Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!bbn!oberon!nunki.usc.edu!jeenglis
From: jeenglis@nunki.usc.edu (Joe English)
Newsgroups: comp.lang.c
Subject: Re: Definition of boolean type
Message-ID: <2892@nunki.usc.edu>
Date: 1 Mar 89 22:40:28 GMT
References: <10@dbase.UUCP> <1989Feb10.092449.20875@sq.uucp> <6849@pogo.GPID.TEK.COM> <9753@smoke.BRL.MIL>
Reply-To: jeenglis@nunki.usc.edu (Joe English)
Organization: University of Southern California, Los Angeles, CA
Lines: 39


gwyn@brl.arpa (Doug Gwyn (VLD/VMB) <gwyn>) writes:
>In article <6849@pogo.GPID.TEK.COM> rickc@pogo.GPID.TEK.COM (Rick Clements) writes:
>>With the compiler I am currently using, I use "if (x == FALSE)" or
>>"if (x != FALSE).  ...  The compiler I am using generates LESS code
>>this way.  ("if (x)" causes it to go to the work of converting it to
>>a 1 or 0 with some less than efficient code.
>
>That's pretty strange -- "if(x)" means no more and no less than
>"if x is nonzero", which nearly all instruction sets support directly.
>It's easy to imagine a dumb compiler that produces MORE code for
>"if(x!=0)" but not one that produces LESS code.

I once had to work with a compiler that was broken in a
similar way: boolean expressions would calculate either a 0
or a 1, then test that result and branch accordingly.  This
generated *three unnecessary instructions* for every
relational expression.  (And the fourth pass of the compiler
was called the "optimizer!"  I don't think it did a very
good job of optimizing...)  

If I recall correctly, it generated the same amount of code
for "if(x)" and "if(x!=0)," but it went something like:

	Compare x and zero; 
	move 1 into AX; 
	if test yielded equal, skip next instruction; 
	move 0 into AX;

Then it would do an "OR AX,AX" and branch accordingly.  
Shocking, but true.

Before you ask (so you can avoid purchasing it :-), the
compiler in question is an old (c. 1984) version of Computer
Innovation's C86, I think somewhere around version 1.2.

--Joe English

  jeenglis@nunki.usc.edu