compiler_types.h: Optimize __unqual_scalar_typeof compilation time

If the compiler supports C11's _Generic, use it to speed up compilation
times of __unqual_scalar_typeof(). GCC version 4.9 or later and
all supported versions of Clang support the feature (the oldest
supported compiler that doesn't support _Generic is GCC 4.8, for which
we use the slower alternative).

The non-_Generic variant relies on multiple expansions of
__pick_integer_type -> __pick_scalar_type -> __builtin_choose_expr,
which increases pre-processed code size, and can cause compile times to
increase in files with numerous expansions of READ_ONCE(), or other
users of __unqual_scalar_typeof().

Summary of compile-time benchmarking done by Arnd Bergmann:

  <baseline normalized time>  clang-11   gcc-9
  this patch                      0.78    0.91
  ideal                           0.76    0.86

See https://lkml.kernel.org/r/CAK8P3a3UYQeXhiufUevz=rwe09WM_vSTCd9W+KvJHJcOeQyWVA@mail.gmail.com

Further compile-testing done with:
  gcc 4.8, 4.9, 5.5, 6.4, 7.5, 8.4;
  clang 9, 10.
Reported-by: NArnd Bergmann <arnd@arndb.de>
Signed-off-by: NMarco Elver <elver@google.com>
Signed-off-by: NBorislav Petkov <bp@suse.de>
Acked-by: NPeter Zijlstra <peterz@infradead.org>
Tested-by: NArnd Bergmann <arnd@arndb.de>
Link: https://lkml.kernel.org/r/20200527103236.148700-1-elver@google.com
Link: https://lkml.kernel.org/r/CAK8P3a0RJtbVi1JMsfik=jkHCNFv+DJn_FeDg-YLW+ueQW3tNg@mail.gmail.com
[will: tweak new macros to make them a bit more readable]
Signed-off-by: NWill Deacon <will@kernel.org>

include/linux/compiler_types.h

@@ -213,7 +213,9 @@ struct ftrace_likely_data {
 /*
  * __unqual_scalar_typeof(x) - Declare an unqualified scalar type, leaving
  *			       non-scalar types unchanged.
- *
+ */
+#if defined(CONFIG_CC_IS_GCC) && CONFIG_GCC_VERSION < 40900
+/*
  * We build this out of a couple of helper macros in a vain attempt to
  * help you keep your lunch down while reading it.
  */
@@ -235,6 +237,25 @@ struct ftrace_likely_data {
 			__pick_integer_type(x, int,				\
 				__pick_integer_type(x, long,			\
 					__pick_integer_type(x, long long, x))))))
+#else
+/*
+ * If supported, prefer C11 _Generic for better compile-times. As above, 'char'
+ * is not type-compatible with 'signed char', and we define a separate case.
+ */
+#define __scalar_type_to_expr_cases(type)				\
+		unsigned type:	(unsigned type)0,			\
+		signed type:	(signed type)0
+
+#define __unqual_scalar_typeof(x) typeof(				\
+		_Generic((x),						\
+			 char:	(char)0,				\
+			 __scalar_type_to_expr_cases(char),		\
+			 __scalar_type_to_expr_cases(short),		\
+			 __scalar_type_to_expr_cases(int),		\
+			 __scalar_type_to_expr_cases(long),		\
+			 __scalar_type_to_expr_cases(long long),	\
+			 default: (x)))
+#endif
 
 /* Is this type a native word size -- useful for atomic operations */
 #define __native_word(t) \