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libstdc++: Simplify constexpr checks in std::char_traits [PR 91488]

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This removes the helper functions added by r8-1294 to detect whether the
char_traits member functions can be evaluated at compile time. Instead,
we can just use __builtin_constant_evaluated directly, which is well
supported by non-GCC compilers by now.

As a result, there is a chance that those members will no longer be
usable in constant expressions when using old versions of non-GCC
compilers. Make the relevant feature test macros depend on the
availability of __builtin_constant_evaluated, so they are defined only
when the feature is actualyl available.

The new testcase from the PR is added to the libitm testsuite, because
that's where we can be sure it's OK to use the -fgnu-tm option.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>

	PR libstdc++/91488

libstdc++-v3/ChangeLog:

	* include/bits/basic_string.h (__cpp_lib_constexpr_string): Only
	define when is_constant_evaluated is available.
	* include/bits/char_traits.h (__cpp_lib_constexpr_char_traits):
	Likewise.
	(__constant_string_p, __constant_array_p): Remove.
	(char_traits): Use is_constant_evaluated directly.
	* include/std/version (__cpp_lib_constexpr_char_traits)
	(__cpp_lib_constexpr_string): Only define when
	is_constant_evaluated is available.

libitm/ChangeLog:

	* testsuite/libitm.c++/libstdc++-pr91488.C: New test.
This commit is contained in:
Jonathan Wakely 2021-06-17 14:11:22 +01:00
parent 331e20a69b
commit b376b1ef38
4 changed files with 46 additions and 92 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

9
libitm/testsuite/libitm.c++/libstdc++-pr91488.C Normal file
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@ -0,0 +1,9 @@
// PR libstdc++/91488 "inlining failed in call to always_inline"
// { dg-do run }
// { dg-additional-options "-O1" }
#include <string>
int main() {
return std::char_traits<char>::length("");
}

7
libstdc++-v3/include/bits/basic_string.h
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@ -52,12 +52,13 @@ namespace std _GLIBCXX_VISIBILITY(default)
{ {
_GLIBCXX_BEGIN_NAMESPACE_VERSION _GLIBCXX_BEGIN_NAMESPACE_VERSION
#if __cplusplus == 201703L #ifdef __cpp_lib_is_constant_evaluated
// Support P1032R1 in C++20 (but not P0980R1 yet).
# define __cpp_lib_constexpr_string 201811L
#elif __cplusplus >= 201703L && _GLIBCXX_HAVE_BUILTIN_IS_CONSTANT_EVALUATED
// Support P0426R1 changes to char_traits in C++17. // Support P0426R1 changes to char_traits in C++17.
# define __cpp_lib_constexpr_string 201611L # define __cpp_lib_constexpr_string 201611L
#elif __cplusplus > 201703L #elif __cplusplus > 201703L
// Also support P1032R1 in C++20 (but not P0980R1 yet).
# define __cpp_lib_constexpr_string 201811L
#endif #endif
#if _GLIBCXX_USE_CXX11_ABI #if _GLIBCXX_USE_CXX11_ABI

