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

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Message-ID: <20250914211243.74bdee2a@pumpkin>
Date: Sun, 14 Sep 2025 21:12:43 +0100
From: David Laight <david.laight.linux@...il.com>
To: Kuan-Wei Chiu <visitorckw@...il.com>
Cc: Caleb Sander Mateos <csander@...estorage.com>, Guan-Chun Wu
 <409411716@....tku.edu.tw>, 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, xiubli@...hat.com
Subject: Re: [PATCH v2 1/5] lib/base64: Replace strchr() for better
 performance

On Fri, 12 Sep 2025 00:38:20 +0800
Kuan-Wei Chiu <visitorckw@...il.com> wrote:

... 
> Or I just realized that since different base64 tables only differ in the
> last two characters, we could allocate a 256 entry reverse table inside
> the base64 function and set the mapping for those two characters. That
> way, users wouldn't need to pass in a reverse table. The downside is that
> this would significantly increase the function's stack size.

How many different variants are there?
IIRC there are only are two common ones.
(and it might not matter is the decoder accepted both sets since I'm
pretty sure the issue is that '/' can't be used because it has already
been treated as a separator.)

Since the code only has to handle in-kernel users - which presumably
use a fixed table for each call site, they only need to pass in
an identifier for the table.
That would mean they can use the same identifier for encode and decode,
and the tables themselves wouldn't be replicated and would be part of
the implementation.

	David