Date: Sat, 5 Mar 2016 00:56:05 -0500 From: Rich Felker <dalias@...c.org> To: musl@...ts.openwall.com Subject: Re: [RFC PATCH] micro-optimize __procfdname 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
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