Re: [PATCH] riscv: Use LMUL=8 for RVV strchr

On 7/6/26 7:54 AM, Ning Tian wrote:
> The RVV strchr implementation currently uses LMUL=1.  On VLEN=128
> hardware this processes 16 bytes per iteration, so the fixed cost of
> the fault-only-first load, two byte comparisons, two vfirst.m
> operations, and scalar branch logic is paid every 16 bytes.
> 
> Use LMUL=8 so each iteration processes 128 bytes on VLEN=128 systems.
> The character-match mask is moved from v9 to v16 because v0-v7 is now
> used by the m8 data vector group.  The end-of-string mask remains in
> v8.
> 
> On Sophgo SG2044 (VLEN=128, GCC 12.3.1), bench-strchr over 445 cases
> shows a 4.63x geometric-mean speedup compared with the LMUL=1 RVV
> version.  Full-scan cases improve by 5.52x to 8.96x for lengths from
> 64 to 8192 bytes.  Short strings that complete in the first vector
> chunk regress by about 50% because LMUL=8 has higher vector setup cost;
> a later scalar fast path can address that case.


First off, we are currently in a freeze for the upcoming release, so this
will have to wait until after the release (~Aug 1st).

Secondly, these routines are supposed to work on a wide range of riscv
hardware.  Before making a change like this, I'd like some performance numbers
from other systems.  Preferably the recently available K3.  I do not yet
have access to a K3, so maybe you or someone else can test it there?
If it's a win on the K3, then I'm fine with the change....after the release.

Peter