Re: [PATCH v2] lib/crypto: blake2b: Roll up BLAKE2b round loop on 32-bit

[Eric Biggers <ebiggers@kernel.org>]

On Thu, Dec 04, 2025 at 09:03:30PM -0800, Eric Biggers wrote:
> BLAKE2b has a state of 16 64-bit words.  Add the message data in and
> there are 32 64-bit words.  With the current code where all the rounds
> are unrolled to enable constant-folding of the blake2b_sigma values,
> this results in a very large code size on 32-bit kernels, including a
> recurring issue where gcc uses a large amount of stack.
> 
> There's just not much benefit to this unrolling when the code is already
> so large.  Let's roll up the rounds when !CONFIG_64BIT.
> 
> To avoid having to duplicate the code, just write code once using a
> loop, and conditionally use 'unrolled_full' from <linux/unroll.h>.
> 
> Then, fold the now-unneeded ROUND() macro into the loop.  Finally, also
> remove the now-unneeded override of the stack frame size warning.
> 
> Code size improvements for blake2b_compress_generic():
> 
>                   Size before (bytes)    Size after (bytes)
>                   -------------------    ------------------
>     i386, gcc           27584                 3632
>     i386, clang         18208                 3248
>     arm32, gcc          19912                 2860
>     arm32, clang        21336                 3344
> 
> Running the BLAKE2b benchmark on a !CONFIG_64BIT kernel on an x86_64
> processor shows a 16384B throughput change of 351 => 340 MB/s (gcc) or
> 442 MB/s => 375 MB/s (clang).  So clearly not much of a slowdown either.
> But also that microbenchmark also effectively disregards cache usage,
> which is important in practice and is far better in the smaller code.
> 
> Note: If we rolled up the loop on x86_64 too, the change would be
> 7024 bytes => 1584 bytes and 1960 MB/s => 1396 MB/s (gcc), or
> 6848 bytes => 1696 bytes and 1920 MB/s => 1263 MB/s (clang).
> Maybe still worth it, though not quite as clearly beneficial.
> 
> Fixes: 91d689337fe8 ("crypto: blake2b - add blake2b generic implementation")
> Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
> Signed-off-by: Eric Biggers <ebiggers@kernel.org>

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

- Eric