Support constexpr for char type via builtins or mem-functions (#34, thanks to @mcskatkat) (#37)

Author: martinmoene

include/nonstd/string_view.hpp

@@ -299,9 +299,24 @@ using std::operator<<;
 
 // Presence of compiler intrinsics:
 
-#define nssv_HAVE_BUILTIN_FN    ( nssv_COMPILER_MSVC_VERSION >= 142 || nssv_COMPILER_CLANG_VERSION >= 400 || nssv_COMPILER_APPLECLANG_VERSION >= 900 )
-#define nssv_HAVE_BUILTIN_MEMCMP  nssv_HAVE_BUILTIN_FN
-#define nssv_HAVE_BUILTIN_STRLEN  nssv_HAVE_BUILTIN_FN
+#define nssv_HAVE_BUILTIN_VER   ( nssv_COMPILER_MSVC_VERSION >= 142 || nssv_COMPILER_GNUC_VERSION > 0 ||  nssv_COMPILER_CLANG_VERSION >= 400 || nssv_COMPILER_APPLECLANG_VERSION >= 900 )
+#ifdef __has_builtin
+#define nssv_HAVE_BUILTIN( x )  __has_builtin( x )
+#else
+#define nssv_HAVE_BUILTIN( x )  0
+#endif
+
+#if nssv_HAVE_BUILTIN(__builtin_memcmp) || nssv_HAVE_BUILTIN_VER
+#define nssv_BUILTIN_MEMCMP  __builtin_memcmp
+#else
+#define nssv_BUILTIN_MEMCMP  memcmp
+#endif
+
+#if nssv_HAVE_BUILTIN(__builtin_strlen) || nssv_HAVE_BUILTIN_VER
+#define nssv_BUILTIN_STRLEN  __builtin_strlen
+#else
+#define nssv_BUILTIN_STRLEN  strlen
+#endif
 
 // C++ feature usage:
 
@@ -439,7 +454,7 @@ inline nssv_constexpr14 int compare( CharT const * s1, CharT const * s2, std::si
 
 inline nssv_constexpr14 int compare( char const * s1, char const * s2, std::size_t count )
 {
-    return __builtin_memcmp( s1, s2, count );
+    return nssv_BUILTIN_MEMCMP( s1, s2, count );
 }
 
 #endif
@@ -448,9 +463,9 @@ inline nssv_constexpr14 int compare( char const * s1, char const * s2, std::size
 
 // specialization of length() for char, see also generic length() further below:
 
-inline nssv_constexpr int length( char const * s )
+inline nssv_constexpr std::size_t length( char const * s )
 {
-    return __builtin_strlen( s );
+    return nssv_BUILTIN_STRLEN( s );
 }
 
 #endif