From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id E178F2E8B8E; Tue, 7 Oct 2025 17:11:38 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1759857099; cv=none; b=VGomJfdmREsFiq18R0rjhYLJlXEASH9EXTswllTyMoOAG9ncC9R8vzZaMGHMrzNYf+CySUGreWMFjGrswjNxtBZyiS0yj9rQzDCDVaq28vFzLlBs/la8GEPbWz17Ak2p9puW9q2CdYLiacjfR/fWUNxISchAUpKF/Fhr6FsqJ0E= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1759857099; c=relaxed/simple; bh=qB0CeIsBa+dsxVohjzzl7Hm4/75YJ2F3WRhsBvhR0FM=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=UX2hd/7ab9uEIPw7Ryk1eGTjJOSsPbRg9tXuCgj/z0pQynpmXKmTrbp+/LLuAyxBu6zwHYmO4xVTUzVEA0aOmGZd3SdHjy5CNZv2062BJc5IWh2yRqn2qy4nfzPWBZTmU3Xin7d8eloJW480D0g+XlEF7nkQqYSgQboFz8taj/k= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=e34NBtAz; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="e34NBtAz" Received: by smtp.kernel.org (Postfix) with ESMTPSA id C97EFC4CEF1; Tue, 7 Oct 2025 17:11:37 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1759857098; bh=qB0CeIsBa+dsxVohjzzl7Hm4/75YJ2F3WRhsBvhR0FM=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=e34NBtAzpJjCXgycFsm17YgoM4R1NKDmdI2Bu48/8tZDv5+6UqQuyS+6dEbsABeOD Dx5m8qA0964rdOSroPyXlo0PlqAHOfQXHFaFqI/f93jDTdsdaNOPKQFLKo41qwo3kh xxBmcPpUfeCPFsuhkSIVYuBN2/uD1FWMCJbJXwrRuRX0n/e7jodiaOxTlwDvYO79f4 Rc71M0yOJDyZ7L4Wafb/gKS9yJZ6IMo99pyfJiTUItq/QeCudlo9CH//lTGaFImMBm 548wtXvcRjOXK+JUXwTI80MSSU2fIZPcq9lRfmepkt2CQy9WKLKA15Co8bkjbmN0PN p4JrQfZ2P5HCQ== Date: Tue, 7 Oct 2025 17:11:36 +0000 From: Eric Biggers To: Caleb Sander Mateos Cc: Guan-Chun Wu <409411716@gms.tku.edu.tw>, David Laight , akpm@linux-foundation.org, axboe@kernel.dk, ceph-devel@vger.kernel.org, hch@lst.de, home7438072@gmail.com, idryomov@gmail.com, jaegeuk@kernel.org, kbusch@kernel.org, linux-fscrypt@vger.kernel.org, linux-kernel@vger.kernel.org, linux-nvme@lists.infradead.org, sagi@grimberg.me, tytso@mit.edu, visitorckw@gmail.com, xiubli@redhat.com Subject: Re: [PATCH v3 2/6] lib/base64: Optimize base64_decode() with reverse lookup tables Message-ID: <20251007171136.GA2094443@google.com> References: <20250926065235.13623-1-409411716@gms.tku.edu.tw> <20250926065556.14250-1-409411716@gms.tku.edu.tw> <20251005181803.0ba6aee4@pumpkin> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Tue, Oct 07, 2025 at 07:57:16AM -0700, Caleb Sander Mateos wrote: > On Tue, Oct 7, 2025 at 1:28 AM Guan-Chun Wu <409411716@gms.tku.edu.tw> wrote: > > > > On Sun, Oct 05, 2025 at 06:18:03PM +0100, David Laight wrote: > > > On Wed, 1 Oct 2025 09:20:27 -0700 > > > Caleb Sander Mateos wrote: > > > > > > > On Wed, Oct 1, 2025 at 3:18 AM Guan-Chun Wu <409411716@gms.tku.edu.tw> wrote: > > > > > > > > > > On Fri, Sep 26, 2025 at 04:33:12PM -0700, Caleb Sander Mateos wrote: > > > > > > On Thu, Sep 25, 2025 at 11:59 PM Guan-Chun Wu <409411716@gms.tku.edu.tw> wrote: > > > > > > > > > > > > > > From: Kuan-Wei Chiu > > > > > > > > > > > > > > Replace the use of strchr() in base64_decode() with precomputed reverse > > > > > > > lookup tables for each variant. This avoids repeated string scans and > > > > > > > improves performance. Use -1 in the tables to mark invalid characters. > > > > > > > > > > > > > > Decode: > > > > > > > 64B ~1530ns -> ~75ns (~20.4x) > > > > > > > 1KB ~27726ns -> ~1165ns (~23.8x) > > > > > > > > > > > > > > Signed-off-by: Kuan-Wei Chiu > > > > > > > Co-developed-by: Guan-Chun Wu <409411716@gms.tku.edu.tw> > > > > > > > Signed-off-by: Guan-Chun Wu <409411716@gms.tku.edu.tw> > > > > > > > --- > > > > > > > lib/base64.c | 66 ++++++++++++++++++++++++++++++++++++++++++++++++---- > > > > > > > 1 file changed, 61 insertions(+), 5 deletions(-) > > > > > > > > > > > > > > diff --git a/lib/base64.c b/lib/base64.c > > > > > > > index 1af557785..b20fdf168 100644 > > > > > > > --- a/lib/base64.c > > > > > > > +++ b/lib/base64.c > > > > > > > @@ -21,6 +21,63 @@ static const char base64_tables[][65] = { > > > > > > > [BASE64_IMAP] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+,", > > > > > > > }; > > > > > > > > > > > > > > +static