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libstdc++: Begin lifetime of chars in constexpr std::string [PR103295]

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Clang gives errors for constexpr std::string because the memory returned
by std::allocator<T>::allocate does not contain any objects yet, and
attempting to set them using char_traits::assign or char_traits::copy
fails with:

assignment to object outside its lifetime is not allowed in a constant expression
              *__result = *__first;
                        ^
This adds code to std::char_traits to use std::construct_at to begin
lifetimes when called during constant evaluation. To support
specializations of std::basic_string that don't use std::char_traits
there is now another layer of wrapper around the allocator_traits, so
that the lifetime of characters is begun as soon as the memory is
allocated. By doing it in the char traits and allocator traits, the rest
of basic_string can ignore the problem.

While modifying char_traits::copy and char_traits::assign to begin
lifetimes for the constexpr cases, I also replaced their uses of
std::copy and std::fill_n respectively. That means we don't need
<bits/stl_algobase.h> for char_traits.

libstdc++-v3/ChangeLog:

	PR libstdc++/103295
	* include/bits/basic_string.h (_Alloc_traits): Replace typedef
	with struct for C++20 mode.
	* include/bits/basic_string.tcc (_M_replace): Use _Alloc_traits
	for allocation.
	* include/bits/char_traits.h (__gnu_cxx::char_traits::assign):
	Use std::_Construct during constant evaluation.
	(__gnu_cxx::char_traits::assign(CharT*, const CharT*, size_t)):
	Likewise. Replace std::fill_n with memset or manual loop.
	(__gnu_cxx::char_traits::copy): Likewise, replacing std::copy
	with memcpy.
	* include/ext/vstring.h: Include <bits/stl_algobase.h> for
	std::min.
	* include/std/string_view: Likewise.
	* testsuite/21_strings/basic_string/capacity/char/resize_and_overwrite.cc:
	Add constexpr test.
This commit is contained in:
Jonathan Wakely 2021-11-18 10:33:14 +00:00
parent 458d2c6899
commit 2d76292bd6
6 changed files with 141 additions and 15 deletions
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40
libstdc++-v3/include/bits/basic_string.h
View file

@ -87,7 +87,37 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
{
typedef typename __gnu_cxx::__alloc_traits<_Alloc>::template
rebind<_CharT>::other _Char_alloc_type;
#if __cpp_lib_constexpr_string < 201907L
typedef __gnu_cxx::__alloc_traits<_Char_alloc_type> _Alloc_traits;
#else
template<typename _Traits2, typename _Dummy_for_PR85282>
struct _Alloc_traits_impl : __gnu_cxx::__alloc_traits<_Char_alloc_type>
{
typedef __gnu_cxx::__alloc_traits<_Char_alloc_type> _Base;
[[__gnu__::__always_inline__]]
static constexpr typename _Base::pointer
allocate(_Char_alloc_type& __a, typename _Base::size_type __n)
{
pointer __p = _Base::allocate(__a, __n);
if (__builtin_is_constant_evaluated())
// Begin the lifetime of characters in allocated storage.
for (size_type __i = 0; __i < __n; ++__i)
std::construct_at(__builtin_addressof(__p[__i]));
return __p;
}
};
template<typename _Dummy_for_PR85282>
struct _Alloc_traits_impl<char_traits<_CharT>, _Dummy_for_PR85282>
: __gnu_cxx::__alloc_traits<_Char_alloc_type>
{
// std::char_traits begins the lifetime of characters.
};
using _Alloc_traits = _Alloc_traits_impl<_Traits, void>;
#endif
// Types:
public:
@ -485,7 +515,10 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
basic_string()
_GLIBCXX_NOEXCEPT_IF(is_nothrow_default_constructible<_Alloc>::value)
: _M_dataplus(_M_local_data())
{ _M_set_length(0); }
{
_M_use_local_data();
_M_set_length(0);
}
/**
* @brief Construct an empty string using allocator @a a.
@ -494,7 +527,10 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
explicit
basic_string(const _Alloc& __a) _GLIBCXX_NOEXCEPT
: _M_dataplus(_M_local_data(), __a)
{ _M_set_length(0); }
{
_M_use_local_data();
_M_set_length(0);
}
/**
* @brief Construct string with copy of value of @a __str.

