linux-kernel - Re: Fast memcpy patch

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Date:	Wed, 23 Nov 2011 13:07:40 +0100
From:	"N. Coesel" <nico@...dev.nl>
To:	Mihai DonÈu <mihai.dontu@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Fast memcpy patch

Nihai,

At 12:45 23-11-2011, Mihai DonÈu wrote:
>On Wed, 23 Nov 2011 12:25:46 +0100 N. Coesel wrote:
> > Dear readers,
> > I noticed the Linux kernel still uses a byte-by-byte copy method for
> > memcpy. Since most memory allocations are aligned to the integer size
> > of a cpu it is often faster to copy by using the CPU's native word
> > size. The patch below does that. The code is already at work in many
> > 16 and 32 bit embedded products. It should also work for 64 bit
> > platforms. So far I only tested 16 and 32 bit platforms.
> >
>
>Could you run checkpatch.pl on this an fix all the warnings? Or, if you
>wish, I could do it for you.

If you can, yes please. I used diff -uprN to make 
the patch. It cross-compiles without warnings using gcc4.4 for ARM.

Nico COesel

> > --- lib/string.c.orig   2010-08-20 20:55:55.000000000 +0200
> > +++ lib/string.c        2011-11-23 12:29:02.000000000 +0100
> > @@ -565,14 +565,47 @@ EXPORT_SYMBOL(memset);
> >    * You should not use this function to access IO space, use
> > memcpy_toio()
> >    * or memcpy_fromio() instead.
> >    */
> > -void *memcpy(void *dest, const void *src, size_t count)
> > +
> > +void *memcpy(void *dst, const void *src, size_t length)
> >   {
> > -       char *tmp = dest;
> > -       const char *s = src;
> > +       void *p=dst;
> >
> > -       while (count--)
> > -               *tmp++ = *s++;
> > -       return dest;
> > +       //check alignment
> > +       if (( (int) dst & (sizeof(int) -1)) != ( (int) src &
> > (sizeof(int) -1) ))
> > +               {
> > +               //unaligned. This will never align so copy
> > byte-by-byte
> > +               goto copyrest;
> > +               }
> > +
> > +       //seek aligment (lower bits should become 0). Because
> > +       //we already tested the lower bits are equal, we only need
> > +       //to test source or destination for matching alignment.
> > +       while ( (length !=0) && (((int) src & (sizeof(int)-1 ))!=0) )
> > +               {
> > +
> > +                *((char*) dst++)=*((char*)src++);
> > +               length--;
> > +               }
> > +
> > +       //copy words
> > +       while(length> (sizeof(int)-1) )
> > +               {
> > +               *((int*) dst)=*((int*)src);
> > +               dst+=sizeof(int);
> > +               src+=sizeof(int);
> > +               length-=sizeof(int);
> > +               }
> > +
> > +copyrest:
> > +
> > +       //now copy the rest byte-by-byte
> > +       while(length !=0)
> > +               {
> > +               *((char*) dst++)=*((char*) src++);
> > +               length--;
> > +               }
> > +
> > +       return p;
> >   }
> >   EXPORT_SYMBOL(memcpy);
> >   #endif
> >
>
>--
>Mihai DonÈu

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