linux-kernel - Re: [PATCH net-next v5 06/21] bitops: let the compiler optimize {__,}assign

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Date: Wed, 28 Feb 2024 08:23:09 -0800
From: Yury Norov <yury.norov@...il.com>
To: Alexander Lobakin <aleksander.lobakin@...el.com>
Cc: "David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Michal Swiatkowski <michal.swiatkowski@...ux.intel.com>,
	Marcin Szycik <marcin.szycik@...ux.intel.com>,
	Wojciech Drewek <wojciech.drewek@...el.com>,
	Andy Shevchenko <andy@...nel.org>,
	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Alexander Potapenko <glider@...gle.com>,
	Jiri Pirko <jiri@...nulli.us>, Ido Schimmel <idosch@...dia.com>,
	Przemek Kitszel <przemyslaw.kitszel@...el.com>,
	Simon Horman <horms@...nel.org>, linux-btrfs@...r.kernel.org,
	dm-devel@...hat.com, ntfs3@...ts.linux.dev,
	linux-s390@...r.kernel.org, intel-wired-lan@...ts.osuosl.org,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Subject: Re: [PATCH net-next v5 06/21] bitops: let the compiler optimize
 {__,}assign_bit()

On Thu, Feb 01, 2024 at 01:22:01PM +0100, Alexander Lobakin wrote:
> 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 bitops) 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.
> 
> Atomic assign_bit() is less affected due to its nature, but let's
> convert it to a macro as well to keep the code consistent and not
> leave a place for possible suboptimal codegen. Moreover, with certain
> kernel configuration it actually gives some saves (x86):
> 
> do_ip_setsockopt    4154    4099     -55
> 
> Suggested-by: Yury Norov <yury.norov@...il.com> # assign_bit(), too
> 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>

Acked-by: Yury Norov <yury.norov@...il.com>