Subject: Re: [PATCH crypto 1/2] lib/crypto: blake2s-generic: reduce code size on small systems

From: Eric Biggers <[email-masked]>
Date: Wed, 12 Jan 2022 10:31:57 -0800

On Tue, Jan 11, 2022 at 02:49:33PM +0100, Jason A. Donenfeld wrote:
> Re-wind the loops entirely on kernels optimized for code size. This is
> really not good at all performance-wise. But on m68k, it shaves off 4k
> of code size, which is apparently important.
> 
> Cc: Geert Uytterhoeven <[email-masked]>
> Cc: Herbert Xu <[email-masked]>
> Cc: Ard Biesheuvel <[email-masked]>
> Signed-off-by: Jason A. Donenfeld <[email-masked]>
> ---
>  lib/crypto/blake2s-generic.c | 30 ++++++++++++++++++------------
>  1 file changed, 18 insertions(+), 12 deletions(-)
> 
> diff --git a/lib/crypto/blake2s-generic.c b/lib/crypto/blake2s-generic.c
> index 75ccb3e633e6..990f000e22ee 100644
> --- a/lib/crypto/blake2s-generic.c
> +++ b/lib/crypto/blake2s-generic.c
> @@ -46,7 +46,7 @@ void blake2s_compress_generic(struct blake2s_state *state, const u8 *block,
>  {
>  	u32 m[16];
>  	u32 v[16];
> -	int i;
> +	int i, j;
>  
>  	WARN_ON(IS_ENABLED(DEBUG) &&
>  		(nblocks > 1 && inc != BLAKE2S_BLOCK_SIZE));
> @@ -86,17 +86,23 @@ void blake2s_compress_generic(struct blake2s_state *state, const u8 *block,
>  	G(r, 6, v[2], v[ 7], v[ 8], v[13]); \
>  	G(r, 7, v[3], v[ 4], v[ 9], v[14]); \
>  } while (0)
> -		ROUND(0);
> -		ROUND(1);
> -		ROUND(2);
> -		ROUND(3);
> -		ROUND(4);
> -		ROUND(5);
> -		ROUND(6);
> -		ROUND(7);
> -		ROUND(8);
> -		ROUND(9);
> -
> +		if (IS_ENABLED(CONFIG_CC_OPTIMIZE_FOR_SIZE)) {
> +			for (i = 0; i < 10; ++i) {
> +				for (j = 0; j < 8; ++j)
> +					G(i, j, v[j % 4], v[((j + (j / 4)) % 4) + 4], v[((j + 2 * (j / 4)) % 4) + 8], v[((j + 3 * (j / 4)) % 4) + 12]);
> +			}

How about unrolling the inner loop but not the outer one?  Wouldn't that give
most of the benefit, without hurting performance as much?

If you stay with this approach and don't unroll either loop, can you use 'r' and
'i' instead of 'i' and 'j', to match the naming in G()?

Also, please wrap lines at 80 columns.

- Eric