Re: {RFC/PATCH] micro-optimize get_sha1_hex()

From: Jeff Garzik <jeff@garzik.org>
Date: Sep 9, 2006, 22:33 UTC
Message-ID: <45034143.4050308@garzik.org>
In-Reply-To: <7vzmd8vh6q.fsf@assigned-by-dhcp.cox.net>

Junio C Hamano wrote:

Show 18 quoted lines

>  int get_sha1_hex(const char *hex, unsigned char *sha1)
>  {
>  	int i;
>  	for (i = 0; i < 20; i++) {
> -		unsigned int val = (hexval(hex[0]) << 4) | hexval(hex[1]);
> -		if (val & ~0xff)
> +		unsigned int v, w, val;
> +		v = *hex++;
> +		if ((v < '0') || ('f' < v) ||
> +		    ((v = hexval[v-'0']) == 255))
> +			return -1;
> +		w = *hex++;
> +		if ((w < '0') || ('f' < w) ||
> +		    ((w = hexval[w-'0']) == 255))
>  			return -1;
> -		*sha1++ = val;
> -		hex += 2;
> +		*sha1++ = (v << 4) | w;

Why not just make the table include the range of all possible characters? That would eliminate some comparisons and subtractions against '0'.

The end result would look more like the current (unpatched) form, except with a function replaced by the table.

	Jeff