const s8 base64_rev_tables[][256] = { > > > > > > > + [BASE64_STD] = { > > > > > > > + -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, > > > > > > > + -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, > > > > > > > + -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 62, -1, -1, -1, 63, > > > > > > > + 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, -1, -1, -1, -1, -1, -1, > > > > > > > + -1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, > > > > > > > + 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, -1, -1, -1, -1, -1, > > > > > > > + -1, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, > > > > > > > + 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, -1, -1, -1, -1, -1, > > > > > > > + -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, > > > > > > > + -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, > > > > > > > + -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, > > > > > > > + -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, > > > > > > > + -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, > > > > > > > + -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, > > > > > > > + -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, > > > > > > > + -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, > > > > > > > + }, > > > > > > > + [BASE64_URLSAFE] = { > > > > > > > + -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, > > > > > > > + -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, > > > > > > > + -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 62, -1, -1, > > > > > > > + 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, -1, -1, -1, -1, -1, -1, > > > > > > > + -1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, > > > > > > > + 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, -1, -1, -1, -1, 63, > > > > > > > + -1, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, > > > > > > > + 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, -1, -1, -1, -1, -1, > > > > > > > + -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, > > > > > > > + -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, > > > > > > > + -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, > > > > > > > + -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, > > > > > > > + -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, > > > > > > > + -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, > > > > > > > + -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, > > > > > > > + -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, > > > > > > > + }, > > > > > > > + [BASE64_IMAP] = { > > > > > > > + -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, > > > > > > > + -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, > > > > > > > + -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 62, 63, -1, -1, -1, > > > > > > > + 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, -1, -1, -1, -1, -1, -1, > > > > > > > + -1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, > > > > > > > + 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, -1, -1, -1, -1, -1, > > > > > > > + -1, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, > > > > > > > + 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, -1, -1, -1, -1, -1, > > > > > > > + -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, > > > > > > > + -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, > > > > > > > + -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, > > > > > > > + -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, > > > > > > > + -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, > > > > > > > + -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, > > > > > > > + -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, > > > > > > > + -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, > > > > > > > + }, > > > > > > > > > > > > Do we actually