[PATCH v5 6/9] bitops: let optimize out non-atomic bitops on compile-time constants

Alexander Lobakin
2022-06-24T12:13:10Z

Currently, many architecture-specific non-atomic bitop
implementations use inline asm or other hacks which are faster or
more robust when working with "real" variables (i.e. fields from
the structures etc.), but the compilers have no clue how to optimize
them out when called on compile-time constants. That said, the
following code:

DECLARE_BITMAP(foo, BITS_PER_LONG) = { }; // -> unsigned long foo[1];
unsigned long bar = BIT(BAR_BIT);
unsigned long baz = 0;

__set_bit(FOO_BIT, foo);
baz |= BIT(BAZ_BIT);

BUILD_BUG_ON(!__builtin_constant_p(test_bit(FOO_BIT, foo));
BUILD_BUG_ON(!__builtin_constant_p(bar & BAR_BIT));
BUILD_BUG_ON(!__builtin_constant_p(baz & BAZ_BIT));

triggers the first assertion on x86_64, which means that the
compiler is unable to evaluate it to a compile-time initializer
when the architecture-specific bitop is used even if it's obvious.
In order to let the compiler optimize out such cases, expand the
bitop() macro to use the "constant" C non-atomic bitop
implementations when all of the arguments passed are compile-time
constants, which means that the result will be a compile-time
constant as well, so that it produces more efficient and simple
code in 100% cases, comparing to the architecture-specific
counterparts.

The savings are architecture, compiler and compiler flags dependent,
for example, on x86_64 -O2:

GCC 12: add/remove: 78/29 grow/shrink: 332/525 up/down: 31325/-61560 (-30235)
LLVM 13: add/remove: 79/76 grow/shrink: 184/537 up/down: 55076/-141892 (-86816)
LLVM 14: add/remove: 10/3 grow/shrink: 93/138 up/down: 3705/-6992 (-3287)

and ARM64 (courtesy of Mark):

GCC 11: add/remove: 92/29 grow/shrink: 933/2766 up/down: 39340/-82580 (-43240)
LLVM 14: add/remove: 21/11 grow/shrink: 620/651 up/down: 12060/-15824 (-3764)

Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Alexander Lobakin <alexandr.lobakin@intel.com>
Reviewed-by: Marco Elver <elver@google.com>
---
include/linux/bitops.h | 18 +++++++++++++++++-
1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/include/linux/bitops.h b/include/linux/bitops.h
index 3c3afbae1533..cf9bf65039f2 100644
--- a/include/linux/bitops.h
+++ b/include/linux/bitops.h
@@ -33,8 +33,24 @@ extern unsigned long __sw_hweight64(__u64 w);

#include <asm-generic/bitops/generic-non-atomic.h>

+/*
+ * Many architecture-specific non-atomic bitops contain inline asm code and due
+ * to that the compiler can't optimize them to compile-time expressions or
+ * constants. In contrary, generic_*() helpers are defined in pure C and
+ * compilers optimize them just well.
+ * Therefore, to make `unsigned long foo = 0; __set_bit(BAR, &foo)` effectively
+ * equal to `unsigned long foo = BIT(BAR)`, pick the generic C alternative when
+ * the arguments can be resolved at compile time. That expression itself is a
+ * constant and doesn't bring any functional changes to the rest of cases.
+ * The casts to `uintptr_t` are needed to mitigate `-Waddress` warnings when
+ * passing a bitmap from .bss or .data (-> `!!addr` is always true).
+ */
#define bitop(op, nr, addr) \
- op(nr, addr)
+ ((__builtin_constant_p(nr) && \
+ __builtin_constant_p((uintptr_t)(addr) != (uintptr_t)NULL) && \
+ (uintptr_t)(addr) != (uintptr_t)NULL && \
+ __builtin_constant_p(*(const unsigned long *)(addr))) ? \
+ const##op(nr, addr) : op(nr, addr))

#define __set_bit(nr, addr) bitop(___set_bit, nr, addr)
#define __clear_bit(nr, addr) bitop(___clear_bit, nr, addr)
--
2.36.1