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From: bs@alice.UucP (Bjarne Stroustrup)
Newsgroups: net.lang.c++
Subject: Call timings
Message-ID: <6181@alice.uUCp>
Date: Thu, 16-Oct-86 13:05:17 EDT
Article-I.D.: alice.6181
Posted: Thu Oct 16 13:05:17 1986
Date-Received: Thu, 16-Oct-86 23:14:12 EDT
Organization: Bell Labs, Murray Hill
Lines: 60

/*
	here are some more timings showing function call costs:
*/
struct s {
	int a;
	virtual f1();			// virtual function
	f3() { a++; return a; }		// calls will be inlined
	f4();				// member function
};

s::f1() { a++; return a;}
s::f4() { a++; return a;}

f2(s* p) {p->a++; return p->a; }		// ``normal'' function
f5(register s* p) {p->a++; return p->a; }	// register argument
int (*pf)(s*) = &f2;

main() {
	s a;
	register s* p = &a;

//	for (register int i = 100000; 0<i; i--) p->f1();
//	for (register int i = 100000; 0<i; i--) f2(p);
//	for (register int i = 100000; 0<i; i--) p->f3();
//	for (register int i = 100000; 0<i; i--) ;
//	for (register int i = 100000; 0<i; i--) p->f4();
//	for (register int i = 100000; 0<i; i--) (*pf)(p);
	for (register int i = 100000; 0<i; i--) f5(p);
}

/*

Times in seconds, that is, the time needed per iteration in microseconds:

		virtual			member			empty
		fct	fct	inline	fct			loop

		p->f1()	f2(p)	p->f3()	p->f4()	pf(p)	f5(p)	;

VAX11/750:	41	32	5	41	31	40	3
VAX11/785:	21	15	2	20	16	20	1	
sun3:		8	6	1	8	7	7	1
AT&T 3B20:	20	16	4	18	18	18	2
Amdahl5860:	1.1	1.0	.2	1.0	1.1	1.0	.2
AT&T6300+ (s)	30	26	12	28	28	27	10
AT&T6300+ (l)	46	42	20	42	42	41	10

A 6300+ is an AT clone (small and large models used for pointers, long i).
Release 1.1 was used except for the 6300+. There I used a toy.

So
	(1) A virtual member call is as fast as a non-virtual member call.
	(2) A member call is as fast as an equivalent non-member call.
	(3) Inlining function calls can be very effective in speeding up code.
	(4) Declaring "this" a register argument in a function with only little
	    use of "this" (as in the examples above)  is a loss.
	(5) VAX function calls are relatively expensive (not news).

Thanks to all who help time the examples.
*/