7
libstdc++-v3/include/bits/basic_string.tcc
View file

@ -490,7 +490,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
#if __cpp_lib_is_constant_evaluated
if (__builtin_is_constant_evaluated())
{
auto __newp = this->_M_get_allocator().allocate(__new_size);
auto __newp = _Alloc_traits::allocate(_M_get_allocator(),
__new_size);
_S_copy(__newp, this->_M_data(), __pos);
_S_copy(__newp + __pos, __s, __len2);
_S_copy(__newp + __pos + __len2, __p + __len1, __how_much);
@ -569,6 +570,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
{
__p = _M_create(__n, __capacity);
this->_S_copy(__p, _M_data(), length()); // exclude trailing null
#if __cpp_lib_is_constant_evaluated
if (__builtin_is_constant_evaluated())
traits_type::assign(__p + length(), __n - length(), _CharT());
#endif
_M_dispose();
_M_data(__p);
_M_capacity(__n);

92
libstdc++-v3/include/bits/char_traits.h
View file

@ -36,11 +36,11 @@
#pragma GCC system_header
#include <bits/stl_algobase.h> // std::copy, std::fill_n
#include <bits/postypes.h> // For streampos
#include <cwchar> // For WEOF, wmemmove, wmemset, etc.
#if __cplusplus > 201703L
#if __cplusplus >= 202002L
# include <compare>
# include <bits/stl_construct.h>
#endif
#ifndef _GLIBCXX_ALWAYS_INLINE
@ -100,7 +100,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
static _GLIBCXX14_CONSTEXPR void
assign(char_type& __c1, const char_type& __c2)
{ __c1 = __c2; }
{
#if __cpp_constexpr_dynamic_alloc && __cpp_lib_is_constant_evaluated
if (std::is_constant_evaluated())
std::construct_at(__builtin_addressof(__c1), __c2);
else
#endif
__c1 = __c2;
}
static _GLIBCXX_CONSTEXPR bool
eq(const char_type& __c1, const char_type& __c2)
@ -240,8 +247,16 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
char_traits<_CharT>::
copy(char_type* __s1, const char_type* __s2, std::size_t __n)
{
// NB: Inline std::copy so no recursive dependencies.
std::copy(__s2, __s2 + __n, __s1);
#if __cpp_lib_is_constant_evaluated
if (std::is_constant_evaluated())
{
for (std::size_t __i = 0; __i < __n; ++__i)
std::construct_at(__s1 + __i, __s2[__i]);
return __s1;
}
#endif
__builtin_memcpy(__s1, __s2, __n * sizeof(char_type));
return __s1;
}
@ -251,8 +266,26 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
char_traits<_CharT>::
assign(char_type* __s, std::size_t __n, char_type __a)
{
// NB: Inline std::fill_n so no recursive dependencies.
std::fill_n(__s, __n, __a);
#if __cpp_lib_is_constant_evaluated
if (std::is_constant_evaluated())
{
for (std::size_t __i = 0; __i < __n; ++__i)
std::construct_at(__s + __i, __a);
return __s;
}
#endif
if _GLIBCXX17_CONSTEXPR (sizeof(_CharT) == 1 && __is_trivial(_CharT))
{
unsigned char __c;
__builtin_memcpy(&__c, __builtin_addressof(__a), 1);
__builtin_memset(__s, __c, __n);
}
else
{
for (std::size_t __i = 0; __i < __n; ++__i)
__s[__i] = __a;
}
return __s;
}
@ -304,7 +337,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
static _GLIBCXX17_CONSTEXPR void
assign(char_type& __c1, const char_type& __c2) _GLIBCXX_NOEXCEPT
{ __c1 = __c2; }
{
#if __cpp_constexpr_dynamic_alloc && __cpp_lib_is_constant_evaluated
if (std::is_constant_evaluated())
std::construct_at(__builtin_addressof(__c1), __c2);
else
#endif
__c1 = __c2;
}
static _GLIBCXX_CONSTEXPR bool
eq(const char_type& __c1, const char_type& __c2) _GLIBCXX_NOEXCEPT
@ -435,7 +475,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
static _GLIBCXX17_CONSTEXPR void
assign(char_type& __c1, const char_type& __c2) _GLIBCXX_NOEXCEPT
{ __c1 = __c2; }
{
#if __cpp_constexpr_dynamic_alloc && __cpp_lib_is_constant_evaluated
if (std::is_constant_evaluated())
std::construct_at(__builtin_addressof(__c1), __c2);
else
#endif
__c1 = __c2;
}
static _GLIBCXX_CONSTEXPR bool
eq(const char_type& __c1, const char_type& __c2) _GLIBCXX_NOEXCEPT
@ -556,7 +603,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
static _GLIBCXX17_CONSTEXPR void
assign(char_type& __c1, const char_type& __c2) _GLIBCXX_NOEXCEPT
{ __c1 = __c2; }
{
#if __cpp_constexpr_dynamic_alloc && __cpp_lib_is_constant_evaluated
if (std::is_constant_evaluated())
std::construct_at(__builtin_addressof(__c1), __c2);
else
#endif
__c1 = __c2;
}
static _GLIBCXX_CONSTEXPR bool
eq(const char_type& __c1, const char_type& __c2) _GLIBCXX_NOEXCEPT
@ -692,7 +746,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
static _GLIBCXX17_CONSTEXPR void
assign(char_type& __c1, const char_type& __c2) noexcept
{ __c1 = __c2; }
{
#if __cpp_constexpr_dynamic_alloc && __cpp_lib_is_constant_evaluated
if (std::is_constant_evaluated())
std::construct_at(__builtin_addressof(__c1), __c2);
else
#endif
__c1 = __c2;
}
static constexpr bool
eq(const char_type& __c1, const char_type& __c2) noexcept
@ -806,7 +867,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
static _GLIBCXX17_CONSTEXPR void
assign(char_type& __c1, const char_type& __c2) noexcept
{ __c1 = __c2; }
{
#if __cpp_constexpr_dynamic_alloc && __cpp_lib_is_constant_evaluated
if (std::is_constant_evaluated())
std::construct_at(__builtin_addressof(__c1), __c2);
else
#endif
__c1 = __c2;
}
static constexpr bool
eq(const char_type& __c1, const char_type& __c2) noexcept

