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From: henry@utzoo.UUCP (Henry Spencer)
Newsgroups: comp.lang.c
Subject: Re: goto's in C: an opinion...
Message-ID: <8361@utzoo.UUCP>
Date: Sun, 2-Aug-87 20:29:20 EDT
Article-I.D.: utzoo.8361
Posted: Sun Aug  2 20:29:20 1987
Date-Received: Sun, 2-Aug-87 20:29:20 EDT
References: <3289@bigburd.PRC.Unisys.COM>, <28700017@ccvaxa>
Organization: U of Toronto Zoology
Lines: 32

> 	When you've completed an application nicely in C,
> 	using functions everywhere, and it's still too slow,
> 	which would you rather do?:
> 
> 	(1) Rewrite parts of it in assembler,
> 
>      	or (2) Use gotos?

Neither.  First, you *profile* it, to find out where the time is really
going; human guessing (even if dignified by names like "intuition" or
"professional judgement") is usually wrong.  Then, you revise the code
in the hot spots to use better algorithms.  If you are really greasing
the code hard, it may be necessary to merge previously-separate functions
into their callers, to reduce call overhead.  The big wins come from
reorganization to avoid work entirely, not from twiddling to do the work
a little bit faster.

For example, C News is now 1.5 orders of magnitude faster than B News,
and will probably be still faster when Geoff and I release it, even though
it contains no gotos and no assembler.  We both dislike gotos, and avoid
assembler on the grounds that it's unnecessary and a maintenance nightmare
for portable code.  (There ARE occasions when small assembler functions can
be a win, if you are trying to perform operations that C isn't too good at,
but often a radically new algorithm -- in C -- is still faster.)  Geoff
does a lot of profiling and code analysis and rewriting (I do some too, but
his parts of the code are the hardest and most speed-critical ones), and we
both think a lot about ways to avoid overhead.  For the gory details, see
our paper in the Winter 1987 Usenix proceedings.  We may do a followup
paper someday about all the things we've thought of since.
-- 
Support sustained spaceflight: fight |  Henry Spencer @ U of Toronto Zoology
the soi-disant "Planetary Society"!  | {allegra,ihnp4,decvax,utai}!utzoo!henry