PR libstdc++/71044 optimize std::filesystem::path construction

This new implementation has a smaller footprint than the previous
implementation, due to replacing std::vector<_Cmpt> with a custom pimpl
type that only needs a single pointer. The _M_type enumeration is also
combined with the pimpl type, by using a tagged pointer, reducing
sizeof(path) further still.

Construction and modification of paths is now done more efficiently, by
splitting the input into a stack-based buffer of string_view objects
instead of a dynamically-allocated vector containing strings. Once the
final size is known only a single allocation is needed to reserve space
for it.  The append and concat operations no longer require constructing
temporary path objects, nor re-parsing the entire native pathname.

This results in algorithmic improvements to path construction, and
working with large paths is much faster.

	PR libstdc++/71044
	* include/bits/fs_path.h (path::path(path&&)): Add noexcept when
	appropriate. Move _M_cmpts instead of reparsing the native pathname.
	(path::operator=(const path&)): Do not define as defaulted.
	(path::operator/=, path::append): Call _M_append.
	(path::concat): Call _M_concat.
	(path::path(string_type, _Type): Change type of first parameter to
	basic_string_view<value_type>.
	(path::_M_append(basic_string_view<value_type>)): New member function.
	(path::_M_concat(basic_string_view<value_type>)): New member function.
	(_S_convert(value_type*, __null_terminated)): Return string view.
	(_S_convert(const value_type*, __null_terminated)): Return string view.
	(_S_convert(value_type*, value_type*))
	(_S_convert(const value_type*, const value_type*)): Add overloads for
	pairs of pointers.
	(_S_convert(_InputIterator, __null_terminated)): Construct string_type
	explicitly, for cases where _S_convert returns a string view.
	(path::_S_is_dir_sep): Replace with non-member is_dir_sep.
	(path::_M_trim, path::_M_add_root_name, path::_M_add_root_dir)
	(path::_M_add_filename): Remove.
	(path::_M_type()): New member function to replace _M_type data member.
	(path::_List): Define new struct type instead of using std::vector.
	(path::_Cmpt::_Cmpt(string_type, _Type, size_t)): Change type of
	first parameter to basic_string_view<value_type>.
	(path::operator+=(const path&)): Do not define inline.
	(path::operator+=(const string_type&)): Call _M_concat.
	(path::operator+=(const value_type*)): Likewise.
	(path::operator+=(value_type)): Likewise.
	(path::operator+=(basic_string_view<value_type>)): Likewise.
	(path::operator/=(const path&)): Do not define inline.
	(path::_M_append(path)): Remove.
	* python/libstdcxx/v6/printers.py (StdPathPrinter): New printer that
	understands the new path::_List type.
	* src/filesystem/std-path.cc (is_dir_sep): New function to replace
	path::_S_is_dir_sep.
	(path::_Parser): New helper class to parse strings as paths.
	(path::_List::_Impl): Define container type for path components.
	(path::_List): Define members.
	(path::operator=(const path&)): Define explicitly, to provide the
	strong exception safety guarantee.
	(path::operator/=(const path&)): Implement manually by processing
	each component of the argument, rather than using _M_split_cmpts
	to parse the entire string again.
	(path::_M_append(string_type)): Likewise.
	(path::operator+=(const path&)): Likewise.
	(path::_M_concat(string_type)): Likewise.
	(path::remove_filename()): Perform trim directly instead of calling
	_M_trim().
	(path::_M_split_cmpts()): Rewrite in terms of _Parser class.
	(path::_M_trim, path::_M_add_root_name, path::_M_add_root_dir)
	(path::_M_add_filename): Remove.
	* testsuite/27_io/filesystem/path/append/source.cc: Test appending a
	string view that aliases the path.
	testsuite/27_io/filesystem/path/concat/strings.cc: Test concatenating
	a string view that aliases the path.

