From mboxrd@z Thu Jan  1 00:00:00 1970
Message-ID: <9d0beeb37aae80180882f07b494170f3@plan9.bell-labs.com>
To: 9fans@cse.psu.edu
Subject: Re: [9fans] plan or side effect
From: "rob pike" <rob@plan9.bell-labs.com>
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Date: Thu,  7 Mar 2002 08:45:53 -0500
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> Note that under reasonable assumptions the macro saves at least a
> dozen function calls for each redundant test of byte equality, so
> the more conservative version is still a big performance win.

Function calls are cheap.  They used to be expensive, but they're
really not any more.  I tried the attached program and got a factor of
about 2.5 between the two programs (mips or x86).  Given that our
strcmp is not clever, one might have expected a bigger difference.

No denying the macro makes a difference, but given their dangers,
you'd have to have a pretty special program before this change could
make a worthwhile improvement.  I tried inlining (by hand) strcmp -
something gcc does just fine - and got exactly the same speedup.
Surely inlining is safer than this sort of hack, if you can inline.

-rob


#include <u.h>
#include <libc.h>

#define StrEq(a,b) (*(a)==*(b) && strcmp((a),(b))==0)

void
main(int argc, char *argv[])
{
	char *a, *b;
	int i, j;

	a = "asdfadfdsf";
	b = "bsdfzxcvvx";
	for(i=0; i<1000*1000*100; i++)
//		if(StrEq(a,b))
		if(strcmp(a,b)==0)
			j++;
}


our strcmp:

#include <u.h>
#include <libc.h>

int
strcmp(char *s1, char *s2)
{
	unsigned c1, c2;

	for(;;) {
		c1 = *s1++;
		c2 = *s2++;
		if(c1 != c2) {
			if(c1 > c2)
				return 1;
			return -1;
		}
		if(c1 == 0)
			return 0;
	}
}