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 [1]:
<baseline normalized time> clang-11 gcc-9
this patch 0.78 0.91
ideal 0.76 0.86
[1]
https://lkml.kernel.org/r/CAK8P3a3UYQeXhiufUevz=rwe09wm_vstcd9w+kvjhjcoeqy...@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 <[email protected]>
Signed-off-by: Marco Elver <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Nick Desaulniers <[email protected]>
Cc: Paul E. McKenney <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Stephen Rothwell <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Will Deacon <[email protected]>
Link:
https://lkml.kernel.org/r/CAK8P3a0RJtbVi1JMsfik=jkhcnfv+djn_fedg-ylw+ueqw3...@mail.gmail.com
---
Same version as in:
https://lkml.kernel.org/r/[email protected]
---
include/linux/compiler_types.h | 22 +++++++++++++++++++++-
1 file changed, 21 insertions(+), 1 deletion(-)
diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
index 5faf68eae204..a529fa263906 100644
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -245,7 +245,9 @@ 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.
*/
@@ -267,6 +269,24 @@ 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) \
+ type: (type)0, unsigned type: (unsigned type)0
+
+#define __unqual_scalar_typeof(x) typeof( \
+ _Generic((x), \
+ __scalar_type_to_expr_cases(char), \
+ signed char: (signed char)0, \
+ __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) \
--
2.27.0.rc0.183.gde8f92d652-goog
