stpecpy(3) vs strlcpy(3) benchmark (was: [PATCH 1/1] string: Add stpecpy(3))

[Alejandro Colomar <alx.manpages@gmail.com>]

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Hi Wilco,

Here goes benchmark code for comparing performance of stpecpy(3) against 
strlcpy(3).  I used my_strlcpy(3), with your definition, because I suspect 
libbsd's version won't be optimized.  stpecpy(3) was also updated to be as 
optimized as I can write it.

I won't be showing exact numbers, as that's probably very system- and 
version-dependent.  Test yourself for exact numbers.  However, I'll share the 
general results.

At first I was a little bit surprised (and disappointed) because strlcpy(3) 
seemed faster under GCC.  When forming the very short string "Hello, " "worlds" 
"!", in just 3 consecutive calls, my_strlcpy(3) outperforms (~10%) stpecpy() in 
GCC.  In Clang they had both similar times.

When I added a long string of 'x's, the results more or less were the same.

When I added a few calls by repeating "worlds" "!", that changed the balance 
very significantly for stpecpy(3).  When chaining more than just a couple calls, 
strlcpy(3) starts showing a performance decrease.  With the code below, I saw 
stpecpy(3) consistently outperform strlcpy(3) by around 3%.

I guess the advantage of strlcpy(3) over stpecpy(3) may be due to the overhead 
of setting 'end = buf + BUFSIZ;' for stpecpy(3), and to the fact that the first 
strlcpy(3) call doesn't have to do the usual '+=', ' + l', and ' - l', which can 
be significant given how negligible the internal difference between the two is.

Cheers,
Alex

---

alx@debian:~/tmp$ cat foo.c
#include <err.h>
#include <stdio.h>

#include <stp/stpe/stpecpy.h>

void
foo(char *buf, size_t *len)
{
	char    *p, *end;

	end = buf + BUFSIZ;
	p = buf;
	p = stpecpy(p, end, "Heylo, xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx");
	p = stpecpy(p, end, "worlds");
	p = stpecpy(p, end, "!");
	p = stpecpy(p, end, "worlds");
	p = stpecpy(p, end, "!");
	p = stpecpy(p, end, "worlds");
	p = stpecpy(p, end, "!");
	p = stpecpy(p, end, "worlds");
	p = stpecpy(p, end, "!");
	if (p == end) {
		p--;
		warnx("Truncated");
	}
	*len = p - buf;
}

void
bar(char *buf, size_t *len)
{
	size_t l;

	l = my_strlcpy(buf, "Hello, xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx", BUFSIZ);
	if (l >= BUFSIZ)
		goto toolong;
	l += my_strlcpy(buf + l, "worlds", BUFSIZ - l);
	if (l >= BUFSIZ)
		goto toolong;
	l += my_strlcpy(buf + l, "!", BUFSIZ - l);
	if (l >= BUFSIZ)
		goto toolong;
	l += my_strlcpy(buf + l, "worlds", BUFSIZ - l);
	if (l >= BUFSIZ)
		goto toolong;
	l += my_strlcpy(buf + l, "!", BUFSIZ - l);
	if (l >= BUFSIZ)
		goto toolong;
	l += my_strlcpy(buf + l, "worlds", BUFSIZ - l);
	if (l >= BUFSIZ)
		goto toolong;
	l += my_strlcpy(buf + l, "!", BUFSIZ - l);
	if (l >= BUFSIZ)
		goto toolong;
	l += my_strlcpy(buf + l, "worlds", BUFSIZ - l);
	if (l >= BUFSIZ)
		goto toolong;
	l += my_strlcpy(buf + l, "!", BUFSIZ - l);
	if (l >= BUFSIZ) {
toolong:
		l = BUFSIZ - 1;
		warnx("Truncated");
	}

	*len = l;
}

alx@debian:~/tmp$ cat bench.c
#include <stdio.h>

int foo(char *buf, size_t *len);
int bar(char *buf, size_t *len);

int
main(void)
{
	char    buf[BUFSIZ];
	size_t  len;

	for (size_t i = 0; i < 10000000; i++)
#if 1
		foo(buf, &len);
#else
		bar(buf, &len);
#endif

	printf("%zu: %s\n", len, buf);
}


-- 
<http://www.alejandro-colomar.es/>

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