Fix usage of char type builtins or mem-functions wrt constexpr-ness (#34, thanks to @mcskatkat) (#38)

Add reporting of various builtin-related macros

Author: martinmoene

include/nonstd/string_view.hpp

@@ -299,24 +299,41 @@ using std::operator<<;
 
 // Presence of compiler intrinsics:
 
-#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 )
+// Providing char-type specializations for compare() and length() that
+// use compiler intrinsics can improve compile- and run-time performance.
+//
+// The challenge is in using the right combinations of builtin availablity
+// and its constexpr-ness.
+//
+// | compiler | __builtin_memcmp (constexpr) | memcmp  (constexpr) |
+// |----------|------------------------------|---------------------|
+// | clang    | 4.0              (>= 4.0   ) | any     (?        ) |
+// | clang-a  | 9.0              (>= 9.0   ) | any     (?        ) |
+// | gcc      | any              (constexpr) | any     (?        ) |
+// | msvc     | >= 14.2          (>= 14.2  ) | any     (?        ) |
+
+#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 )
+#define nssv_HAVE_BUILTIN_CE        nssv_HAVE_BUILTIN_VER
+
+#define nssv_HAVE_BUILTIN_MEMCMP  ( (nssv_HAVE_CONSTEXPR_14 && nssv_HAVE_BUILTIN_CE) || !nssv_HAVE_CONSTEXPR_14 )
+#define nssv_HAVE_BUILTIN_STRLEN  ( (nssv_HAVE_CONSTEXPR_11 && nssv_HAVE_BUILTIN_CE) || !nssv_HAVE_CONSTEXPR_11 )
 
 #ifdef __has_builtin
-#define nssv_HAVE_BUILTIN( x )  __has_builtin( x )
+# define nssv_HAVE_BUILTIN( x )  __has_builtin( x )
 #else
-#define nssv_HAVE_BUILTIN( x )  0
+# define nssv_HAVE_BUILTIN( x )  0
 #endif
 
 #if nssv_HAVE_BUILTIN(__builtin_memcmp) || nssv_HAVE_BUILTIN_VER
-#define nssv_BUILTIN_MEMCMP  __builtin_memcmp
+# define nssv_BUILTIN_MEMCMP  __builtin_memcmp
 #else
-#define nssv_BUILTIN_MEMCMP  memcmp
+# define nssv_BUILTIN_MEMCMP  memcmp
 #endif
 
 #if nssv_HAVE_BUILTIN(__builtin_strlen) || nssv_HAVE_BUILTIN_VER
-#define nssv_BUILTIN_STRLEN  __builtin_strlen
+# define nssv_BUILTIN_STRLEN  __builtin_strlen
 #else
-#define nssv_BUILTIN_STRLEN  strlen
+# define nssv_BUILTIN_STRLEN  strlen
 #endif
 
 // C++ feature usage:

test/string-view-main.t.cpp

@@ -101,6 +101,18 @@ CASE( "presence of C++ library features" "[.stdlibrary]" )
 #endif
 }
 
+CASE( "usage of compiler intrinsics" "[.intrinsics]" )
+{
+#if nssv_USES_STD_STRING_VIEW
+    std::cout << "(Compiler version not available: using std::string_view)\n";
+#else
+    nssv_PRESENT( nssv_HAVE_BUILTIN_VER    );
+    nssv_PRESENT( nssv_HAVE_BUILTIN_CE     );
+    nssv_PRESENT( nssv_HAVE_BUILTIN_MEMCMP );
+    nssv_PRESENT( nssv_HAVE_BUILTIN_STRLEN );
+#endif
+}
+
 int main( int argc, char * argv[] )
 {
     return lest::run( specification(), argc, argv );