From: Kuan-Wei Chiu <visitorckw@gmail.com>
To: Caleb Sander Mateos <csander@purestorage.com>
Cc: Guan-Chun Wu <409411716@gms.tku.edu.tw>,
akpm@linux-foundation.org, axboe@kernel.dk,
ceph-devel@vger.kernel.org, ebiggers@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, xiubli@redhat.com
Subject: Re: [PATCH v3 2/6] lib/base64: Optimize base64_decode() with reverse lookup tables
Date: Sun, 28 Sep 2025 14:37:17 +0800 [thread overview]
Message-ID: <aNjXnYorbInXF6FM@visitorckw-System-Product-Name> (raw)
In-Reply-To: <CADUfDZruZWyrsjRCs_Y5gjsbfU7dz_ALGG61pQ8qCM7K2_DjmA@mail.gmail.com>
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 <visitorckw@gmail.com>
> >
> > 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 <visitorckw@gmail.com>
> > 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().
Ah, David also mentioned this earlier, but I forgot about it while
writing the code. Sorry for that. I'll rectify it.
Regards,
Kuan-Wei
>
> > +};
> > +
> > /**
> > * base64_encode() - Base64-encode some binary data
> > * @src: the binary data to encode
> > @@ -82,11 +139,9 @@ int base64_decode(const char *src, int srclen, u8 *dst, bool padding, enum base6
> > int bits = 0;
> > int i;
> > u8 *bp = dst;
> > - const char *base64_table = base64_tables[variant];
> > + s8 ch;
> >
> > for (i = 0; i < srclen; i++) {
> > - const char *p = strchr(base64_table, src[i]);
> > -
> > if (src[i] == '=') {
> > ac = (ac << 6);
> > bits += 6;
> > @@ -94,9 +149,10 @@ int base64_decode(const char *src, int srclen, u8 *dst, bool padding, enum base6
> > bits -= 8;
> > continue;
> > }
> > - if (p == NULL || src[i] == 0)
> > + ch = base64_rev_tables[variant][(u8)src[i]];
> > + if (ch == -1)
>
> Checking for < 0 can save an additional comparison here.
>
> Best,
> Caleb
>
> > return -1;
> > - ac = (ac << 6) | (p - base64_table);
> > + ac = (ac << 6) | ch;
> > bits += 6;
> > if (bits >= 8) {
> > bits -= 8;
> > --
> > 2.34.1
> >
> >
next prev parent reply other threads:[~2025-09-28 6:37 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-09-26 6:52 [PATCH v3 0/6] lib/base64: add generic encoder/decoder, migrate users Guan-Chun Wu
2025-09-26 6:55 ` [PATCH v3 1/6] lib/base64: Add support for multiple variants Guan-Chun Wu
2025-09-30 23:56 ` Caleb Sander Mateos
2025-10-01 14:09 ` Guan-Chun Wu
2025-09-26 6:55 ` [PATCH v3 2/6] lib/base64: Optimize base64_decode() with reverse lookup tables Guan-Chun Wu
2025-09-26 23:33 ` Caleb Sander Mateos
2025-09-28 6:37 ` Kuan-Wei Chiu [this message]
2025-10-01 10:18 ` Guan-Chun Wu
2025-10-01 16:20 ` Caleb Sander Mateos
2025-10-05 17:18 ` David Laight
2025-10-07 8:28 ` Guan-Chun Wu
2025-10-07 14:57 ` Caleb Sander Mateos
2025-10-07 17:11 ` Eric Biggers
2025-10-07 18:23 ` David Laight
2025-10-09 12:25 ` Guan-Chun Wu
2025-10-10 9:51 ` David Laight
2025-10-13 9:49 ` Guan-Chun Wu
2025-10-14 8:14 ` David Laight
2025-10-16 10:07 ` Guan-Chun Wu
2025-10-27 13:12 ` Guan-Chun Wu
2025-10-27 14:18 ` David Laight
2025-10-28 6:58 ` Guan-Chun Wu
2025-09-28 18:57 ` David Laight
2025-09-26 6:56 ` [PATCH v3 3/6] lib/base64: rework encode/decode for speed and stricter validation Guan-Chun Wu
2025-10-01 0:11 ` Caleb Sander Mateos
2025-10-01 9:39 ` Guan-Chun Wu
2025-10-06 20:52 ` David Laight
2025-10-07 8:34 ` Guan-Chun Wu
2025-09-26 6:56 ` [PATCH v3 4/6] lib: add KUnit tests for base64 encoding/decoding Guan-Chun Wu
2025-09-26 6:56 ` [PATCH v3 5/6] fscrypt: replace local base64url helpers with lib/base64 Guan-Chun Wu
2025-09-26 6:57 ` [PATCH v3 6/6] ceph: replace local base64 " Guan-Chun Wu
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