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

[David Laight <david.laight.linux@gmail.com>]

On Thu, 11 Sep 2025 15:32:04 +0800
Guan-Chun Wu <409411716@gms.tku.edu.tw> wrote:

> From: Kuan-Wei Chiu <visitorckw@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@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 | 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;
> +}

That's still going to be really horrible with random data.
You'll get a lot of mispredicted branch penalties.
I think they are about 20 clocks each on my Zen-5.
A 256 byte lookup table might be better.
However if you assume ascii then 'ch' can be split 3:5 bits and
the top three used to determine the valid values for the low bits
(probably using shifts of constants rather than actual arrays).
So apart from the outlying '+' and '/' (and IIRC there is a variant
that uses different characters) which can be picked up in the error
path; it ought to be possible to code with no conditionals at all.

To late at night to write (and test) an implementation.

	David




> +
>  /**
>   * 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);