Re: faster strcpy()

[Alexander Kjeldaas <astor@guardian.no>]

On Fri, Apr 24, 1998 at 11:26:09AM +0100, Philip Blundell wrote:
> >It's something like subtracting 0x01010101 from the dword and oring with 
> >0x80808080 to detect the carry, but that's not quite it. Perhaps you do 
> 
> /* The following magic check was designed by A. Mycroft. It yields a     */
> /* nonzero value if the argument w has a zero byte in it somewhere. The  */
> /* messy constants have been unfolded a bit in this code so that they    */
> /* get preloaded into registers before relevant loops.                   */
> 
> #ifdef _copywords
> #  define ONES_WORD   0x01010101
> #  define EIGHTS_WORD (ones_word << 7)
> #  define nullbyte_prologue_() \
>       int ones_word = ONES_WORD;
> #  define word_has_nullbyte(w) (((w) - ones_word) & ~(w) & EIGHTS_WORD)
> #endif
> 

I'm not sure why Mycroft used '& ~(w)' in the above. Unless something
obvious escapes me, you can optimize the above down to 3
instructions. Both codepaths have a dependency chain of 3
instructions.

#define word_has_nullbyte(w) ((((w) - 0x01010101) ^ (w)) & 0x80808080)

astor

-- 
 Alexander Kjeldaas, Guardian Networks AS, Trondheim, Norway
 http://www.guardian.no/

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