108
libstdc++-v3/include/bits/char_traits.h
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@ -235,62 +235,12 @@ namespace std _GLIBCXX_VISIBILITY(default)
{ {
_GLIBCXX_BEGIN_NAMESPACE_VERSION _GLIBCXX_BEGIN_NAMESPACE_VERSION
#if __cplusplus >= 201703L #ifdef __cpp_lib_is_constant_evaluated
// Unofficial macro indicating P1032R1 support in C++20
#if __cplusplus == 201703L
// Unofficial macro indicating P0426R1 support
# define __cpp_lib_constexpr_char_traits 201611L
#else
// Also support P1032R1 in C++20
# define __cpp_lib_constexpr_char_traits 201811L # define __cpp_lib_constexpr_char_traits 201811L
#endif #elif __cplusplus >= 201703L && _GLIBCXX_HAVE_BUILTIN_IS_CONSTANT_EVALUATED
// Unofficial macro indicating P0426R1 support in C++17
/** # define __cpp_lib_constexpr_char_traits 201611L
* @brief Determine whether the characters of a NULL-terminated
* string are known at compile time.
* @param __s The string.
*
* Assumes that _CharT is a built-in character type.
*/
template<typename _CharT>
static _GLIBCXX_ALWAYS_INLINE constexpr bool
__constant_string_p(const _CharT* __s)
{
#ifdef _GLIBCXX_HAVE_BUILTIN_IS_CONSTANT_EVALUATED
(void) __s;
// In constexpr contexts all strings should be constant.
return __builtin_is_constant_evaluated();
#else
while (__builtin_constant_p(*__s) && *__s)
__s++;
return __builtin_constant_p(*__s);
#endif
}
/**
* @brief Determine whether the characters of a character array are
* known at compile time.
* @param __a The character array.
* @param __n Number of characters.
*
* Assumes that _CharT is a built-in character type.
*/
template<typename _CharT>
static _GLIBCXX_ALWAYS_INLINE constexpr bool
__constant_char_array_p(const _CharT* __a, size_t __n)
{
#ifdef _GLIBCXX_HAVE_BUILTIN_IS_CONSTANT_EVALUATED
(void) __a;
(void) __n;
// In constexpr contexts all character arrays should be constant.
return __builtin_is_constant_evaluated();
#else
size_t __i = 0;
while (__i < __n && __builtin_constant_p(__a[__i]))
__i++;
return __i == __n;
#endif
}
#endif #endif
// 21.1 // 21.1
@ -345,10 +295,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
{ {
if (__n == 0) if (__n == 0)
return 0; return 0;
#if __cplusplus >= 201703L #if __cplusplus >= 201703L && _GLIBCXX_HAVE_BUILTIN_IS_CONSTANT_EVALUATED
if (__builtin_constant_p(__n) if (__builtin_is_constant_evaluated())
&& __constant_char_array_p(__s1, __n)
&& __constant_char_array_p(__s2, __n))
{ {
for (size_t __i = 0; __i < __n; ++__i) for (size_t __i = 0; __i < __n; ++__i)
if (lt(__s1[__i], __s2[__i])) if (lt(__s1[__i], __s2[__i]))
@ -364,8 +312,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
static _GLIBCXX17_CONSTEXPR size_t static _GLIBCXX17_CONSTEXPR size_t
length(const char_type* __s) length(const char_type* __s)
{ {
#if __cplusplus >= 201703L #if __cplusplus >= 201703L && _GLIBCXX_HAVE_BUILTIN_IS_CONSTANT_EVALUATED
if (__constant_string_p(__s)) if (__builtin_is_constant_evaluated())
return __gnu_cxx::char_traits<char_type>::length(__s); return __gnu_cxx::char_traits<char_type>::length(__s);
#endif #endif
return __builtin_strlen(__s); return __builtin_strlen(__s);
@ -376,10 +324,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
{ {
if (__n == 0) if (__n == 0)
return 0; return 0;
#if __cplusplus >= 201703L #if __cplusplus >= 201703L && _GLIBCXX_HAVE_BUILTIN_IS_CONSTANT_EVALUATED
if (__builtin_constant_p(__n) if (__builtin_is_constant_evaluated())
&& __builtin_constant_p(__a)
&& __constant_char_array_p(__s, __n))
return __gnu_cxx::char_traits<char_type>::find(__s, __n, __a); return __gnu_cxx::char_traits<char_type>::find(__s, __n, __a);
#endif #endif
return static_cast<const char_type*>(__builtin_memchr(__s, __a, __n)); return static_cast<const char_type*>(__builtin_memchr(__s, __a, __n));
@ -476,10 +422,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
{ {
if (__n == 0) if (__n == 0)
return 0; return 0;
#if __cplusplus >= 201703L #if __cplusplus >= 201703L && _GLIBCXX_HAVE_BUILTIN_IS_CONSTANT_EVALUATED
if (__builtin_constant_p(__n) if (__builtin_is_constant_evaluated())
&& __constant_char_array_p(__s1, __n)
&& __constant_char_array_p(__s2, __n))
return __gnu_cxx::char_traits<char_type>::compare(__s1, __s2, __n); return __gnu_cxx::char_traits<char_type>::compare(__s1, __s2, __n);
#endif #endif
return wmemcmp(__s1, __s2, __n); return wmemcmp(__s1, __s2, __n);
@ -488,8 +432,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
static _GLIBCXX17_CONSTEXPR size_t static _GLIBCXX17_CONSTEXPR size_t
length(const char_type* __s) length(const char_type* __s)
{ {
#if __cplusplus >= 201703L #if __cplusplus >= 201703L && _GLIBCXX_HAVE_BUILTIN_IS_CONSTANT_EVALUATED
if (__constant_string_p(__s)) if (__builtin_is_constant_evaluated())
return __gnu_cxx::char_traits<char_type>::length(__s); return __gnu_cxx::char_traits<char_type>::length(__s);
#endif #endif
return wcslen(__s); return wcslen(__s);
@ -500,10 +444,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
{ {
if (__n == 0) if (__n == 0)
return 0; return 0;
#if __cplusplus >= 201703L #if __cplusplus >= 201703L && _GLIBCXX_HAVE_BUILTIN_IS_CONSTANT_EVALUATED
if (__builtin_constant_p(__n) if (__builtin_is_constant_evaluated())
&& __builtin_constant_p(__a)
&& __constant_char_array_p(__s, __n))
return __gnu_cxx::char_traits<char_type>::find(__s, __n, __a); return __gnu_cxx::char_traits<char_type>::find(__s, __n, __a);
#endif #endif
return wmemchr(__s, __a, __n); return wmemchr(__s, __a, __n);
@ -597,10 +539,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
{ {
if (__n == 0) if (__n == 0)
return 0; return 0;
#if __cplusplus > 201402 #if __cplusplus >= 201703L && _GLIBCXX_HAVE_BUILTIN_IS_CONSTANT_EVALUATED
if (__builtin_constant_p(__n) if (__builtin_is_constant_evaluated())
&& __constant_char_array_p(__s1, __n)
&& __constant_char_array_p(__s2, __n))
return __gnu_cxx::char_traits<char_type>::compare(__s1, __s2, __n); return __gnu_cxx::char_traits<char_type>::compare(__s1, __s2, __n);
#endif #endif
return __builtin_memcmp(__s1, __s2, __n); return __builtin_memcmp(__s1, __s2, __n);
@ -609,8 +549,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
static _GLIBCXX17_CONSTEXPR size_t static _GLIBCXX17_CONSTEXPR size_t
length(const char_type* __s) length(const char_type* __s)
{ {
#if __cplusplus > 201402 #if __cplusplus >= 201703L && _GLIBCXX_HAVE_BUILTIN_IS_CONSTANT_EVALUATED
if (__constant_string_p(__s)) if (__builtin_is_constant_evaluated())
return __gnu_cxx::char_traits<char_type>::length(__s); return __gnu_cxx::char_traits<char_type>::length(__s);
#endif #endif
size_t __i = 0; size_t __i = 0;
@ -624,10 +564,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
{ {
if (__n == 0) if (__n == 0)
return 0; return 0;
#if __cplusplus > 201402 #if __cplusplus >= 201703L && _GLIBCXX_HAVE_BUILTIN_IS_CONSTANT_EVALUATED
if (__builtin_constant_p(__n) if (__builtin_is_constant_evaluated())
&& __builtin_constant_p(__a)
&& __constant_char_array_p(__s, __n))
return __gnu_cxx::char_traits<char_type>::find(__s, __n, __a); return __gnu_cxx::char_traits<char_type>::find(__s, __n, __a);
#endif #endif
return static_cast<const char_type*>(__builtin_memchr(__s, __a, __n)); return static_cast<const char_type*>(__builtin_memchr(__s, __a, __n));

