linux-kernel - Re: [PATCH] crypto: aegis: fix badly optimized clang output

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Date:   Thu, 18 Jul 2019 14:17:05 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>,
        Ondrej Mosnacek <omosnacek@...il.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        linux-crypto@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] crypto: aegis: fix badly optimized clang output

On Thu, Jul 18, 2019 at 6:50 AM Arnd Bergmann <arnd@...db.de> wrote:
>
> Clang sometimes makes very different inlining decisions from gcc.
> In case of the aegis crypto algorithms, it decides to turn the innermost
> primitives (and, xor, ...) into separate functions but inline most of
> the rest.
>
> This results in a huge amount of variables spilled on the stack, leading
> to rather slow execution as well as kernel stack usage beyond the 32-bit
> warning limit when CONFIG_KASAN is enabled:
>
> crypto/aegis256.c:123:13: warning: stack frame size of 648 bytes in function 'crypto_aegis256_encrypt_chunk' [-Wframe-larger-than=]
> crypto/aegis256.c:366:13: warning: stack frame size of 1264 bytes in function 'crypto_aegis256_crypt' [-Wframe-larger-than=]
> crypto/aegis256.c:187:13: warning: stack frame size of 656 bytes in function 'crypto_aegis256_decrypt_chunk' [-Wframe-larger-than=]
> crypto/aegis128l.c:135:13: warning: stack frame size of 832 bytes in function 'crypto_aegis128l_encrypt_chunk' [-Wframe-larger-than=]
> crypto/aegis128l.c:415:13: warning: stack frame size of 1480 bytes in function 'crypto_aegis128l_crypt' [-Wframe-larger-than=]
> crypto/aegis128l.c:218:13: warning: stack frame size of 848 bytes in function 'crypto_aegis128l_decrypt_chunk' [-Wframe-larger-than=]
> crypto/aegis128.c:116:13: warning: stack frame size of 584 bytes in function 'crypto_aegis128_encrypt_chunk' [-Wframe-larger-than=]
> crypto/aegis128.c:351:13: warning: stack frame size of 1064 bytes in function 'crypto_aegis128_crypt' [-Wframe-larger-than=]
> crypto/aegis128.c:177:13: warning: stack frame size of 592 bytes in function 'crypto_aegis128_decrypt_chunk' [-Wframe-larger-than=]
>
> Forcing the primitives to all get inlined avoids the issue and the
> resulting code is similar to what gcc produces.
>
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
>  crypto/aegis.h | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/crypto/aegis.h b/crypto/aegis.h
> index 41a3090cda8e..efed7251c49d 100644
> --- a/crypto/aegis.h
> +++ b/crypto/aegis.h
> @@ -34,21 +34,21 @@ static const union aegis_block crypto_aegis_const[2] = {
>         } },
>  };
>
> -static void crypto_aegis_block_xor(union aegis_block *dst,
> +static __always_inline void crypto_aegis_block_xor(union aegis_block *dst,

`static inline` would be more concise and expand to the same
attribute, IIRC.  Not sure if that's worth sending a v2. But for now,
thanks for the patch and:
Acked-by: Nick Desaulniers <ndesaulniers@...gle.com>

-- 
Thanks,
~Nick Desaulniers