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Path: utzoo!utgpu!water!watmath!clyde!burl!codas!killer!jfh
From: jfh@killer.UUCP
Newsgroups: comp.unix.questions,comp.lang.c
Subject: Re: Wierd Compilers
Message-ID: <1988@killer.UUCP>
Date: Wed, 4-Nov-87 12:17:47 EST
Article-I.D.: killer.1988
Posted: Wed Nov  4 12:17:47 1987
Date-Received: Sat, 7-Nov-87 17:24:03 EST
References: <367@white.gcm> <18964@amdcad.AMD.COM> <44177@beno.seismo.CSS.GOV>
Organization: Big "D" Home for Wayward Hackers
Lines: 50
Keywords: Sun
Xref: utgpu comp.unix.questions:4269 comp.lang.c:5021
Summary: But there is a difference.

In article <44177@beno.seismo.CSS.GOV>, rick@seismo.CSS.GOV (Rick Adams) writes:
> Even a mediocre compiler should be able to recognize that
> 	a++;
> is equivalent to
> 	++a;
> if there is no assignment, etc.
> 
> There is really no excuse for not making this trivial optimization. You should
> not have to depend on the user doing it.
> 
> --rick

I really hadn't expected Rick to come up with that one.  Guess working for
the government was gone to his brain ... (Sorry, couldn't pass up a
anti-government dig ;-).

I have recapped the story for the comp.lang.c folks.

The code in question was

	while (a--)		vs.		while (--a)
		;					;

which is the same as

	while (a-- != 0)	vs.		while (--a != 0)
		;					;

the pre-decrement loop makes one less trip since the value of `a' is
decremented _prior_ to the test (not actually though).  Consider the
case where a == 1.  The --a will equal 0 and the test will fail.  In
the case of a--, the value _prior_ to the decrement is used and the
test will pass, so the loop gets executed.  To be the same, assuming
`a' is not used in the loop, you would have to code

	a++;
	while (--a)
		;

to get the same number of trips (assuming overflow was not a problem).
This would be a big win if the code in the loop was very small and
the value of `a' was large.  I wouldn't do it since the time savings
would be small, and the code a kluge.  Of course, documentation helps.

- John.
-- 
John F. Haugh II		HECI Exploration Co. Inc.
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