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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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