Re: [PATCH] riscv: fix memmove and optimise memcpy when misalign

[Gary Guo]

On Sun, 23 May 2021 17:12:23 +0000
David Laight <David.Laight@xxxxxxxxxx> wrote:

> From: Palmer Dabbelt
> > Sent: 23 May 2021 02:47  
> ...
> > IMO the right way to go here is to just move to C-based string
> > routines, at least until we get to the point where we're seriously
> > optimizing for specific processors.  We went with the C-based
> > string rountines in glibc as part of the upstreaming process and
> > found only some small performance differences when compared to the
> > hand-written assembly, and they're way easier to maintain.

I prefer C versions as well, and actually before commit 04091d6 we are
indeed using the generic C version. The issue is that 04091d6
introduces an assembly version that's very broken. It does not offer
and performance improvement to the C version, and breaks all processors
without hardware misalignment support (yes, firmware is expected to
trap and handle these, but they are painfully slow).

I noticed the issue because I ran Linux on my own firmware and found
that kernel couldn't boot. I didn't implement misalignment emulation at
that time (and just send the trap to the supervisor).

Because 04091d6 is accepted, my assumption is that we need an assembly
version. So I spent some time writing, testing and optimising the
assembly.

> > 
> > IIRC Linux only has trivial C string routines in lib, I think the
> > best way to go about that would be to higher performance versions
> > in there. That will allow other ports to use them.  
> 
> I certainly wonder how much benefit these massively unrolled
> loops have on modern superscaler processors - especially those
> with any form of 'out of order' execution.
> 
> It is often easy to write assembler where all the loop
> control instructions happen in parallel with the memory
> accesses - which cannot be avoided.
> Loop unrolling is so 1970s.
> 
> Sometimes you need to unroll once.
> And maybe interleave the loads and stores.
> But after that you can just be trashing the i-cache.

I didn't introduce the loop unrolling though. The loop unrolled
assembly is there before this patch, and I didn't even change the
unroll factor. I only added a path to handle misaligned case.

There are a lot of diffs because I did made some changes to the
register allocation so that the code is more optimal. I also made a few
cleanups and added a few comments. It might be easier to review if you
apply the patch locally and just look at the file.

- Gary