Re: [RFC PATCH] micro-optimize __procfdname

[Rich Felker <dalias@libc.org>]

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