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From: ado@elsie.UUCP (Arthur David Olson)
Newsgroups: net.bugs.4bsd
Subject: Re: /lib/c2 optimizes calls instructions
Message-ID: <5207@elsie.UUCP>
Date: Thu, 22-Aug-85 11:22:21 EDT
Article-I.D.: elsie.5207
Posted: Thu Aug 22 11:22:21 1985
Date-Received: Sat, 24-Aug-85 18:42:19 EDT
References: <2064@ukma.UUCP> <1326@umcp-cs.UUCP>
Organization: NIH-LEC, Bethesda, MD
Lines: 79
Summary: Is "calls r*,_subr" truly faster than "call $*,_subr"?

> >Index: /lib/c2, BRL-Unix release 3, likely appears in other 4.?BSD.
> >Problem: Calls instructions have a format "calls <const-#-of-args>,<addr>".
> >	The <const-#-of-args> argument will sometimes be "optimized" into
> >	a register reference.  This isn't necessary. . .
> 
> It saves a little teensy bit of time. . .

Hmmm. . .let's check this out with the following program on a VAX 11/750
(4.1bsd, cp_urev = 94, cp_hrev = 72):

	main(argc, argv)
	int	argc;
	char *	argv[];
	{
		register int	i;

		i = atoi(argv[1]);
		do {
			one();
			two();
		} while (--i > 0);
	}

	dummy1() { dummy1(); dummy1(); dummy1(); dummy1(); dummy1(); }

	one()
	{
		register int	i, j;

		i = 1000;
		do {
			j = 0;
			subr();
		} while (--i > 0);
	}

	dummy2() { dummy2(); dummy2(); dummy2(); dummy2(); dummy2(); }

	two()
	{
		register int	i, j;

		i = 1000;
		do {
			subr();
			j = 0;
		} while (--i > 0);
	}

	dummy3() { dummy3(); dummy3(); dummy3(); dummy3(); dummy3(); }

	subr() {}

If I name the above "try.c" and
	cc -c -O try.c ; cc -p -O try.o ; a.out 10000 ; prof
I get this output:
	 %time  cumsecs  #call  ms/call  name
	  60.8   325.10                  _subr
	  20.2   433.28                  _one
	  18.9   534.22                  _two
	   0.1   534.73                  _main
which says that the function "one", with its
	L33:clrl	r10
	calls	r10,_subr
	sobgtr	r11,L33
takes more time than function "two", with its
	L46:calls	$0,_subr
	clrl	r10
	sobgtr	r11,L46
loop.

Comments?
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