[PATCH] copy_n for input iterator, avoid in-loop jumps

[Petr Ovtchenkov]

Reword loop in copy_n specialization for input iterator.
Avoid condition check and jumps within loop.
Pay attention that input iterator incremented n - 1 times,
while output iterator incremented n times.

See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=50119 and
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81857
---
 libstdc++-v3/include/bits/stl_algo.h | 22 +++++++++++-----------
 1 file changed, 11 insertions(+), 11 deletions(-)

diff --git a/libstdc++-v3/include/bits/stl_algo.h 
b/libstdc++-v3/include/bits/stl_algo.h
index c2ac031..bf043cd 100644
--- a/libstdc++-v3/include/bits/stl_algo.h
+++ b/libstdc++-v3/include/bits/stl_algo.h
@@ -757,17 +757,17 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
             _OutputIterator __result, input_iterator_tag)
     {
       if (__n > 0)
-       {
-         while (true)
-           {
-             *__result = *__first;
-             ++__result;
-             if (--__n > 0)
-               ++__first;
-             else
-               break;
-           }
-       }
+        {
+          *__result = *__first;
+          ++__result;
+          --__n;
+        }
+      for (; __n > 0; --__n)
+        {
+          ++__first;
+          *__result = *__first;
+          ++__result;
+        }
       return __result;
     }
 
-- 
2.10.1