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Message-ID: <aNz/+xLDnc2mKsKo@wu-Pro-E500-G6-WS720T>
Date: Wed, 1 Oct 2025 18:18:35 +0800
From: Guan-Chun Wu <409411716@....tku.edu.tw>
To: Caleb Sander Mateos <csander@...estorage.com>
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 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@....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().
>
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.
For example:
static const s8 base64_rev_common[256] = {
[0 ... 255] = -1,
['A'] = 0, ['B'] = 1, /* ... */, ['Z'] = 25,
['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?
Best regards,
Guan-Chun
> > +};
> > +
> > /**
> > * 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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