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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
Message-ID: <20231009151026.66145-4-aleksander.lobakin@intel.com>
In-Reply-To: <20231009151026.66145-1-aleksander.lobakin@intel.com>
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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