[PATCH v5 2/6] lib/base64: Optimize base64_decode() with reverse lookup tables

Guan-Chun Wu
2025-11-14T06:01:14Z

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 -> ~80ns (~19.1x)
1KB ~27726ns -> ~1239ns (~22.4x)

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 | 51 +++++++++++++++++++++++++++++++++++++++++++++++----
1 file changed, 47 insertions(+), 4 deletions(-)

diff --git a/lib/base64.c b/lib/base64.c
index a7c20a8e8e98..9d1074bb821c 100644
--- a/lib/base64.c
+++ b/lib/base64.c
@@ -21,6 +21,49 @@ static const char base64_tables[][65] = {
[BASE64_IMAP] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+,",
};

+/**
+ * Initialize the base64 reverse mapping for a single character
+ * This macro maps a character to its corresponding base64 value,
+ * returning -1 if the character is invalid.
+ * char 'A'-'Z' maps to 0-25, 'a'-'z' maps to 26-51, '0'-'9' maps to 52-61,
+ * ch_62 maps to 62, ch_63 maps to 63, and other characters return -1
+ */
+#define INIT_1(v, ch_62, ch_63) \
+ [v] = (v) >= 'A' && (v) <= 'Z' ? (v) - 'A' \
+ : (v) >= 'a' && (v) <= 'z' ? (v) - 'a' + 26 \
+ : (v) >= '0' && (v) <= '9' ? (v) - '0' + 52 \
+ : (v) == (ch_62) ? 62 : (v) == (ch_63) ? 63 : -1
+/**
+ * Recursive macros to generate multiple Base64 reverse mapping table entries.
+ * Each macro generates a sequence of entries in the lookup table:
+ * INIT_2 generates 2 entries, INIT_4 generates 4, INIT_8 generates 8, and so on up to INIT_32.
+ */
+#define INIT_2(v, ...) INIT_1(v, __VA_ARGS__), INIT_1((v) + 1, __VA_ARGS__)
+#define INIT_4(v, ...) INIT_2(v, __VA_ARGS__), INIT_2((v) + 2, __VA_ARGS__)
+#define INIT_8(v, ...) INIT_4(v, __VA_ARGS__), INIT_4((v) + 4, __VA_ARGS__)
+#define INIT_16(v, ...) INIT_8(v, __VA_ARGS__), INIT_8((v) + 8, __VA_ARGS__)
+#define INIT_32(v, ...) INIT_16(v, __VA_ARGS__), INIT_16((v) + 16, __VA_ARGS__)
+
+#define BASE64_REV_INIT(ch_62, ch_63) { \
+ [0 ... 0x1f] = -1, \
+ INIT_32(0x20, ch_62, ch_63), \
+ INIT_32(0x40, ch_62, ch_63), \
+ INIT_32(0x60, ch_62, ch_63), \
+ [0x80 ... 0xff] = -1 }
+
+static const s8 base64_rev_maps[][256] = {
+ [BASE64_STD] = BASE64_REV_INIT('+', '/'),
+ [BASE64_URLSAFE] = BASE64_REV_INIT('-', '_'),
+ [BASE64_IMAP] = BASE64_REV_INIT('+', ',')
+};
+
+#undef BASE64_REV_INIT
+#undef INIT_32
+#undef INIT_16
+#undef INIT_8
+#undef INIT_4
+#undef INIT_2
+#undef INIT_1
/**
* base64_encode() - Base64-encode some binary data
* @src: the binary data to encode
@@ -84,10 +127,9 @@ 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 (padding) {
if (src[i] == '=') {
ac = (ac << 6);
@@ -97,9 +139,10 @@ int base64_decode(const char *src, int srclen, u8 *dst, bool padding, enum base6
continue;
}
}
- if (p == NULL || src[i] == 0)
+ ch = base64_rev_maps[variant][(u8)src[i]];
+ if (ch == -1)
return -1;
- ac = (ac << 6) | (p - base64_table);
+ ac = (ac << 6) | ch;
bits += 6;
if (bits >= 8) {
bits -= 8;
--
2.34.1