Date: Sat, 5 Mar 2016 08:42:16 +0300 (MSK) From: Alexander Monakov <amonakov@...ras.ru> To: musl@...ts.openwall.com Subject: Re: [RFC PATCH] micro-optimize __procfdname 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. > > +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. > > +#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 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. > 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. Alexander
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