[C++ PATCH] Speed up inplace_merge algorithm & fix inefficient logic(PR c++/83938)

[chang jc]

Current std::inplace_merge() suffers from performance issue by inefficient
logic under limited memory,

It leads to performance downgrade.

Please help to review it.

Index: include/bits/stl_algo.h
===================================================================
--- include/bits/stl_algo.h     (revision 256871)
+++ include/bits/stl_algo.h     (working copy)
@@ -2437,7 +2437,7 @@
          _BidirectionalIterator __second_cut = __middle;
          _Distance __len11 = 0;
          _Distance __len22 = 0;
-         if (__len1 > __len2)
+         if (__len1 < __len2)
            {
              __len11 = __len1 / 2;
              std::advance(__first_cut, __len11);
@@ -2539,9 +2539,15 @@
       const _DistanceType __len1 = std::distance(__first, __middle);
       const _DistanceType __len2 = std::distance(__middle, __last);

+
       typedef _Temporary_buffer<_BidirectionalIterator, _ValueType> _TmpBuf;
-      _TmpBuf __buf(__first, __last);
-
+      _BidirectionalIterator __start, __end;
+      if (__len1 < __len2) {
+       __start = __first; __end = __middle;
+      } else {
+       __start = __middle; __end = __last;
+      }
+      _TmpBuf __buf(__start, ___end);
       if (__buf.begin() == 0)
        std::__merge_without_buffer
          (__first, __middle, __last, __len1, __len2, __comp);
Index: include/bits/stl_tempbuf.h
===================================================================
--- include/bits/stl_tempbuf.h  (revision 256871)
+++ include/bits/stl_tempbuf.h  (working copy)
@@ -95,7 +95,7 @@
                                                        std::nothrow));
          if (__tmp != 0)
            return std::pair<_Tp*, ptrdiff_t>(__tmp, __len);
-         __len /= 2;
+         __len = (__len + 1) / 2;
        }
       return std::pair<_Tp*, ptrdiff_t>(static_cast<_Tp*>(0), 0);
     }




Thanks