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libstdc++: Avoid unwanted allocations in filesystem::path 

When using COW strings, accessing _M_pathname[0] and similar non-const
accessors can cause the string to "leak", meaning it reallocates itself
if it shares ownership with another string object.

This causes test failures for --enable-fully-dynamic-string builds:
/home/jwakely/src/gcc/libstdc++-v3/testsuite/experimental/filesystem/path/construct/90634.cc:62: void test01(): Assertion 'bytes_allocated == 0' failed.
FAIL: experimental/filesystem/path/construct/90634.cc execution test

This FAIL happens because the fully-dynamic move constructor results in
shared ownership, so for path(std::move(std::string("foo"))) the
_M_pathname member shares ownership with the temporary, and the
non-const accesses in _M_split_cmpts() cause a new copy of the string to
be allocated. This un-sharing is wasteful, and entirely unnecessary when
sharing ownership with an rvalue that is about to release its ownership
anyway. Even for lvalues, sharing ownership is not a problem and
reallocating a unique copy of the string is wasteful.

This removes non-const accesses of _M_pathname in the
path::_M_split_cmpts() members.

libstdc++-v3/ChangeLog:

	* src/c++17/fs_path.cc (path::_M_split_cmpts()): Remove
	micro-optimization for "/" path.
	* src/filesystem/path.cc (path::_M_split_cmpts()): Only access
	the contents of _M_pathname using const member functions.
This commit is contained in:
Jonathan Wakely 2021-11-30 16:18:23 +00:00
parent 1e625a44f6
commit e9089e4fa9
2 changed files with 17 additions and 19 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

5
libstdc++-v3/src/c++17/fs_path.cc
View File

@ -1872,11 +1872,6 @@ path::_M_split_cmpts()
_M_cmpts.type(_Type::_Filename);
return;
}
if (_M_pathname.length() == 1 && _M_pathname[0] == preferred_separator)
{
_M_cmpts.type(_Type::_Root_dir);
return;
}
_Parser parser(_M_pathname);

31
libstdc++-v3/src/filesystem/path.cc
View File

@ -337,15 +337,18 @@ path::_M_split_cmpts()
_M_type = _Type::_Multi;
_M_cmpts.clear();
if (_M_pathname.empty())
// Use const-reference to access _M_pathname, to avoid "leaking" COW string.
const auto& pathname = _M_pathname;
if (pathname.empty())
return;
{
// Approximate count of components, to reserve space in _M_cmpts vector:
int count = 1;
bool saw_sep_last = _S_is_dir_sep(_M_pathname[0]);
bool saw_sep_last = _S_is_dir_sep(pathname[0]);
bool saw_non_sep = !saw_sep_last;
for (value_type c : _M_pathname)
for (value_type c : pathname)
{
if (_S_is_dir_sep(c))
saw_sep_last = true;
@ -363,13 +366,13 @@ path::_M_split_cmpts()
}
size_t pos = 0;
const size_t len = _M_pathname.size();
const size_t len = pathname.size();
// look for root name or root directory
if (_S_is_dir_sep(_M_pathname[0]))
if (_S_is_dir_sep(pathname[0]))
{
// look for root name, such as "//" or "//foo"
if (len > 1 && _M_pathname[1] == _M_pathname[0])
if (len > 1 && pathname[1] == pathname[0])
{
if (len == 2)
{
@ -378,11 +381,11 @@ path::_M_split_cmpts()
return;
}
if (!_S_is_dir_sep(_M_pathname[2]))
if (!_S_is_dir_sep(pathname[2]))
{
// got root name, find its end
pos = 3;
while (pos < len && !_S_is_dir_sep(_M_pathname[pos]))
while (pos < len && !_S_is_dir_sep(pathname[pos]))
++pos;
if (pos == len)
{
@ -409,7 +412,7 @@ path::_M_split_cmpts()
++pos;
}
#ifdef _GLIBCXX_FILESYSTEM_IS_WINDOWS
else if (len > 1 && _M_pathname[1] == L':')
else if (len > 1 && pathname[1] == L':')
{
// got disk designator
if (len == 2)
@ -418,7 +421,7 @@ path::_M_split_cmpts()
return;
}
_M_add_root_name(2);
if (len > 2 && _S_is_dir_sep(_M_pathname[2]))
if (len > 2 && _S_is_dir_sep(pathname[2]))
_M_add_root_dir(2);
pos = 2;
}
@ -426,9 +429,9 @@ path::_M_split_cmpts()
else
{
size_t n = 1;
for (; n < _M_pathname.size() && !_S_is_dir_sep(_M_pathname[n]); ++n)
for (; n < pathname.size() && !_S_is_dir_sep(pathname[n]); ++n)
{ }
if (n == _M_pathname.size())
if (n == pathname.size())
{
_M_type = _Type::_Filename;
return;
@ -438,7 +441,7 @@ path::_M_split_cmpts()
size_t back = pos;
while (pos < len)
{
if (_S_is_dir_sep(_M_pathname[pos]))
if (_S_is_dir_sep(pathname[pos]))
{
if (back != pos)
_M_add_filename(back, pos - back);
@ -450,7 +453,7 @@ path::_M_split_cmpts()
if (back != pos)
_M_add_filename(back, pos - back);
else if (_S_is_dir_sep(_M_pathname.back()))
else if (_S_is_dir_sep(pathname.back()))
{
// [path.itr]/8
// "Dot, if one or more trailing non-root slash characters are present."