[PATCH] riscv: avoid vrgather in RVV memcmp mismatch path

[Kito Cheng]

Ack, the patch seems good to me, plan to put to my test queue then push :)

Pincheng Wang <pincheng.plct@isrc.iscas.ac.cn> 於 2026年6月11日週四 下午10:01寫道：

> Hi all,
>
> Gentle ping. :)
>
> BR,
> Pincheng Wang
>
> On 2026/5/29 9:29, Pincheng Wang wrote:
> > vfirst.m already returns the byte offset of the first mismatch in the
> > current vector chunk. Use that offset to reload the two differing bytes
> > with lbu instead of extracting them with vrgather.vx and vmv.x.s.
> >
> > The vector gather path can be more expensive on some implementations and
> > also increases vector register pressure. This keeps the mismatch path
> > shorter while preserving the memcmp result.
> >
> > Signed-off-by: Pincheng Wang <pincheng.plct@isrc.iscas.ac.cn>
> > ---
> >   newlib/libc/machine/riscv/memcmp-asm.S | 10 ++++------
> >   1 file changed, 4 insertions(+), 6 deletions(-)
> >
> > diff --git a/newlib/libc/machine/riscv/memcmp-asm.S
> b/newlib/libc/machine/riscv/memcmp-asm.S
> > index b05df9521..1cf1680e2 100644
> > --- a/newlib/libc/machine/riscv/memcmp-asm.S
> > +++ b/newlib/libc/machine/riscv/memcmp-asm.S
> > @@ -28,12 +28,10 @@ memcmp:
> >     li a0, 0
> >     ret
> >   .Lfound:
> > -  vrgather.vx v16, v0, a4
> > -  vrgather.vx v24, v8, a4
> > -  vmv.x.s a0, v16
> > -  vmv.x.s a4, v24
> > -  andi a0, a0, 0xff
> > -  andi a4, a4, 0xff
> > +  add a0, a0, a4
> > +  add a1, a1, a4
> > +  lbu a0, 0(a0)
> > +  lbu a4, 0(a1)
> >     sub a0, a0, a4
> >     ret
> >   .size memcmp, .-memcmp
>
>
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