From mboxrd@z Thu Jan 1 00:00:00 1970 X-Msuck: nntp://news.gmane.org/gmane.linux.lib.musl.general/9470 Path: news.gmane.org!not-for-mail From: Rich Felker Newsgroups: gmane.linux.lib.musl.general Subject: Re: [RFC PATCH] micro-optimize __procfdname Date: Sat, 5 Mar 2016 00:56:05 -0500 Message-ID: <20160305055605.GF9349@brightrain.aerifal.cx> References: <20160305052459.GD9349@brightrain.aerifal.cx> Reply-To: musl@lists.openwall.com NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii X-Trace: ger.gmane.org 1457157383 8441 80.91.229.3 (5 Mar 2016 05:56:23 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Sat, 5 Mar 2016 05:56:23 +0000 (UTC) To: musl@lists.openwall.com Original-X-From: musl-return-9483-gllmg-musl=m.gmane.org@lists.openwall.com Sat Mar 05 06:56:23 2016 Return-path: Envelope-to: gllmg-musl@m.gmane.org Original-Received: from mother.openwall.net ([195.42.179.200]) by plane.gmane.org with smtp (Exim 4.69) (envelope-from ) id 1ac5CO-0001zZ-Vh for gllmg-musl@m.gmane.org; Sat, 05 Mar 2016 06:56:21 +0100 Original-Received: (qmail 7195 invoked by uid 550); 5 Mar 2016 05:56:18 -0000 Mailing-List: contact musl-help@lists.openwall.com; run by ezmlm Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: List-ID: Original-Received: (qmail 7174 invoked from network); 5 Mar 2016 05:56:18 -0000 Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) Original-Sender: Rich Felker Xref: news.gmane.org gmane.linux.lib.musl.general:9470 Archived-At: On Sat, Mar 05, 2016 at 08:42:16AM +0300, Alexander Monakov wrote: > On Sat, 5 Mar 2016, Rich Felker wrote: > > I really doubt this makes any major improvement, but it might help > > size a bit and it might be cleaner/more readable, so it's interesting. > > Yeah, this precedes a syscall so speed-wise it doesn't matter; I just > noticed two div-10 loops and saw a chance to improve size. *nod* > > > +char *__procfdname_impl(char *, unsigned); > > > + > > > +#define procfdbufsize sizeof "/proc/self/fd/0123456789" + (3 * (sizeof(int)-4)) > > > > What is the motivation behind changing the size expression to use the > > "012...9" part? It's nonobvious to me. > > It just makes it obvious that there are 10 decimal places, which is how much a > 32-bit unsigned int can occupy at most. I don't mind using any other style. I tend to like 3*sizeof(int) just because it's an idiom I know (pessimistic bound as if each byte could hold 0...999 range rather than just 0...255) but your version is slightly sharper. > > > +#define procfdname(buf, fd) __procfdname_impl(buf + procfdbufsize - 1, fd) > > > > I suppose the idea of putting the offset to the end in a macro in the > > header rather than in the callee is both optimization and allowing the > > compiler to detect out-of-bounds pointer arithmetic? > > Hm, the latter is rather theoretical given the uses, right? I just made it to I meant I thought the compiler might be able to catch if a callee accidentally used the wrong buffer size. Shouldn't happen anyway, but it'd be nice to have an extra layer of verification. > make it really obvious that __procfdname_impl fills in reverse; it might be a > very minor size optimization. I don't mind dropping this add adjusting buf > with '+= procfdbufsize - 1' in the callee. Yes, making it obvious what's going on is nice too. Actually it would be even nicer if we could use a compound literal inside the macro as the buffer, but that would pessimize with unnecessary initialization and eliminate a lot of the code-size benefit, I think. > > Here using the return value directly is nice but at some other call > > points might we need to introduce a pointer variable to store the > > pointer returned? I haven't checked yet. > > Yes, I went through the call sites and they are all easy to adjust; I think a > couple needed a pointer, like you said. OK. Rich