Re: [patch] reload1.c: Very minor speedup.

[Mike Stump <mrs at apple dot com>]

On Friday, February 6, 2004, at 02:34  PM, Paolo Carlini wrote:
> > Attached is a patch to micro-optimize the reset of can_eliminate in
> > reload1.c.
>
> > - if (ep->from_rtx == x && ep->can_eliminate)
> > + if (ep->from_rtx == x)
> > ep->can_eliminate = 0;
>
> if you have got two spare minutes, could you possibly explain a bit?
>
> I mean, it's because the cost of a test (&& ep->can_eliminate) is
> comparable to that of an assignment (ep->can_eliminate = 0), never
> much smaller? Is it true on every architecture?

Smaller?  He did say speedup, not smaller!

Modern machines are fascinating.  You really want to grab a high level 
tool like Shark (Mac developer tool) and run it on your favorite code 
and then take a look at the results.  Once you train up for a few days, 
you'll discover just what you've been missing.  Neat results like, out 
of these 133,000 instructions in this one file, 4 of them, no more, no 
less account for 90% of the time.

The change assumes a load store unit that isn't bandwidth limited and a 
conditional branch that will be slow, mispredicted.  Seems like it 
might be the right choice, though, of course, I'd almost want to fire 
it up and watch it, but I think this is so far down in the noise that 
I'll pass up the opportunity.

[ pause ]

Ok, so I tried this, and found:

0.873 for V1 and 0.876 for V2:

struct S {
  int pad[3];
  int can_eliminate;
  int pad2[4];
  int rtx;
  int tortx;
} er[1000];

main() {
  int i, j;
  struct S *s = er;

  for (i= 0; i<100000; ++i) {
    for (s = er; s<&er[1000]; ++s) {
#ifdef V1
      if (s->rtx == 12 && s->can_eliminate)
#else
      if (s->rtx == 12)
#endif
        s->can_eliminate = 0;
    }
  }
}

and using a much larger er that might get us out of cache more:

V1: 0.448  V2: 0.459

So, the proposed patch looks like on my machine, it is slower.  Kazu, 
did you measure a speedup?  On what machine?  One the whole, changing 
code for a near zero win, is probably not very interesting, otot, V2 is 
12 bytes smaller, which tilts towards V2, plus it compiles faster!  :-)