Re: [PATCH V3 13/26] csky: Library functions

[Guo Ren <ren_guo@c-sky.com>]

On Thu, Sep 06, 2018 at 04:24:59PM +0200, Arnd Bergmann wrote:
> On Wed, Sep 5, 2018 at 2:08 PM Guo Ren <ren_guo@c-sky.com> wrote:
> 
> > --- /dev/null
> > +++ b/arch/csky/abiv1/memset.c
> > @@ -0,0 +1,38 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +// Copyright (C) 2018 Hangzhou C-SKY Microsystems co.,ltd.
> > +#include <linux/types.h>
> > +
> > +void *memset(void *dest, int c, size_t l)
> > +{
> > +       char *d = dest;
> > +       int ch = c;
> > +       int tmp;
> > +
> > +       if ((long)d & 0x3)
> > +               while (l--) *d++ = ch;
> > +       else {
> > +               ch &= 0xff;
> > +               tmp = (ch | ch << 8 | ch << 16 | ch << 24);
> > +
> > +               while (l >= 16) {
> > +                       *(((long *)d)) = tmp;
> > +                       *(((long *)d)+1) = tmp;
> > +                       *(((long *)d)+2) = tmp;
> > +                       *(((long *)d)+3) = tmp;
> > +                       l -= 16;
> > +                       d += 16;
> > +               }
> > +
> > +               while (l > 3) {
> > +                       *(((long *)d)) = tmp;
> > +                       d = d + 4;
> > +                       l -= 4;
> > +               }
> > +
> > +               while (l) {
> > +                       *d++ = ch;
> > +                       l--;
> > +               }
> > +       }
> > +       return dest;
> > +}
> 
> I see that we have a trivial memset() implementation in lib/string.c, but yours
> seems to be better optimized. Where did you get it from?
We write it for our ck610 to improve the performance, but I think a lot
of other arch done it in asm style.

> Is this a version
> that works particularly well on C-Sky, or is this a generic optimized memset
> that others could use as well?
We only test it on C-SKY, but I think it will also work better on other
arch CPU than current lib/string.c memset implement.

I see that in lib/string.c:
void *memset(void *s, int c, size_t count)
{
	char *xs = s;

	while (count--)
		*xs++ = c;
	return s;
}
The most problem is "char *xs;" and it will cause "st.b" in asm.
"st.b" is very slow.

Our key improvement is:
> > +                       *(((long *)d)) = tmp;
> > +                       *(((long *)d)+1) = tmp;
> > +                       *(((long *)d)+2) = tmp;
> > +                       *(((long *)d)+3) = tmp;
It will cause SOC AXI burst transfer.

> In the latter case, we could add it to
> lib/string.c and let architectures select it in place of the triivial version.
Good idea.

 Guo Ren