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Avoid reallocation for basic_string::clear()

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PR libstdc++/56166
	PR libstdc++/77582
	* include/bits/basic_string.h (basic_string::clear()): Drop reference
	and use empty rep.
	* include/ext/rc_string_base.h (__rc_string_base::_M_clear()):
	Likewise.
	* testsuite/21_strings/basic_string/56166.cc: New.
	* testsuite/ext/vstring/modifiers/clear/56166.cc: New.

From-SVN: r240447
This commit is contained in:
Jonathan Wakely 2016-09-23 18:25:34 +01:00 • committed by Jonathan Wakely
parent a922c5ff6f
commit 1319041924
5 changed files with 216 additions and 1 deletions
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9
libstdc++-v3/ChangeLog
View file

@ -1,5 +1,14 @@
2016-09-23 Jonathan Wakely <jwakely@redhat.com>
PR libstdc++/56166
PR libstdc++/77582
* include/bits/basic_string.h (basic_string::clear()): Drop reference
and use empty rep.
* include/ext/rc_string_base.h (__rc_string_base::_M_clear()):
Likewise.
* testsuite/21_strings/basic_string/56166.cc: New.
* testsuite/ext/vstring/modifiers/clear/56166.cc: New.
* include/bits/basic_string.h [_GLIBCXX_USE_CXX11_ABI]
(basic_string::erase(size_type, size_type)): Add fast path for
truncating the string, by calling _M_set_length directly.

14
libstdc++-v3/include/bits/basic_string.h
View file

@ -3690,10 +3690,24 @@ _GLIBCXX_END_NAMESPACE_CXX11
/**
* Erases the string, making it empty.
*/
#if _GLIBCXX_FULLY_DYNAMIC_STRING == 0
void
clear() _GLIBCXX_NOEXCEPT
{
if (_M_rep()->_M_is_shared())
{
_M_rep()->_M_dispose(this->get_allocator());
_M_data(_S_empty_rep()._M_refdata());
}
else
_M_rep()->_M_set_length_and_sharable(0);
}
#else
// PR 56166: this should not throw.
void
clear()
{ _M_mutate(0, this->size(), 0); }
#endif
/**
* Returns true if the %string is empty. Equivalent to

5
libstdc++-v3/include/ext/rc_string_base.h
View file

@ -354,7 +354,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
void
_M_clear()
{ _M_erase(size_type(0), _M_length()); }
{
_M_dispose();
_M_data(_S_empty_rep._M_refcopy());
}
bool
_M_compare(const __rc_string_base&) const

93
libstdc++-v3/testsuite/21_strings/basic_string/56166.cc Normal file
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@ -0,0 +1,93 @@
// Copyright (C) 2016 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library. This library is free
// software; you can redistribute it and/or modify it under the
// terms of the GNU General Public License as published by the
// Free Software Foundation; either version 3, or (at your option)
// any later version.
// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU General Public License for more details.
// You should have received a copy of the GNU General Public License along
// with this library; see the file COPYING3. If not see
// <http://www.gnu.org/licenses/>.
// { dg-do run { target c++11 } }
// libstdc++/56166
#ifndef _GLIBCXX_USE_CXX11_ABI
# define _GLIBCXX_USE_CXX11_ABI 0
#endif
#include <string>
#include <new>
static int fail_after = -1;
template<typename T>
struct Allocator
{
using value_type = T;
// Need these typedefs because COW string doesn't use allocator_traits.
using pointer = T*;
using const_pointer = const T*;
using reference = T&;
using const_reference = const T&;
using difference_type = long;
using size_type = unsigned long;
template<typename U>
struct rebind {
using other = Allocator<U>;
};
Allocator() { }
template<typename U>
Allocator(const Allocator<U>&) { }
T* allocate(size_type n)
{
if (fail_after >= 0) {
if (fail_after-- == 0) {
throw std::bad_alloc();
}
}
return (T*)new char[n * sizeof(T)];
}
void deallocate(T* p, size_type)
{
delete[] (char*)p;
}
};
template<typename T, typename U>
bool operator==(const Allocator<T>&, const Allocator<U>&) { return true; }
template<typename T, typename U>
bool operator!=(const Allocator<T>&, const Allocator<U>&) { return false; }
using string = std::basic_string<char, std::char_traits<char>, Allocator<char>>;
string f()
{
string s1("xxxxxx");
string s2 = s1;
s1.clear();
return s2;
}
int main()
{
for (int i = 0; i < 10; i++) {
try {
fail_after = i;
f();
break;
} catch (std::bad_alloc) {
}
}
}

96
libstdc++-v3/testsuite/ext/vstring/modifiers/clear/56166.cc Normal file
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@ -0,0 +1,96 @@
// Copyright (C) 2016 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library. This library is free
// software; you can redistribute it and/or modify it under the
// terms of the GNU General Public License as published by the
// Free Software Foundation; either version 3, or (at your option)
// any later version.
// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU General Public License for more details.
// You should have received a copy of the GNU General Public License along
// with this library; see the file COPYING3. If not see
// <http://www.gnu.org/licenses/>.
// { dg-do run { target c++11 } }
// libstdc++/56166
#ifndef _GLIBCXX_USE_CXX11_ABI
# define _GLIBCXX_USE_CXX11_ABI 0
#endif
#include <ext/vstring.h>
#include <new>
static int fail_after = -1;
template<typename T>
struct Allocator
{
using value_type = T;
// Need these typedefs because COW string doesn't use allocator_traits.
using pointer = T*;
using const_pointer = const T*;
using reference = T&;
using const_reference = const T&;
using difference_type = long;
using size_type = unsigned long;
template<typename U>
struct rebind {
using other = Allocator<U>;
};
Allocator() { }
template<typename U>
Allocator(const Allocator<U>&) { }
T* allocate(size_type n)
{
if (fail_after >= 0) {
if (fail_after-- == 0) {
throw std::bad_alloc();
}
}
return (T*)new char[n * sizeof(T)];
}
void deallocate(T* p, size_type)
{
delete[] (char*)p;
}
};
template<typename T, typename U>
bool operator==(const Allocator<T>&, const Allocator<U>&) { return true; }
template<typename T, typename U>
bool operator!=(const Allocator<T>&, const Allocator<U>&) { return false; }
using string = __gnu_cxx::__versa_string<char, std::char_traits<char>,
Allocator<char>,
__gnu_cxx::__rc_string_base>;
string f()
{
string s1("xxxxxx");
string s2 = s1;
s1.clear();
return s2;
}
int main()
{
for (int i = 0; i < 10; i++) {
try {
fail_after = i;
f();
break;
} catch (std::bad_alloc) {
}
}
}