From: Alex Riesen <raa.lkml@gmail.com>
Date: Sun, 07 Oct 2007 16:10:12 GMT
Subject: Re: [PATCH] Make strbuf_cmp inline, constify its arguments and optimize it a bit
Message-ID: <20071007161012.GB3270@steel.home>
In-Reply-To: <85fy0nknnq.fsf@lola.goethe.zz>

David Kastrup, Sun, Oct 07, 2007 16:24:57 +0200:
> Alex Riesen <raa.lkml@gmail.com> writes:
> 
> > It is definitely less code (also object code). It is not always
> > measurably faster (but mostly is).
> 
> > -int strbuf_cmp(struct strbuf *a, struct strbuf *b)
> > -{
> > -	int cmp;
> > -	if (a->len < b->len) {
> > -		cmp = memcmp(a->buf, b->buf, a->len);
> > -		return cmp ? cmp : -1;
> > -	} else {
> > -		cmp = memcmp(a->buf, b->buf, b->len);
> > -		return cmp ? cmp : a->len != b->len;
> > -	}
> > -}
> > -
> 
> > +static inline int strbuf_cmp(const struct strbuf *a, const struct strbuf *b)
> > +{
> > +	int len = a->len < b->len ? a->len: b->len;
> > +	int cmp = memcmp(a->buf, b->buf, len);
> > +	if (cmp)
> > +		return cmp;
> > +	return a->len < b->len ? -1: a->len != b->len;
> > +}
> 
> My guess is that you are conflating two issues about speed here: the
> inlining will like speed the stuff up.  But having to evaluate the
> (a->len < b->len) comparison twice will likely slow it down.


Can't the result of the expression be reused in compiled?
Isn't it a common expression?

> So if you do any profiling, you should do it on both separate angles
> of this patch.
> 

I compared the inlined versions of both.