[Bug libstdc++/71044] Optimize std::filesystem implementation

["redi at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71044

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Author: redi
Date: Thu Dec 13 20:33:55 2018
New Revision: 267106

URL: https://gcc.gnu.org/viewcvs?rev=267106&root=gcc&view=rev
Log:
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.

Modified:
    trunk/libstdc++-v3/ChangeLog
    trunk/libstdc++-v3/include/bits/fs_path.h
    trunk/libstdc++-v3/python/libstdcxx/v6/printers.py
    trunk/libstdc++-v3/src/filesystem/std-path.cc
    trunk/libstdc++-v3/testsuite/27_io/filesystem/path/append/source.cc
    trunk/libstdc++-v3/testsuite/27_io/filesystem/path/concat/strings.cc