Re: Fast memcpy patch

From: N. Coesel
Date: Wed Nov 23 2011 - 07:07:41 EST


Nihai,

At 12:45 23-11-2011, Mihai DonÈ?u 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 DonÈ?u

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