need 3 separate lookup tables? It looks like all 3 > > > > > > variants agree on the value of any characters they have in common. So > > > > > > we could combine them into a single lookup table that would work for a > > > > > > valid base64 string of any variant. The only downside I can see is > > > > > > that base64 strings which are invalid in some variants might no longer > > > > > > be rejected by base64_decode(). > > > > > > > > > > > > > > > > In addition to the approach David mentioned, maybe we can use a common > > > > > lookup table for A–Z, a–z, and 0–9, and then handle the variant-specific > > > > > symbols with a switch. > > > > > > It is certainly possible to generate the initialiser from a #define to > > > avoid all the replicated source. > > > > > > > > > > > > > For example: > > > > > > > > > > static const s8 base64_rev_common[256] = { > > > > > [0 ... 255] = -1, > > > > > ['A'] = 0, ['B'] = 1, /* ... */, ['Z'] = 25, > > > > > > If you assume ASCII (I doubt Linux runs on any EBCDIC systems) you > > > can assume the characters are sequential and miss ['B'] = etc to > > > reduce the the line lengths. > > > (Even EBCDIC has A-I J-R S-Z and 0-9 as adjacent values) > > > > > > > > ['a'] = 26, /* ... */, ['z'] = 51, > > > > > ['0'] = 52, /* ... */, ['9'] = 61, > > > > > }; > > > > > > > > > > static inline int base64_rev_lookup(u8 c, enum base64_variant variant) { > > > > > s8 v = base64_rev_common[c]; > > > > > if (v != -1) > > > > > return v; > > > > > > > > > > switch (variant) { > > > > > case BASE64_STD: > > > > > if (c == '+') return 62; > > > > > if (c == '/') return 63; > > > > > break; > > > > > case BASE64_IMAP: > > > > > if (c == '+') return 62; > > > > > if (c == ',') return 63; > > > > > break; > > > > > case BASE64_URLSAFE: > > > > > if (c == '-') return 62; > > > > > if (c == '_') return 63; > > > > > break; > > > > > } > > > > > return -1; > > > > > } > > > > > > > > > > What do you think? > > > > > > > > That adds several branches in the hot loop, at least 2 of which are > > > > unpredictable for valid base64 input of a given variant (v != -1 as > > > > well as the first c check in the applicable switch case). > > > > > > I'd certainly pass in the character values for 62 and 63 so they are > > > determined well outside the inner loop. > > > Possibly even going as far as #define BASE64_STD ('+' << 8 | '/'). > > > > > > > That seems like it would hurt performance, no? > > > > I think having 3 separate tables > > > > would be preferable to making the hot loop more branchy. > > > > > > Depends how common you think 62 and 63 are... > > > I guess 63 comes from 0xff bytes - so might be quite common. > > > > > > One thing I think you've missed is that the decode converts 4 characters > > > into 24 bits - which then need carefully writing into the output buffer. > > > There is no need to check whether each character is valid. > > > After: > > > val_24 = t[b[0]] | t[b[1]] << 6 | t[b[2]] << 12 | t[b[3]] << 18; > > > val_24 will be negative iff one of b[0..3] is invalid. > > > So you only need to check every 4 input characters, not for every one. > > > That does require separate tables. > > > (Or have a decoder that always maps "+-" to 62 and "/,_" to 63.) > > > > > > David > > > > > > > Thanks for the feedback. > > For the next revision, we’ll use a single lookup table that maps both + > > and - to 62, and /, _, and , to 63. > > Does this approach sound good to everyone? > > Sounds fine to me. Perhaps worth pointing out that the decision to > accept any base64 variant in the decoder would likely be permanent, > since users may come to depend on it. But I don't see any issue with > it as long as all the base64 variants agree on the values of their > common symbols. No thanks. fs/crypto/ needs to have a correct Base64 decoder which rejects invalid inputs, so that multiple filenames aren't accepted for the same file. If lib/ won't provide that, then please keep fs/crypto/ as-is. - Eric