Re: [PATCH v2 2/2] arm64/crc32: Implement 4-way interleave using PMULL

[Eric Biggers <ebiggers@kernel.org>]

On Wed, Oct 16, 2024 at 09:26:43PM +0200, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb@kernel.org>
> 
> Now that kernel mode NEON no longer disables preemption, using FP/SIMD
> in library code which is not obviously part of the crypto subsystem is
> no longer problematic, as it will no longer incur unexpected latencies.
> 
> So accelerate the CRC-32 library code on arm64 to use a 4-way
> interleave, using PMULL instructions to implement the folding.
> 
> On Apple M2, this results in a speedup of 2 - 2.8x when using input
> sizes of 1k - 8k. For smaller sizes, the overhead of preserving and
> restoring the FP/SIMD register file may not be worth it, so 1k is used
> as a threshold for choosing this code path.
> 
> The coefficient tables were generated using code provided by Eric. [0]
> 
> [0] https://github.com/ebiggers/libdeflate/blob/master/scripts/gen_crc32_multipliers.c
> 
> Cc: Eric Biggers <ebiggers@kernel.org>
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> ---
>  arch/arm64/lib/Makefile     |   2 +-
>  arch/arm64/lib/crc32-4way.S | 242 ++++++++++++++++++++
>  arch/arm64/lib/crc32-glue.c |  48 ++++
>  3 files changed, 291 insertions(+), 1 deletion(-)

Reviewed-by: Eric Biggers <ebiggers@google.com>

> +	/* Process up to 64 blocks of 64 bytes at a time */
> +.La\@:	mov	x3, #64
> +	cmp	len, #64
> +	csel	x3, x3, len, hi		// x3 := max(len, 64)

The comment should say min(len, 64), not max(len, 64).

- Eric