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From: gwyn@brl-smoke.ARPA (Doug Gwyn )
Newsgroups: net.lang.c
Subject: Re: Re: fast code and no morals
Message-ID: <978@brl-smoke.ARPA>
Date: Sun, 16-Feb-86 00:26:33 EST
Article-I.D.: brl-smok.978
Posted: Sun Feb 16 00:26:33 1986
Date-Received: Tue, 18-Feb-86 04:21:44 EST
References: <842@megaron.UUCP> <1820@brl-tgr.ARPA>
Reply-To: gwyn@brl.ARPA
Organization: /usr/local/lib/news/organization
Lines: 35

>Even if 'fast but no morals' doesn't optimize as well on the VAX as it does
>on the 68K, it still runs.  The examples are common profiler hot spots.
>bcopy(dest,src,len)
>REG char *dest,*src;
>REG USHORT len;
>{
>while(len--)
>   *dest++=*src++;
>}
>
>may look prettier than
>
>bcopy(dest,src,len)
>REG char *dest,*src;
>REG USHORT len;
>{
>if(len)
>do{
>  *dest++=*src++;
>  }while(--len)
>}
>
>but when you are calling bcopy to move 2k before the next
>interrupt hits, the extra 10us per iteration (dbgt vs. test,bra)
>can get real costly.  Adding automatic backward detection (used in editors)
>would be difficult in assembler, but not in C.

Neither of these examples uses obscure code (except for your
redefining C keywords for no good reason).  Also, neither
one is the best implementation of bcopy() on most machines.
One can do much better than byte transfers within a loop.
If you're going to worry about efficiency, do it right; use
the memcpy() library routine which some system programmer
has tuned for fastest possible block transfer, rather than
writing your own.