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

Alexander Lobakin
2025-02-10T15:53:14Z

From: Simon Horman <horms@kernel.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@oracle.com> # pragmas
>> Signed-off-by: Jose E. Marchesi <jose.marchesi@oracle.com>
>> Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
>> Signed-off-by: Alexander Lobakin <aleksander.lobakin@intel.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