From-SVN: r267106

libstdc++-v3/ChangeLog

 2018-12-13  Jonathan Wakely  <jwakely@redhat.com>
 
+	PR libstdc++/71044
+	* include/bits/fs_path.h (path::path(path&&)): Add noexcept when
+	appropriate. Move _M_cmpts instead of reparsing the native pathname.
+	(path::operator=(const path&)): Do not define as defaulted.
+	(path::operator/=, path::append): Call _M_append.
+	(path::concat): Call _M_concat.
+	(path::path(string_type, _Type): Change type of first parameter to
+	basic_string_view<value_type>.
+	(path::_M_append(basic_string_view<value_type>)): New member function.
+	(path::_M_concat(basic_string_view<value_type>)): New member function.
+	(_S_convert(value_type*, __null_terminated)): Return string view.
+	(_S_convert(const value_type*, __null_terminated)): Return string view.
+	(_S_convert(value_type*, value_type*))
+	(_S_convert(const value_type*, const value_type*)): Add overloads for
+	pairs of pointers.
+	(_S_convert(_InputIterator, __null_terminated)): Construct string_type
+	explicitly, for cases where _S_convert returns a string view.
+	(path::_S_is_dir_sep): Replace with non-member is_dir_sep.
+	(path::_M_trim, path::_M_add_root_name, path::_M_add_root_dir)
+	(path::_M_add_filename): Remove.
+	(path::_M_type()): New member function to replace _M_type data member.
+	(path::_List): Define new struct type instead of using std::vector.
+	(path::_Cmpt::_Cmpt(string_type, _Type, size_t)): Change type of
+	first parameter to basic_string_view<value_type>.
+	(path::operator+=(const path&)): Do not define inline.
+	(path::operator+=(const string_type&)): Call _M_concat.
+	(path::operator+=(const value_type*)): Likewise.
+	(path::operator+=(value_type)): Likewise.
+	(path::operator+=(basic_string_view<value_type>)): Likewise.
+	(path::operator/=(const path&)): Do not define inline.
+	(path::_M_append(path)): Remove.
+	* python/libstdcxx/v6/printers.py (StdPathPrinter): New printer that
+	understands the new path::_List type.
+	* src/filesystem/std-path.cc (is_dir_sep): New function to replace
+	path::_S_is_dir_sep.
+	(path::_Parser): New helper class to parse strings as paths.
+	(path::_List::_Impl): Define container type for path components.
+	(path::_List): Define members.
+	(path::operator=(const path&)): Define explicitly, to provide the
+	strong exception safety guarantee.
+	(path::operator/=(const path&)): Implement manually by processing
+	each component of the argument, rather than using _M_split_cmpts
+	to parse the entire string again.
+	(path::_M_append(string_type)): Likewise.
+	(path::operator+=(const path&)): Likewise.
+	(path::_M_concat(string_type)): Likewise.
+	(path::remove_filename()): Perform trim directly instead of calling
+	_M_trim().
+	(path::_M_split_cmpts()): Rewrite in terms of _Parser class.
+	(path::_M_trim, path::_M_add_root_name, path::_M_add_root_dir)
+	(path::_M_add_filename): Remove.
+	* testsuite/27_io/filesystem/path/append/source.cc: Test appending a
+	string view that aliases the path.
+	testsuite/27_io/filesystem/path/concat/strings.cc: Test concatenating
+	a string view that aliases the path.
+
 	* testsuite/27_io/filesystem/path/generation/proximate.cc: Use
 	preferred directory separators for normalized paths.
 	* testsuite/27_io/filesystem/path/generation/relative.cc: Likewise.

libstdc++-v3/python/libstdcxx/v6/printers.py

@@ -1244,6 +1244,77 @@ class StdExpPathPrinter:
     def children(self):
         return self._iterator(self.val['_M_cmpts'])
 
