Re: Add dg-require-wchars to libstdc++ testsuite#

[Jonathan Wakely <jwakely@redhat.com>]

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On 14/01/21 13:41 +0000, Jonathan Wakely wrote:
><aside>
>Is it the case that the wchar_t type is defined on this target, it's
>just that libc doesn't have support for wcslen etc?  Because we should
>probably audit all our uses of _GLIBCXX_USE_WCHAR_T and find which
>ones actually need libc support and which just need the wchar_t type
>to exist. Some things really do need the libc support, but I suspect
>many others don't.
>
>It seems wrong that we can provide full support for char16_t and
>char32_t but not wchar_t, just because the former two don't depend on
>anything being present in libc. Why can't we just implement the same
>functionality for wchar_t without using libc?
>
>In fact, if we just define std::char_traits<wchar_t> generically
>without using any libc functions (or just using them as optimisations)
>we might be able to support std::basic_string<wchar_t> and iostream
>classes with almost no work. But that's something to consider in the
>future.
></aside>

Oops, I considered it already.

This untested patch should define std::char_traits<wchar_t> so it is
available if wchar_t is defined by the front end (which I assume is
always true, is that right?), only using optimized libc routines if
available.

This would be the first step to enabling std::wstring etc for targets
with no wchar_t support in libc.



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diff --git a/libstdc++-v3/include/bits/char_traits.h b/libstdc++-v3/include/bits/char_traits.h
index ea1e036f721..3a60478ea32 100644
--- a/libstdc++-v3/include/bits/char_traits.h
+++ b/libstdc++-v3/include/bits/char_traits.h
@@ -438,7 +438,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   };
 
 
-#ifdef _GLIBCXX_USE_WCHAR_T
+#ifdef __SIZEOF_WCHAR_T__
   /// 21.1.3.2  char_traits specializations
   template<>
     struct char_traits<wchar_t>
@@ -469,23 +469,21 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       {
 	if (__n == 0)
 	  return 0;
-#if __cplusplus >= 201703L
-	if (__builtin_constant_p(__n)
-	    && __constant_char_array_p(__s1, __n)
-	    && __constant_char_array_p(__s2, __n))
-	  return __gnu_cxx::char_traits<char_type>::compare(__s1, __s2, __n);
+#ifdef _GLIBCXX_USE_WCHAR_T
+	if (!__builtin_is_constant_evaluated())
+	  return wmemcmp(__s1, __s2, __n);
 #endif
-	return wmemcmp(__s1, __s2, __n);
+	return __gnu_cxx::char_traits<char_type>::compare(__s1, __s2, __n);
       }
 
       static _GLIBCXX17_CONSTEXPR size_t
       length(const char_type* __s)
       {
-#if __cplusplus >= 201703L
-	if (__constant_string_p(__s))
-	  return __gnu_cxx::char_traits<char_type>::length(__s);
+#ifdef _GLIBCXX_USE_WCHAR_T
+	if (!__builtin_is_constant_evaluated())
+	  return wcslen(__s);
 #endif
-	return wcslen(__s);
+	return __gnu_cxx::char_traits<char_type>::length(__s);
       }
 
       static _GLIBCXX17_CONSTEXPR const char_type*
@@ -493,13 +491,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       {
 	if (__n == 0)
 	  return 0;
-#if __cplusplus >= 201703L
-	if (__builtin_constant_p(__n)
-	    && __builtin_constant_p(__a)
-	    && __constant_char_array_p(__s, __n))
-	  return __gnu_cxx::char_traits<char_type>::find(__s, __n, __a);
+#ifdef _GLIBCXX_USE_WCHAR_T
+	if (!__builtin_is_constant_evaluated())
+	  return wmemchr(__s, __a, __n);
 #endif
-	return wmemchr(__s, __a, __n);
+	return __gnu_cxx::char_traits<char_type>::find(__s, __n, __a);
       }
 
       static _GLIBCXX20_CONSTEXPR char_type*
@@ -507,11 +503,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       {
 	if (__n == 0)
 	  return __s1;
-#ifdef __cpp_lib_is_constant_evaluated
-	if (std::is_constant_evaluated())
-	  return __gnu_cxx::char_traits<char_type>::move(__s1, __s2, __n);
+#ifdef _GLIBCXX_USE_WCHAR_T
+	if (!__builtin_is_constant_evaluated())
+	  return wmemmove(__s1, __s2, __n);
 #endif
-	return wmemmove(__s1, __s2, __n);
+	return __gnu_cxx::char_traits<char_type>::move(__s1, __s2, __n);
       }
 
       static _GLIBCXX20_CONSTEXPR char_type*
@@ -519,11 +515,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       {
 	if (__n == 0)
 	  return __s1;
-#ifdef __cpp_lib_is_constant_evaluated
-	if (std::is_constant_evaluated())
-	  return __gnu_cxx::char_traits<char_type>::copy(__s1, __s2, __n);
+#ifdef _GLIBCXX_USE_WCHAR_T
+	if (!__builtin_is_constant_evaluated())
+	  return wmemcpy(__s1, __s2, __n);
 #endif
-	return wmemcpy(__s1, __s2, __n);
+	return __gnu_cxx::char_traits<char_type>::copy(__s1, __s2, __n);
       }
 
       static _GLIBCXX20_CONSTEXPR char_type*
@@ -531,11 +527,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       {
 	if (__n == 0)
 	  return __s;
-#ifdef __cpp_lib_is_constant_evaluated
-	if (std::is_constant_evaluated())
-	  return __gnu_cxx::char_traits<char_type>::assign(__s, __n, __a);
+#ifdef _GLIBCXX_USE_WCHAR_T
+	if (!__builtin_is_constant_evaluated())
+	  return wmemset(__s, __a, __n);
 #endif
-	return wmemset(__s, __a, __n);
+	return __gnu_cxx::char_traits<char_type>::assign(__s, __n, __a);
       }
 
       static _GLIBCXX_CONSTEXPR char_type
@@ -558,7 +554,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       not_eof(const int_type& __c) _GLIBCXX_NOEXCEPT
       { return eq_int_type(__c, eof()) ? 0 : __c; }
   };
-#endif //_GLIBCXX_USE_WCHAR_T
+#endif // __SIZEOF_WCHAR_T__
 
 #ifdef _GLIBCXX_USE_CHAR8_T
   template<>