Re: [PATCH] Optimize prefixcmp()

From: Marco Costalba <mcostalba@gmail.com>
Date: Dec 30, 2007, 14:50 UTC
Message-ID: <e5bfff550712300650j2ea70032jaca893b734592184@mail.gmail.com>
In-Reply-To: <20071230135820.GB25917@artemis.madism.org>

On Dec 30, 2007 2:58 PM, Pierre Habouzit <madcoder@debian.org> wrote:

Show 12 quoted lines

> >
> >   This code doesn't work if prefix is "". You want something like:
> >
> >     for (; *prefix; prefix++, str++) {
> >         if (*str != *prefix)
> >             return *(unsigned const char *)prefix - *(unsigned const char *)str;
> >     }
> >     return 0;
>
>   Which happens to be basically the same than what Dscho wrote, though I
> suppose the compiler can compile that more efficiently than his code.
>

Yes, your version covers the *prefix == "" case too. If this case is important for us we could use something as

static inline int prefixcmp(const char *str, const char *prefix)
{
	do {
		if (*str != *prefix)
			return (!*prefix ? 0 : *(unsigned const char *)prefix - *(unsigned
const char *)str);

		if (!*(++prefix))
			return 0;

		str++;

	} while (1);
}

But your code is *surely* nicer then this one. But, for unknown reasons, this code happens to be faster, probably as you say the compiler optimizes away the second check in the return statement so that this version is slightly faster then the 'for' loop one, but admitelly we are going to much in the academic now.

If *prefix == "" case is to be considered I vote for your/Johannes version because it's "better code" (tm).

Marco