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: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Tested-by: Arnd 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: Will Deacon <will@kernel.org>

diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
index 6ed0612..31416b6 100644 (file)
--- a/include/linux/compiler_types.h
+++ b/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) \