Explanation, please!

[Eric S. Raymond]

(Code below reproduced so that comp.arch people seeing this followup only
won't get terminally frustrated. This is *really neat*, gang...)

In article <638 at paris.ics.uci.edu> Douglas C. Schmidt writes:
> 
> void send(int *to,int *from, int count) {
>    int n = (count + 7) / 8;
>    
>    switch(count % 8) {
>       case 0:  do { *to++ = *from++;
>       case 7:       *to++ = *from++;
>       case 6:       *to++ = *from++;
>       case 5:       *to++ = *from++;
>       case 4:       *to++ = *from++;
>       case 3:       *to++ = *from++;
>       case 2:       *to++ = *from++;
>       case 1:       *to++ = *from++;
>                } while (--n > 0);
>    }
> 
> }
> 
> Finally, Stroustrup asks the rhetorical question ``why would anyone
> want to write something like this.''  Any guesses?!

Yeah. That's the most hackish way of trying to write a portable optimized
copy routine I've ever seen. I gather the whole point of the shenanigans
is to get all the *from++ -> *to++ instructions in the generated code to be
adjacent.

This only makes if the author knows he's got a hardware instruction pipeline
or cache that's no less than 8 and no more than 9 byte-copy instruction widths
long, and stuff executing out of the pipeline is a lot faster than if the
copies are interleaved with control transfers. Dollars to doughnuts this code
was written on a RISC machine.

(Gawrsh. That sounded just like one of the big boys on comp.arch tawkin'. I
think I'll cross-post over there just to see if I get shot down in flames...)

-- 
      Eric S. Raymond                     (the mad mastermind of TMN-Netnews)
      UUCP: ...!{uunet,att,rutgers}!snark!eric = eric at snark.UUCP
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