algorithms/max/constrained.cc
@@ -73,10 +73,35 @@ test03()
   VERIFY( ranges::max({2,3,1,4}, ranges::greater{}, std::negate<>{}) == 4 );
 }
 
+void
+test04()
+{
+  // PR libstdc++/112349 - ranges::max/min make unnecessary copies
+  static int copies, moves;
+  struct A {
+    A(int m) : m(m) { }
+    A(const A& other) : m(other.m) { ++copies; }
+    A(A&& other) : m(other.m) { ++moves; }
+    A& operator=(const A& other) { m = other.m; ++copies; return *this; }
+    A& operator=(A&& other) { m = other.m; ++moves; return *this; }
+    int m;
+  };
+  A r[5] = {5, 4, 3, 2, 1};
+  ranges::max(r, ranges::less{}, &A::m);
+  VERIFY( copies == 1 );
+  VERIFY( moves == 0 );
+  copies = moves = 0;
+  A s[5] = {1, 2, 3, 4, 5};
+  ranges::max(s, ranges::less{}, &A::m);
+  VERIFY( copies == 5 );
+  VERIFY( moves == 0 );
+}
+
 int
 main()
 {
   test01();
   test02();
   test03();
+  test04();
 }
diff --git a/libstdc++-v3/testsuite/25_algorithms/min/constrained.cc b/libstdc++-v3/testsuite/25_algorithms/min/constrained.cc
index 7198df69adf7..17048fda6394 100644
--- a/libstdc++-v3/testsuite/25_algorithms/min/constrained.cc
+++ b/libstdc++-v3/testsuite/25_algorithms/min/constrained.cc
@@ -73,10 +73,35 @@ test03()
   VERIFY( ranges::min({2,3,1,4}, ranges::greater{}, std::negate<>{}) == 1 );
 }
 
+void
+test04()
+{
+  // PR libstdc++/112349 - ranges::max/min make unnecessary copies
+  static int copies, moves;
+  struct A {
+    A(int m) : m(m) { }
+    A(const A& other) : m(other.m) { ++copies; }
+    A(A&& other) : m(other.m) { ++moves; }
+    A& operator=(const A& other) { m = other.m; ++copies; return *this; }
+    A& operator=(A&& other) { m = other.m; ++moves; return *this; }
+    int m;
+  };
+  A r[5] = {5, 4, 3, 2, 1};
+  ranges::min(r, ranges::less{}, &A::m);
+  VERIFY( copies == 5 );
+  VERIFY( moves == 0 );
+  copies = moves = 0;
+  A s[5] = {1, 2, 3, 4, 5};
+  ranges::min(s, ranges::less{}, &A::m);
+  VERIFY( copies == 1 );
+  VERIFY( moves == 0 );
+}
+
 int
 main()
 {
   test01();
   test02();
   test03();
+  test04();
 }