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From: ado@elsie.UUCP (Arthur David Olson)
Newsgroups: net.bugs.4bsd
Subject: 4.?BSD C fails to optimize a function's last expression--who has fix?
Message-ID: <5216@elsie.UUCP>
Date: Thu, 5-Sep-85 18:13:53 EDT
Article-I.D.: elsie.5216
Posted: Thu Sep  5 18:13:53 1985
Date-Received: Sat, 7-Sep-85 05:58:35 EDT
Organization: NIH-LEC, Bethesda, MD
Lines: 30
Keywords: C optimizer

Today's quiz:  which of these two functions results in more
code after they've been run through the 4.?BSD optimizer:

	one(i, j, k)				two(i, j, k)
	{					{
		i = j * k;				dummy();
		dummy();				i = j * k;
	}					}

The answer:  you get more code out of "two".  Here are the results:

	_one:.word	L12			_two:.word	L18
	mull3	12(ap),8(ap),4(ap)		calls	$0,_dummy
	calls	$0,_dummy			mull3	12(ap),8(ap),r0
	ret					movl	r0,4(ap)
						ret

And here's the explanation:  in the second case, the optimizer sees that
something's been put in register zero and then there's a return, so
the optimizer figures that the thing put in register zero is a return value
(rather than a temporary value, as it is in this instance) and decides it
had best be left alone.

Has anybody out there figured out how to get better results in situations such
as this?
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