+class StdPathPrinter:
+    "Print a std::filesystem::path"
+
+    def __init__ (self, typename, val):
+        self.val = val
+        self.typename = typename
+        impl = self.val['_M_cmpts']['_M_impl']['_M_t']['_M_t']['_M_head_impl']
+        self.type = impl.cast(gdb.lookup_type('uintptr_t')) & 3
+        if self.type == 0:
+            self.impl = impl
+        else:
+            self.impl = None
+
+    def _path_type(self):
+        t = str(self.type.cast(gdb.lookup_type(self.typename + '::_Type')))
+        if t[-9:] == '_Root_dir':
+            return "root-directory"
+        if t[-10:] == '_Root_name':
+            return "root-name"
+        return None
+
+    def to_string (self):
+        path = "%s" % self.val ['_M_pathname']
+        if self.type != 0:
+            t = self._path_type()
+            if t:
+                path = '%s [%s]' % (path, t)
+        return "filesystem::path %s" % path
+
+    class _iterator(Iterator):
+        def __init__(self, impl, pathtype):
+            if impl:
+                # We can't access _Impl::_M_size because _Impl is incomplete
+                # so cast to int* to access the _M_size member at offset zero,
+                int_type = gdb.lookup_type('int')
+                cmpt_type = gdb.lookup_type(pathtype+'::_Cmpt')
+                char_type = gdb.lookup_type('char')
+                impl = impl.cast(int_type.pointer())
+                size = impl.dereference()
+                #self.capacity = (impl + 1).dereference()
+                if hasattr(gdb.Type, 'alignof'):
+                    sizeof_Impl = max(2 * int_type.sizeof, cmpt_type.alignof)
+                else:
+                    sizeof_Impl = 2 * int_type.sizeof
+                begin = impl.cast(char_type.pointer()) + sizeof_Impl
+                self.item = begin.cast(cmpt_type.pointer())
+                self.finish = self.item + size
+                self.count = 0
+            else:
+                self.item = None
+                self.finish = None
+
+        def __iter__(self):
+            return self
+
+        def __next__(self):
+            if self.item == self.finish:
+                raise StopIteration
+            item = self.item.dereference()
+            count = self.count
+            self.count = self.count + 1
+            self.item = self.item + 1
+            path = item['_M_pathname']
+            t = StdPathPrinter(item.type.name, item)._path_type()
+            if not t:
+                t = count
+            return ('[%s]' % t, path)
+
+    def children(self):
+        return self._iterator(self.impl, self.typename)
+
 
 class StdPairPrinter:
     "Print a std::pair object, with 'first' and 'second' as children"
@@ -1759,9 +1830,9 @@ def build_libstdcxx_dictionary ():
     libstdcxx_printer.add_version('std::experimental::filesystem::v1::__cxx11::',
                                   'path', StdExpPathPrinter)
     libstdcxx_printer.add_version('std::filesystem::',
-                                  'path', StdExpPathPrinter)
+                                  'path', StdPathPrinter)
     libstdcxx_printer.add_version('std::filesystem::__cxx11::',
-                                  'path', StdExpPathPrinter)
+                                  'path', StdPathPrinter)
 
     # C++17 components
     libstdcxx_printer.add_version('std::',

libstdc++-v3/testsuite/27_io/filesystem/path/append/source.cc

@@ -112,6 +112,33 @@ test04()
 #endif
 }
 
+void
+test05()
+{
+  std::basic_string_view<path::value_type> s;
+
+  path p = "0/1/2/3/4/5/6";
+  // The string_view aliases the path's internal string:
+  s = p.native();
+  // Append that string_view, which must work correctly even though the
+  // internal string will be reallocated during the operation:
+  p /= s;
+  VERIFY( p.string() == "0/1/2/3/4/5/6/0/1/2/3/4/5/6" );
+
+  // Same again with a trailing slash:
+  path p2 = "0/1/2/3/4/5/";
+  s = p2.native();
+  p2 /= s;
+  VERIFY( p2.string() == "0/1/2/3/4/5/0/1/2/3/4/5/" );
+
+  // And aliasing one of the components of the path:
+  path p3 = "0/123456789/a";
+  path::iterator second = std::next(p3.begin());
+  s = second->native();
+  p3 /= s;
+  VERIFY( p3.string() == "0/123456789/a/123456789" );
+}
+
 int
 main()
 {
@@ -119,4 +146,5 @@ main()
   test02();
   test03();
   test04();
+  test05();
 }

libstdc++-v3/testsuite/27_io/filesystem/path/concat/strings.cc

@@ -57,8 +57,36 @@ test01()
   VERIFY( p.filename().string() == file );
 }
 
+void
+test02()
+{
+  std::basic_string_view<path::value_type> s;
+
+  path p = "0/1/2/3/4/5/6";
+  // The string_view aliases the path's internal string:
+  s = p.native();
+  // Append that string_view, which must work correctly even though the
+  // internal string will be reallocated during the operation:
+  p += s;
+  VERIFY( p.string() == "0/1/2/3/4/5/60/1/2/3/4/5/6" );
+
+  // Same again with a trailing slash:
+  path p2 = "0/1/2/3/4/5/";
+  s = p2.native();
+  p2 += s;
+  VERIFY( p2.string() == "0/1/2/3/4/5/0/1/2/3/4/5/" );
+
+  // And aliasing one of the components of the path:
+  path p3 = "0/123456789";
+  path::iterator second = std::next(p3.begin());
+  s = second->native();
+  p3 += s;
+  VERIFY( p3.string() == "0/123456789123456789" );
+}
+
 int
 main()
 {
   test01();
+  test02();
 }