[PATCH] Huge performance boost for recursion (cp, du, find, ls, rm, , mv)

[Rolf Eike Beer]

On Mittwoch, 10. April 2024 21:36:06 CEST Jody Bruchon wrote:
> This patch uses pre-calculated name lengths to massively speed up various
> recursive operations. Three new *_fast variant functions are added along
> with get_d_namlen copied from libjodycode. Passing lengths allows use of
> memcpy() instead of strcpy()/strcat() and replacement of a particularly
> hot xasprintf(). Cachegrind shows CPU instructions on Linux x86_64 drop
> by 24% to 67% with similar reductions in data reads and writes.
> 
> Anything in BusyBox that uses a while(readdir()) loop or that calls
> concat_*path_file() or last_char_is() might benefit from adopting this
> optimization framework.

Completely untested, but how about this:

char* FAST_FUNC concat_path_file_fast(const char *path, const struct dirent 
*dirp)
{
	const char *filename = dirp->d_name;
	char *buf;
	int end_offset;
	int pathlen, namelen;

	if (!path) {
		path = "";
		pathlen = 0;
	} else {
		pathlen = strlen(path);
		if (last_char_is_fast(path, '/', pathlen) == NULL) 
pathlen--;
	}
	namelen = get_d_namlen(dirp);
	while (*filename == '/') {
		filename++;
		namelen--;
	}
	end_offset = pathlen + 1 + namelen;
	buf = (char *)malloc(end_offset + 1);
	if (!buf) return NULL;
	memcpy(buf, path, pathlen);
	*(buf + pathlen) = '/';
	memcpy(buf + pathlen + 1, filename, namelen);
	*(buf + end_offset) = '\0';
	return buf;
}

This avoids scanning an empty path for the trailing slash, and avoids the 
check for a trailing slash later entirely, saving one instruction and 2 
variables.

You also have quite some places where you have the old code still around and 
just commented out, I would have just removed them.

Generally I wonder if the length variables shouldn't be size_t or the like, 
which would not affect this function but all of them.

Eike
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