Re: [PATCH] lib/crypto: x86/chacha: Eliminate more never-taken branches

[Eric Biggers <ebiggers@kernel.org>]

On Wed, Nov 04, 2026 at 02:55:26PM -0800, Eric Biggers wrote:
> The x86_64 optimized ChaCha code has assembly functions that generate 1,
> 2, 4, 8, or 16 keystream blocks and XOR them with up to that number of
> data blocks.
> 
> However, to avoid unnecessary overhead, the 4, 8, and 16-block
> functions, and the AVX2 2-block function, are never called with a length
> that is supported by the next-smaller function (that is, <= N/2 blocks,
> or <= 1 block for the SSSE3 4-block function), since a smaller function
> is called to handle such cases instead.
> 
> As a result, all the branches in those assembly functions that handle
> XOR'ing fewer than that minimum number of data blocks are dead code.
> 
> Remove these branches to eliminate dead code and overhead.  Also rename
> the assembly functions to make it clear which cases they handle.
> 
> Some register moves in chacha_1to4blocks_ssse3() are also eliminated, as
> they were needed only because of the support for less than 1 block.
> 
> Note: many of the functions could use exclusive rather than inclusive
> lower bounds.  Inclusive is used for simplicity.
> 
> Signed-off-by: Eric Biggers <ebiggers@kernel.org>
> ---
>  lib/crypto/x86/chacha-avx2-x86_64.S   | 73 +++++++--------------
>  lib/crypto/x86/chacha-avx512-x86_64.S | 94 +++++++++------------------
>  lib/crypto/x86/chacha-ssse3-x86_64.S  | 51 ++++++---------
>  lib/crypto/x86/chacha.h               | 72 +++++++++++++-------
>  4 files changed, 119 insertions(+), 171 deletions(-)

Applied to https://git.kernel.org/pub/scm/linux/kernel/git/ebiggers/linux.git/log/?h=libcrypto-next

- Eric