Re: Allocate variables within code blocks.

From	Ben Bacarisse <ben.usenet@bsb.me.uk>
Newsgroups	comp.lang.c
Subject	Re: Allocate variables within code blocks.
Date	2018-04-09 12:17 +0100
Organization	A noiseless patient Spider
Message-ID	<87muyc6341.fsf@bsb.me.uk> (permalink)
References	<pacshc$18uv$1@gioia.aioe.org> <kfnlgdxe23l.fsf@x-alumni2.alumni.caltech.edu> <paeo53$a2k$1@gioia.aioe.org>

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"itsme.susila" <itsme.susila@gmx.com> writes:
<snip>
> In fact, chess programmers are a lot who talk about speed. I did go to
> talkchess.com (now no more) where there were much talk about coding
> practice. A simple example that novice may not know: use integer(4
> bytes) data rather than short(2 bytes)...chess programming is almost
> all with integer operations and there are overheads for modern cpu to
> operate on char and short types.

I don't think that's good advice.  It may be true in some cases (it
certainly used to be on some CPUs) but it may not be true in others.

I used to be quite good at guessing what code changes would improve
performance, but that was 25 years ago.  With every new generation of
CPU, and every new compiler release, I have become used being surprised
and I've thrown out almost every "rule of thumb" that I used to use.

For example, I just tried a nested loop calculation using short, int and
long (long being 64-bit here) and the short version was faster than the
(32-bit) int version.  But that was un-optimised.  With gcc -O2 I can't
detect any difference between short and int.  This was designed to
avoid cache issues so it's possible that short might even be faster in
some cases where the memory saving turns out to be helpful.

And if someone now posts that they've also done a quick test and int was
faster than short I would not be at all surprised.  I have, in the past,
been so frequently surprised by what clever chips and compilers do that
I am rarely surprised any more.

> In fact the experts would also do profiling (there is compiler auto
> profiling optimizations too) and then to optimize at the particular
> spots; but we already know it i recursive search which is the critical
> function.

That sounds odd.  To me, a chess playing program /is/ recursive search
with very little else!  It sounds like saying that the program is the
critical part of the program.

When I wrote a chess program, it was the function that evaluated
positions that was the bottle-neck.  If I remember correctly, there was
some measurable component from the function that generated valid moves,
but I don't remember it being worth working on.  And despite knowing the
most expensive function, the best improvements came from re-ordering the
move list because of the way the tree pruning worked.

(I suspect the design of chess programs is more sophisticated these
days.  Even for the time, mine was crude, having no book of openings and
no special end-game mode.)

> But I think it is correct to start off with what you say,
> good choice of data structures, etc...

Yes.  For macro-optimisation, think data structures and algorithms (and
measure).  For micro-optimisation, measure (don't try to guess).

-- 
Ben.

