linux-kernel - Re: [PATCH net-next 1/4] unroll: add generic loop unroll helpers

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Message-ID: <fa01e28e-b75d-4d60-b10a-ccf3e544ff1e@intel.com>
Date: Mon, 10 Feb 2025 16:49:14 +0100
From: Alexander Lobakin <aleksander.lobakin@...el.com>
To: Simon Horman <horms@...nel.org>
CC: Andrew Lunn <andrew+netdev@...n.ch>, "David S. Miller"
	<davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski
	<kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Alexei Starovoitov
	<ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, John Fastabend
	<john.fastabend@...il.com>, Andrii Nakryiko <andrii@...nel.org>, "Jose E.
 Marchesi" <jose.marchesi@...cle.com>,
	Toke Høiland-Jørgensen <toke@...hat.com>, "Magnus
 Karlsson" <magnus.karlsson@...el.com>, Maciej Fijalkowski
	<maciej.fijalkowski@...el.com>, Przemek Kitszel
	<przemyslaw.kitszel@...el.com>, Jason Baron <jbaron@...mai.com>, "Casey
 Schaufler" <casey@...aufler-ca.com>, Nathan Chancellor <nathan@...nel.org>,
	<bpf@...r.kernel.org>, <netdev@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next 1/4] unroll: add generic loop unroll helpers

From: Simon Horman <horms@...nel.org>
Date: Sun, 9 Feb 2025 11:07:25 +0000

> On Thu, Feb 06, 2025 at 07:26:26PM +0100, Alexander Lobakin wrote:
>> There are cases when we need to explicitly unroll loops. For example,
>> cache operations, filling DMA descriptors on very high speeds etc.
>> Add compiler-specific attribute macros to give the compiler a hint
>> that we'd like to unroll a loop.
>> Example usage:
>>
>>  #define UNROLL_BATCH 8
>>
>> 	unrolled_count(UNROLL_BATCH)
>> 	for (u32 i = 0; i < UNROLL_BATCH; i++)
>> 		op(priv, i);
>>
>> Note that sometimes the compilers won't unroll loops if they think this
>> would have worse optimization and perf than without unrolling, and that
>> unroll attributes are available only starting GCC 8. For older compiler
>> versions, no hints/attributes will be applied.
>> For better unrolling/parallelization, don't have any variables that
>> interfere between iterations except for the iterator itself.
>>
>> Co-developed-by: Jose E. Marchesi <jose.marchesi@...cle.com> # pragmas
>> Signed-off-by: Jose E. Marchesi <jose.marchesi@...cle.com>
>> Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@...el.com>
>> Signed-off-by: Alexander Lobakin <aleksander.lobakin@...el.com>
> 
> Hi Alexander,
> 
> This patch adds four variants of the unrolled helper.  But as far as I can
> tell the patch-set only makes use of one of them, unrolled_count().
> 
> I think it would be best if this patch only added helpers that are used.

I thought they might help people in future.
I can remove them if you insist. BTW the original patch from Jose also
added several variants.

Thanks,
Olek