netdev - [PATCH 03/14] bitops: let the compiler optimize __assign

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Date: Mon,  9 Oct 2023 17:10:15 +0200
From: Alexander Lobakin <aleksander.lobakin@...el.com>
To: Yury Norov <yury.norov@...il.com>
Cc: Alexander Lobakin <aleksander.lobakin@...el.com>,
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Alexander Potapenko <glider@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>,
	Eric Dumazet <edumazet@...gle.com>,
	David Ahern <dsahern@...nel.org>,
	Przemek Kitszel <przemyslaw.kitszel@...el.com>,
	Simon Horman <simon.horman@...igine.com>,
	netdev@...r.kernel.org,
	linux-btrfs@...r.kernel.org,
	dm-devel@...hat.com,
	ntfs3@...ts.linux.dev,
	linux-s390@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH 03/14] bitops: let the compiler optimize __assign_bit()

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@...ux.intel.com>
Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@...el.com>
Signed-off-by: Alexander Lobakin <aleksander.lobakin@...el.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