netdev - Re: [PATCH net-next v2 1/9] unroll: add generic loop unroll helpers

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Message-ID: <20240820175539.6b1cec2b@kernel.org>
Date: Tue, 20 Aug 2024 17:55:39 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Tony Nguyen <anthony.l.nguyen@...el.com>
Cc: davem@...emloft.net, pabeni@...hat.com, edumazet@...gle.com,
 netdev@...r.kernel.org, Alexander Lobakin <aleksander.lobakin@...el.com>,
 przemyslaw.kitszel@...el.com, joshua.a.hay@...el.com,
 michal.kubiak@...el.com, nex.sw.ncis.osdt.itp.upstreaming@...el.com, "Jose
 E . Marchesi" <jose.marchesi@...cle.com>
Subject: Re: [PATCH net-next v2 1/9] unroll: add generic loop unroll helpers

On Mon, 19 Aug 2024 15:34:33 -0700 Tony Nguyen 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.

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