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Thread

Allocate variables within code blocks. "itsme.susila" <itsme.susila@gmx.com> - 2018-04-08 18:56 +0800
  Re: Allocate variables within code blocks. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2018-04-08 12:29 +0100
    Re: Allocate variables within code blocks. Steven Petruzzellis <frelwizzen@gmail.com> - 2018-04-08 05:06 -0700
    Re: Allocate variables within code blocks. Spiros Bousbouras <spibou@gmail.com> - 2018-04-08 13:00 +0000
      Re: Allocate variables within code blocks. Steven Petruzzellis <frelwizzen@gmail.com> - 2018-04-08 08:14 -0700
    Re: Allocate variables within code blocks. GOTHIER Nathan <nathan.gothier@gmail.com> - 2018-04-08 16:37 +0200
    Re: Allocate variables within code blocks. Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2018-04-08 08:53 -0600
      Re: Allocate variables within code blocks. bartc <bc@freeuk.com> - 2018-04-08 16:32 +0100
        Re: Allocate variables within code blocks. Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2018-04-08 15:19 -0600
          Re: Allocate variables within code blocks. Tim Rentsch <txr@alumni.caltech.edu> - 2018-04-09 07:59 -0700
            Re: Allocate variables within code blocks. Tim Rentsch <txr@alumni.caltech.edu> - 2018-04-11 06:29 -0700
              Re: Allocate variables within code blocks. bartc <bc@freeuk.com> - 2018-04-11 15:58 +0100
                Re: Allocate variables within code blocks. Tim Rentsch <txr@alumni.caltech.edu> - 2018-04-11 11:28 -0700
                Re: Allocate variables within code blocks. Steven Petruzzellis <frelwizzen@gmail.com> - 2018-04-13 00:42 -0700
        Re: Allocate variables within code blocks. Jorgen Grahn <grahn+nntp@snipabacken.se> - 2018-04-11 10:25 +0000
    Re: Allocate variables within code blocks. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2018-04-08 19:27 +0100
  Re: Allocate variables within code blocks. James Kuyper <jameskuyper@verizon.net> - 2018-04-08 07:54 -0400
    Re: Allocate variables within code blocks. Spiros Bousbouras <spibou@gmail.com> - 2018-04-08 12:50 +0000
  Re: Allocate variables within code blocks. bartc <bc@freeuk.com> - 2018-04-08 13:42 +0100
  Re: Allocate variables within code blocks. "itsme.susila" <itsme.susila@gmx.com> - 2018-04-08 22:19 +0800
    Re: Allocate variables within code blocks. "itsme.susila" <itsme.susila@gmx.com> - 2018-04-08 22:27 +0800
      Re: Allocate variables within code blocks. Lew Pitcher <lew.pitcher@digitalfreehold.ca> - 2018-04-08 10:42 -0400
    Re: Allocate variables within code blocks. bartc <bc@freeuk.com> - 2018-04-08 15:37 +0100
    Re: Allocate variables within code blocks. David Brown <david.brown@hesbynett.no> - 2018-04-08 16:45 +0200
      Re: Allocate variables within code blocks. "itsme.susila" <itsme.susila@gmx.com> - 2018-04-08 22:55 +0800
        Re: Allocate variables within code blocks. GOTHIER Nathan <nathan.gothier@gmail.com> - 2018-04-08 17:52 +0200
    Re: Allocate variables within code blocks. Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2018-04-08 09:00 -0600
  Re: Allocate variables within code blocks. Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2018-04-08 08:51 -0600
  Re: Allocate variables within code blocks. Tim Rentsch <txr@alumni.caltech.edu> - 2018-04-08 15:59 -0700
    Re: Allocate variables within code blocks. "itsme.susila" <itsme.susila@gmx.com> - 2018-04-09 11:54 +0800
      Re: Allocate variables within code blocks. David Brown <david.brown@hesbynett.no> - 2018-04-09 09:31 +0200
      Re: Allocate variables within code blocks. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2018-04-09 12:17 +0100
        Re: Allocate variables within code blocks. "itsme.susila" <itsme.susila@gmx.com> - 2018-04-10 00:27 +0800
          Re: Allocate variables within code blocks. Steven Petruzzellis <frelwizzen@gmail.com> - 2018-04-09 09:33 -0700
          Re: Allocate variables within code blocks. Spiros Bousbouras <spibou@gmail.com> - 2018-04-09 16:52 +0000
            Re: Allocate variables within code blocks. "itsme.susila" <itsme.susila@gmx.com> - 2018-04-10 01:17 +0800
          Re: Allocate variables within code blocks. scott@slp53.sl.home (Scott Lurndal) - 2018-04-09 18:03 +0000
            Re: Allocate variables within code blocks. supercat@casperkitty.com - 2018-04-09 13:39 -0700
              Re: Allocate variables within code blocks. scott@slp53.sl.home (Scott Lurndal) - 2018-04-09 20:53 +0000
      Re: Allocate variables within code blocks. bartc <bc@freeuk.com> - 2018-04-09 12:23 +0100
        Re: Allocate variables within code blocks. "itsme.susila" <itsme.susila@gmx.com> - 2018-04-10 00:39 +0800
      Re: Allocate variables within code blocks. Tim Rentsch <txr@alumni.caltech.edu> - 2018-04-09 08:09 -0700
    Re: Allocate variables within code blocks. Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2018-04-09 17:07 -0600
      Re: Allocate variables within code blocks. supercat@casperkitty.com - 2018-04-09 16:28 -0700
      Re: Allocate variables within code blocks. Steven Petruzzellis <frelwizzen@gmail.com> - 2018-04-10 00:46 -0700
      Re: Allocate variables within code blocks. Tim Rentsch <txr@alumni.caltech.edu> - 2018-04-11 07:10 -0700

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