Path: utzoo!utgpu!water!watmath!clyde!rutgers!mit-eddie!uw-beaver!uw-june!pardo
From: pardo@june.cs.washington.edu (David Keppel)
Newsgroups: comp.lang.c
Subject: Re: comma operator
Message-ID: <4018@june.cs.washington.edu>
Date: 20 Jan 88 06:55:59 GMT
References: <3819@sigi.Colorado.EDU> <8599@ism780c.UUCP> <1866@bsu-cs.UUCP> <4006@june.cs.washington.edu> <3586@sdcc6.ucsd.EDU>
Reply-To: pardo@uw-june.UUCP (David Keppel)
Organization: U of Washington, Computer Science, Seattle
Lines: 25

[ how could the comma operator cause slower code? ]

Because the comma operator guarantees an order of evaluation on two
statements that the compiler might otherwise rearrange to produce more
efficient code.

As a brain-dead example:

	    ++x, ++y;
	    if (x % n) { ... }

If the comma was replaced by a semicolon, the compiler could rewrite it as

	    ++y;
	    if (++x % n) { ... }

Which would be faster on many machines.

I'm not saying that this happens all the time or that you should worry
about it in the normal case, but if you are writing code where you need
all the optimization you can get (particularly if your compiler does good
code-motion) then you might want to worry that the compiler can't optimize
that extra cycle (because the comma operator guarantees order of execution).

	;-D on  (speed, like cocaine, is highly illegal)  Pardo