Optimize filesystem::path::parent_path()

Parsing a complete string is more efficient than appending each
component one-by-one.

	* src/c++17/fs_path.cc (path::parent_path()): Create whole path at
	once instead of building it iteratively.

From-SVN: r271754

diff --git a/libstdc++-v3/ChangeLog b/libstdc++-v3/ChangeLog
index b209460..b6b7a05 100644 (file)
--- a/libstdc++-v3/ChangeLog
+++ b/libstdc++-v3/ChangeLog
@@ -1,5 +1,8 @@
 2019-05-29  Jonathan Wakely  <jwakely@redhat.com>
 
+       * src/c++17/fs_path.cc (path::parent_path()): Create whole path at
+       once instead of building it iteratively.
+
        * testsuite/util/testsuite_api.h: Remove names of unused parameters.
 
        PR libstdc++/85494 use rdseed and rand_s in std::random_device

diff --git a/libstdc++-v3/src/c++17/fs_path.cc b/libstdc++-v3/src/c++17/fs_path.cc
index 8e01bf5..c438ddc 100644 (file)
--- a/libstdc++-v3/src/c++17/fs_path.cc
+++ b/libstdc++-v3/src/c++17/fs_path.cc
@@ -1523,11 +1523,9 @@ path::parent_path() const
     __ret = *this;
   else if (_M_cmpts.size() >= 2)
     {
-      for (auto __it = _M_cmpts.begin(), __end = std::prev(_M_cmpts.end());
-          __it != __end; ++__it)
-       {
-         __ret /= *__it;
-       }
+      const auto parent = std::prev(_M_cmpts.end(), 2);
+      const auto len = parent->_M_pos + parent->_M_pathname.length();
+      __ret.assign(_M_pathname.substr(0, len));
     }
   return __ret;
 }