* [committed] libstdc++: Begin lifetime of chars in constexpr std::string [PR103295]
@ 2021-11-19 18:22 Jonathan Wakely
2021-11-19 20:21 ` [committed] libstdc++: One more change for Clang to support " Jonathan Wakely
0 siblings, 1 reply; 2+ messages in thread
From: Jonathan Wakely @ 2021-11-19 18:22 UTC (permalink / raw)
To: libstdc++, gcc-patches
Tested powerpc64le-linux (and clang on x86_64-linux), pushed to trunk.
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.
---
libstdc++-v3/include/bits/basic_string.h | 40 +++++++-
libstdc++-v3/include/bits/basic_string.tcc | 7 +-
libstdc++-v3/include/bits/char_traits.h | 92 ++++++++++++++++---
libstdc++-v3/include/ext/vstring.h | 1 +
libstdc++-v3/include/std/string_view | 2 +
.../capacity/char/resize_and_overwrite.cc | 14 +++
6 files changed, 141 insertions(+), 15 deletions(-)
diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
index d29c9cdc410..6e7de738308 100644
--- a/libstdc++-v3/include/bits/basic_string.h
+++ b/libstdc++-v3/include/bits/basic_string.h
@@ -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.
diff --git a/libstdc++-v3/include/bits/basic_string.tcc b/libstdc++-v3/include/bits/basic_string.tcc
index 9a54b63b933..374406c0e13 100644
--- a/libstdc++-v3/include/bits/basic_string.tcc
+++ b/libstdc++-v3/include/bits/basic_string.tcc
@@ -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);
diff --git a/libstdc++-v3/include/bits/char_traits.h b/libstdc++-v3/include/bits/char_traits.h
index 3c9f4ad9420..b1cdc55ea61 100644
--- a/libstdc++-v3/include/bits/char_traits.h
+++ b/libstdc++-v3/include/bits/char_traits.h
@@ -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
diff --git a/libstdc++-v3/include/ext/vstring.h b/libstdc++-v3/include/ext/vstring.h
index cb5872a7030..331282c0104 100644
--- a/libstdc++-v3/include/ext/vstring.h
+++ b/libstdc++-v3/include/ext/vstring.h
@@ -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)
{
diff --git a/libstdc++-v3/include/std/string_view b/libstdc++-v3/include/std/string_view
index fd92df6e425..f2863c1beb4 100644
--- a/libstdc++-v3/include/std/string_view
+++ b/libstdc++-v3/include/std/string_view
@@ -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
diff --git a/libstdc++-v3/testsuite/21_strings/basic_string/capacity/char/resize_and_overwrite.cc b/libstdc++-v3/testsuite/21_strings/basic_string/capacity/char/resize_and_overwrite.cc
index f0e81126a41..c9087b55190 100644
--- a/libstdc++-v3/testsuite/21_strings/basic_string/capacity/char/resize_and_overwrite.cc
+++ b/libstdc++-v3/testsuite/21_strings/basic_string/capacity/char/resize_and_overwrite.cc
@@ -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() );
}
--
2.31.1
^ permalink raw reply [flat|nested] 2+ messages in thread* [committed] libstdc++: One more change for Clang to support constexpr std::string [PR103295]
2021-11-19 18:22 [committed] libstdc++: Begin lifetime of chars in constexpr std::string [PR103295] Jonathan Wakely
@ 2021-11-19 20:21 ` Jonathan Wakely
0 siblings, 0 replies; 2+ messages in thread
From: Jonathan Wakely @ 2021-11-19 20:21 UTC (permalink / raw)
To: libstdc++, gcc-patches
Tested powerpc64le-linux (and x86_64-linux clang), pushed to trunk.
All writes into the allocated buffer need to be via traits_type::assign
to begin lifetimes.
libstdc++-v3/ChangeLog:
PR libstdc++/103295
* include/bits/basic_string.tcc (_M_construct): Use the
traits assign member to write into allcoated memory.
---
libstdc++-v3/include/bits/basic_string.tcc | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/libstdc++-v3/include/bits/basic_string.tcc b/libstdc++-v3/include/bits/basic_string.tcc
index 374406c0e13..6f619a08f70 100644
--- a/libstdc++-v3/include/bits/basic_string.tcc
+++ b/libstdc++-v3/include/bits/basic_string.tcc
@@ -201,7 +201,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
_M_data(__another);
_M_capacity(__capacity);
}
- _M_data()[__len++] = *__beg;
+ traits_type::assign(_M_data()[__len++], *__beg);
++__beg;
}
--
2.31.1
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2021-11-19 20:21 ` [committed] libstdc++: One more change for Clang to support " Jonathan Wakely
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