[PATCH v2 1/5] lib/base64: Replace strchr() for better performance

[Guan-Chun Wu]

From: Kuan-Wei Chiu <visitorckw at gmail.com>

The base64 decoder previously relied on strchr() to locate each
character in the base64 table. In the worst case, this requires
scanning all 64 entries, and even with bitwise tricks or word-sized
comparisons, still needs up to 8 checks.

Introduce a small helper function that maps input characters directly
to their position in the base64 table. This reduces the maximum number
of comparisons to 5, improving decoding efficiency while keeping the
logic straightforward.

Benchmarks on x86_64 (Intel Core i7-10700 @ 2.90GHz, averaged
over 1000 runs, tested with KUnit):

Decode:
 - 64B input: avg ~1530ns -> ~126ns (~12x faster)
 - 1KB input: avg ~27726ns -> ~2003ns (~14x faster)

Signed-off-by: Kuan-Wei Chiu <visitorckw at gmail.com>
Co-developed-by: Guan-Chun Wu <409411716 at gms.tku.edu.tw>
Signed-off-by: Guan-Chun Wu <409411716 at gms.tku.edu.tw>
---
 lib/base64.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/lib/base64.c b/lib/base64.c
index b736a7a43..9416bded2 100644
--- a/lib/base64.c
+++ b/lib/base64.c
@@ -18,6 +18,21 @@
 static const char base64_table[65] =
 	"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
 
+static inline const char *find_chr(const char *base64_table, char ch)
+{
+	if ('A' <= ch && ch <= 'Z')
+		return base64_table + ch - 'A';
+	if ('a' <= ch && ch <= 'z')
+		return base64_table + 26 + ch - 'a';
+	if ('0' <= ch && ch <= '9')
+		return base64_table + 26 * 2 + ch - '0';
+	if (ch == base64_table[26 * 2 + 10])
+		return base64_table + 26 * 2 + 10;
+	if (ch == base64_table[26 * 2 + 10 + 1])
+		return base64_table + 26 * 2 + 10 + 1;
+	return NULL;
+}
+
 /**
  * base64_encode() - base64-encode some binary data
  * @src: the binary data to encode
@@ -78,7 +93,7 @@ int base64_decode(const char *src, int srclen, u8 *dst)
 	u8 *bp = dst;
 
 	for (i = 0; i < srclen; i++) {
-		const char *p = strchr(base64_table, src[i]);
+		const char *p = find_chr(base64_table, src[i]);
 
 		if (src[i] == '=') {
 			ac = (ac << 6);
-- 
2.34.1