From: Alexander Lobakin <aleksander.lobakin@intel.com>
To: Yury Norov <yury.norov@gmail.com>
Cc: Alexander Lobakin <aleksander.lobakin@intel.com>,
Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
Rasmus Villemoes <linux@rasmusvillemoes.dk>,
Alexander Potapenko <glider@google.com>,
Jakub Kicinski <kuba@kernel.org>,
Eric Dumazet <edumazet@google.com>,
David Ahern <dsahern@kernel.org>,
Przemek Kitszel <przemyslaw.kitszel@intel.com>,
Simon Horman <simon.horman@corigine.com>,
netdev@vger.kernel.org, linux-btrfs@vger.kernel.org,
dm-devel@redhat.com, ntfs3@lists.linux.dev,
linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: [PATCH 03/14] bitops: let the compiler optimize __assign_bit()
Date: Mon, 9 Oct 2023 17:10:15 +0200 [thread overview]
Message-ID: <20231009151026.66145-4-aleksander.lobakin@intel.com> (raw)
In-Reply-To: <20231009151026.66145-1-aleksander.lobakin@intel.com>
Since commit b03fc1173c0c ("bitops: let optimize out non-atomic bitops
on compile-time constants"), the compilers are able to expand inline
bitmap operations to compile-time initializers when possible.
However, during the round of replacement if-__set-else-__clear with
__assign_bit() as per Andy's advice, bloat-o-meter showed +1024 bytes
difference in object code size for one module (even one function),
where the pattern:
DECLARE_BITMAP(foo) = { }; // on the stack, zeroed
if (a)
__set_bit(const_bit_num, foo);
if (b)
__set_bit(another_const_bit_num, foo);
...
is heavily used, although there should be no difference: the bitmap is
zeroed, so the second half of __assign_bit() should be compiled-out as
a no-op.
I either missed the fact that __assign_bit() has bitmap pointer marked
as `volatile` (as we usually do for bitmaps) or was hoping that the
compilers would at least try to look past the `volatile` for
__always_inline functions. Anyhow, due to that attribute, the compilers
were always compiling the whole expression and no mentioned compile-time
optimizations were working.
Convert __assign_bit() to a macro since it's a very simple if-else and
all of the checks are performed inside __set_bit() and __clear_bit(),
thus that wrapper has to be as transparent as possible. After that
change, despite it showing only -20 bytes change for vmlinux (due to
that it's still relatively unpopular), no drastic code size changes
happen when replacing if-set-else-clear for onstack bitmaps with
__assign_bit(), meaning the compiler now expands them to the actual
operations will all the expected optimizations.
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Signed-off-by: Alexander Lobakin <aleksander.lobakin@intel.com>
---
include/linux/bitops.h | 10 ++--------
1 file changed, 2 insertions(+), 8 deletions(-)
diff --git a/include/linux/bitops.h b/include/linux/bitops.h
index e0cd09eb91cd..f98f4fd1047f 100644
--- a/include/linux/bitops.h
+++ b/include/linux/bitops.h
@@ -284,14 +284,8 @@ static __always_inline void assign_bit(long nr, volatile unsigned long *addr,
clear_bit(nr, addr);
}
-static __always_inline void __assign_bit(long nr, volatile unsigned long *addr,
- bool value)
-{
- if (value)
- __set_bit(nr, addr);
- else
- __clear_bit(nr, addr);
-}
+#define __assign_bit(nr, addr, value) \
+ ((value) ? __set_bit(nr, addr) : __clear_bit(nr, addr))
/**
* __ptr_set_bit - Set bit in a pointer's value
--
2.41.0
next prev parent reply other threads:[~2023-10-09 15:13 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-10-09 15:10 [PATCH 00/14] ip_tunnel: convert __be16 tunnel flags to bitmaps Alexander Lobakin
2023-10-09 15:10 ` [PATCH 01/14] bitops: add missing prototype check Alexander Lobakin
2023-10-09 15:10 ` [PATCH 02/14] bitops: make BYTES_TO_BITS() treewide-available Alexander Lobakin
2023-10-09 15:10 ` Alexander Lobakin [this message]
2023-10-09 16:18 ` [PATCH 03/14] bitops: let the compiler optimize __assign_bit() Yury Norov
2023-10-11 7:25 ` Alexander Lobakin
2023-10-09 15:10 ` [PATCH 04/14] linkmode: convert linkmode_{test,set,clear,mod}_bit() to macros Alexander Lobakin
2023-10-09 15:10 ` [PATCH 05/14] s390/cio: rename bitmap_size() -> idset_bitmap_size() Alexander Lobakin
2023-10-09 16:35 ` Yury Norov
2023-10-11 7:28 ` Alexander Lobakin
2023-10-09 15:10 ` [PATCH 06/14] fs/ntfs3: rename bitmap_size() -> ntfs3_bitmap_size() Alexander Lobakin
2023-10-09 16:50 ` Yury Norov
2023-10-11 7:36 ` Alexander Lobakin
2023-10-09 15:10 ` [PATCH 07/14] btrfs: rename bitmap_set_bits() -> btrfs_bitmap_set_bits() Alexander Lobakin
2023-10-09 15:23 ` David Sterba
2023-10-09 15:10 ` [PATCH 08/14] bitmap: introduce generic optimized bitmap_size() Alexander Lobakin
2023-10-09 17:04 ` Yury Norov
2023-10-09 15:10 ` [PATCH 09/14] bitmap: extend bitmap_{get,set}_value8() to bitmap_{get,set}_bits() Alexander Lobakin
2023-10-09 16:31 ` Yury Norov
2023-10-11 9:33 ` Alexander Lobakin
2023-10-11 10:36 ` Andy Shevchenko
2023-10-15 2:20 ` Yury Norov
2023-10-09 15:10 ` [PATCH 10/14] ip_tunnel: use a separate struct to store tunnel params in the kernel Alexander Lobakin
2023-10-09 15:10 ` [PATCH 11/14] ip_tunnel: convert __be16 tunnel flags to bitmaps Alexander Lobakin
2023-10-09 15:10 ` [PATCH 12/14] lib/bitmap: add compile-time test for __assign_bit() optimization Alexander Lobakin
2023-10-09 15:10 ` [PATCH 13/14] lib/bitmap: add tests for bitmap_{get,set}_bits() Alexander Lobakin
2023-10-09 15:10 ` [PATCH 14/14] lib/bitmap: add tests for IP tunnel flags conversion helpers Alexander Lobakin
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