14
libstdc++-v3/include/std/version
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@ -130,8 +130,10 @@
#define __cpp_lib_chrono 201611 #define __cpp_lib_chrono 201611
#define __cpp_lib_clamp 201603 #define __cpp_lib_clamp 201603
#if __cplusplus == 201703L // N.B. updated value in C++20 #if __cplusplus == 201703L // N.B. updated value in C++20
# define __cpp_lib_constexpr_char_traits 201611L # if _GLIBCXX_HAVE_BUILTIN_IS_CONSTANT_EVALUATED
# define __cpp_lib_constexpr_string 201611L # define __cpp_lib_constexpr_char_traits 201611L
# define __cpp_lib_constexpr_string 201611L
# endif
#endif #endif
#define __cpp_lib_enable_shared_from_this 201603 #define __cpp_lib_enable_shared_from_this 201603
#define __cpp_lib_execution 201902L // FIXME: should be 201603L #define __cpp_lib_execution 201902L // FIXME: should be 201603L
@ -213,14 +215,18 @@
// FIXME: #define __cpp_lib_execution 201902L // FIXME: #define __cpp_lib_execution 201902L
#define __cpp_lib_integer_comparison_functions 202002L #define __cpp_lib_integer_comparison_functions 202002L
#define __cpp_lib_constexpr_algorithms 201806L #define __cpp_lib_constexpr_algorithms 201806L
#define __cpp_lib_constexpr_char_traits 201811L #ifdef __cpp_lib_is_constant_evaluated
# define __cpp_lib_constexpr_char_traits 201811L
#endif
#define __cpp_lib_constexpr_complex 201711L #define __cpp_lib_constexpr_complex 201711L
#define __cpp_lib_constexpr_dynamic_alloc 201907L #define __cpp_lib_constexpr_dynamic_alloc 201907L
#define __cpp_lib_constexpr_functional 201907L #define __cpp_lib_constexpr_functional 201907L
#define __cpp_lib_constexpr_iterator 201811L #define __cpp_lib_constexpr_iterator 201811L
#define __cpp_lib_constexpr_memory 201811L #define __cpp_lib_constexpr_memory 201811L
#define __cpp_lib_constexpr_numeric 201911L #define __cpp_lib_constexpr_numeric 201911L
#define __cpp_lib_constexpr_string 201811L #ifdef __cpp_lib_is_constant_evaluated
# define __cpp_lib_constexpr_string 201811L
#endif
#define __cpp_lib_constexpr_string_view 201811L #define __cpp_lib_constexpr_string_view 201811L
#define __cpp_lib_constexpr_tuple 201811L #define __cpp_lib_constexpr_tuple 201811L
#define __cpp_lib_constexpr_utility 201811L #define __cpp_lib_constexpr_utility 201811L