Re: [GIT PULL] CRC updates for 6.14

David Laight
2025-01-23T20:58:13Z

On Thu, 23 Jan 2025 09:07:44 -0500
"Theodore Ts'o" <tytso@mit.edu> wrote:

> On Wed, Jan 22, 2025 at 11:46:18PM -0800, Eric Biggers wrote:
> >
> > Actually, I'm tempted to just provide slice-by-1 (a.k.a. byte-by-byte) as the
> > only generic CRC32 implementation. The generic code has become increasingly
> > irrelevant due to the arch-optimized code existing. The arch-optimized code
> > tends to be 10 to 100 times faster on long messages.
>
> Yeah, that's my intuition as well; I would think the CPU's that
> don't have a CRC32 optimization instruction(s) would probably be the
> most sensitive to dcache thrashing.
>
> But given that Geert ran into this on m68k (I assume), maybe we could
> have him benchmark the various crc32 generic implementation to see if
> we is the best for him? That is, assuming that he cares (which he
> might not. :-).

The difference between the clock speed and main memory speed on an m68k will
be a lot less than on anything more recent.
So I suspect the effect of cache misses is much less (or more likely it is
pretty much always getting a cache miss).
Brain wakes up, does the m68k even have a D-cache?
Checks the m68k user manual section 6 - it only has a I-cache (64 32-bit words).
So the important thing is probably keeping the loop small.
A cpu board might have an external data cache.

For a small memory footprint it might be worth considering 4 bits at a time.
So a 16 word (64 byte) lookup table.
Thinks....
You can xor a data byte onto the crc 'accumulator' and then do two separate
table lookups for each of the high nibbles and xor both onto it before the rotate.
That is probably a reasonable compromise.

David