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Message-ID: <CADUfDZruZWyrsjRCs_Y5gjsbfU7dz_ALGG61pQ8qCM7K2_DjmA@mail.gmail.com>
Date: Fri, 26 Sep 2025 16:33:12 -0700
From: Caleb Sander Mateos <csander@...estorage.com>
To: Guan-Chun Wu <409411716@....tku.edu.tw>
Cc: akpm@...ux-foundation.org, axboe@...nel.dk, ceph-devel@...r.kernel.org,
ebiggers@...nel.org, hch@....de, home7438072@...il.com, idryomov@...il.com,
jaegeuk@...nel.org, kbusch@...nel.org, linux-fscrypt@...r.kernel.org,
linux-kernel@...r.kernel.org, linux-nvme@...ts.infradead.org,
sagi@...mberg.me, tytso@....edu, visitorckw@...il.com, xiubli@...hat.com
Subject: Re: [PATCH v3 2/6] lib/base64: Optimize base64_decode() with reverse
lookup tables
On Thu, Sep 25, 2025 at 11:59 PM Guan-Chun Wu <409411716@....tku.edu.tw> wrote:
>
> From: Kuan-Wei Chiu <visitorckw@...il.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@...il.com>
> Co-developed-by: Guan-Chun Wu <409411716@....tku.edu.tw>
> Signed-off-by: Guan-Chun Wu <409411716@....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().
> +};
> +
> /**
> * 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
>
>
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