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Subject[tip: locking/kcsan] compiler_types.h: Optimize __unqual_scalar_typeof compilation time
The following commit has been merged into the locking/kcsan branch of tip:

Commit-ID: a5dead405f6be1fb80555bdcb77c406bf133fdc8
Gitweb: https://git.kernel.org/tip/a5dead405f6be1fb80555bdcb77c406bf133fdc8
Author: Marco Elver <elver@google.com>
AuthorDate: Wed, 27 May 2020 12:32:36 +02:00
Committer: Borislav Petkov <bp@suse.de>
CommitterDate: Wed, 27 May 2020 14:03:26 +02:00

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
---
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 5faf68e..a529fa2 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) \
\
 
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