1
libstdc++-v3/include/ext/vstring.h
View file

@ -38,6 +38,7 @@
#include <ext/vstring_util.h>
#include <ext/rc_string_base.h>
#include <ext/sso_string_base.h>
#include <bits/stl_algobase.h> // std::min
namespace __gnu_cxx _GLIBCXX_VISIBILITY(default)
{

2
libstdc++-v3/include/std/string_view
View file

@ -39,9 +39,11 @@
#include <iosfwd>
#include <bits/char_traits.h>
#include <bits/functexcept.h>
#include <bits/functional_hash.h>
#include <bits/range_access.h>
#include <bits/ostream_insert.h>
#include <bits/stl_algobase.h>
#include <ext/numeric_traits.h>
#if __cplusplus >= 202002L

14
libstdc++-v3/testsuite/21_strings/basic_string/capacity/char/resize_and_overwrite.cc
View file

@ -105,10 +105,24 @@ test04()
}
}
constexpr bool
test05()
{
std::string s;
s.resize_and_overwrite(20, [](char* p, auto n) {
*p = '!'; // direct assignment should be OK
std::char_traits<char>::copy(p, "constexpr", 9);
return 9;
});
VERIFY( s == "constexpr" );
return true;
}
int main()
{
test01();
test02();
test03();
test04();
static_assert( test05() );
}