Message-Id: <3.0.32.19990814040832.01181ec0@pop.xs4all.nl>
X-Sender: diep AT pop DOT xs4all DOT nl
X-Mailer: Windows Eudora Pro Version 3.0 (32)
Date: Sat, 14 Aug 1999 04:08:34 +0100
To: pgcc AT delorie DOT com
From: Vincent Diepeveen <diep AT xs4all DOT nl>
Subject: Re: optimizing for k6
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Reply-To: pgcc AT delorie DOT com

At 06:34 PM 8/13/99 +0200, you wrote:
>> At 05:45 PM 8/12/99 +0200, you wrote:
>> >> There is a very easy way of optimizing for K6,
>> >> just rewrite everything in 8 bits and you're 2 times faster.
>> >> 
>> >Why? The 8 bit arithmetic is issues to X pipe only, so it ought to be 2
times
>> >slower... many 8 bit insns have longer decoding latencies in 8 bit
versions.
>> 
>> K6 processor 'cracks' 32 bits slower than 8 bits.
>> 8 bits translates in shorter RISC instructions
>Strange... according to docs, same instructions seems to be generated
>except alu is replaced by alux.
>I was even doing experiment with replacing 8 bit arithmetics with 32 bit
>(in same way as gcc does for 16 bits) in order to avoid x pipe dependencies
>and it was a win in my benchmarks.

Don't use GCC for that experiment. GCC is the worst in producing code
using 8 bits dataformat. 

When i had still some 8 bits datastructure left (so not
code but datastructure only, tables containing values 0..255) 
GCC was over 10% slower than MSVC. 

I then rewrote my last datastructures to 32 bits, and my program got 
6% slower under MSVC. Yet the speed difference gcc <==> MSVC was hardly 7%
then.

My conclusion was that gcc optimized my code relatively
worse when there were 8 bits things to do.

But about the speed difference. Programming in assembler you clearly
notice the speed difference. benchmark reference: rebel program. www.rebel.nl

Although my program was way faster when i had a combined 8 bits/32 bits
datastructure (so 8 bits code too), i chose for getting 32 bits completely
in order to get rid of possible casting faults from my side. Let's
keep it simple&easy...

I still don't regret that decision. Especially not if i look to
how many KBs my source code is growing every month.

>Honza
 
>> >Honza
>> >> Greetings,
>> >> Vincent
>> >> 
>> >> /At 11:49 AM 8/7/99 +0200, you wrote:
>> >> >Henrik Berglund SdU wrote:
>> >> >> 
>> >> >>
ftp://ftp.sinica.edu.tw/pub/doc/cpu/www.amd.com/K6/k6docs/pdf/21828a.pdf
>> >> >> 
>> >> >>
>> >>
>>
-----------------------------------------------------------------------------
>> >> >> Henrik DOT Berglund AT mds DOT mdh DOT se
>> >> >> http://www.mds.mdh.se/~adb94hbd/
>> >> >
>> >> >This is a long known document, it does some help in optimizing. But the
>> >> >information is just too incomplete to get really good optimizations.
>> >> >
>> >> >There is also a lot of mistakes in that document. I had a little
>> >> >discussion
>> >> >with AMD technical support, but they did not help :-(
>> >> >AMD Technical Support wrote:
>> >> >> 
>> >> >> >Return-Path: <w DOT formann AT neuss DOT netsurf DOT de>
>> >> >> >Sender: wolfi AT neuss DOT netsurf DOT de
>> >> >> >Date: Fri, 12 Mar 1999 19:10:15 +0100
>> >> >> >From: Wolfgang Formann <w DOT formann AT neuss DOT netsurf DOT de>
>> >> >> >To: AMD Technical Support <blikefet AT pedigree DOT amd DOT com>
>> >> >> >Subject: Re: Some question to your literature, maybe a typo?
>> >> >> >References: <3 DOT 0 DOT 32 DOT 19990303153034 DOT 0074931c AT pedigree DOT amd DOT com>
>> >> >> >
>> >> >> 
>> >> >> Hi,
>> >> >> 
>> >> >> it is the last update of the document. I think you must try it.
>> >> >> 
>> >> >> Kind regards
>> >> >> 
>> >> >> Bernard
>> >> >> 
>> >> >> >AMD Technical Support wrote:
>> >> >> >>
>> >> >> >> >Return-Path: <euro DOT lit AT amd DOT com>
>> >> >> >> >X-Sender: support2 AT pedigree
>> >> >> >> >Date: Thu, 25 Feb 1999 06:39:16 +0100
>> >> >> >> >To: blikefet AT pedigree DOT amd DOT com
>> >> >> >> >From: Wolfgang Formann <w DOT formann AT neuss DOT netsurf DOT de> (by way of
CPA
>> >> <euro DOT lit AT amd DOT com>)
>> >> >> >> >Subject: Some question to your literature, maybe a typo?
>> >> >> >> >
>> >> >> >> >I just downloaded the document
>> >> http://www.amd.com/K6/k6docs/pdf/21828a.pdf.
>> >> >> >> >The table in Chaper 4, Pages 37 to 40 says, that all the shift
>> >> operations
>> >> >> >> >like SHIFT mreg16/32,imm8; SHIFT mreg16/32, 1; SHIFT
mreg16/32, CL;
>> >> where
>> >> >> >> >SHIFT can be replaced by SAR, SHL/SAL and SHR, are executed as
>> >> RISC86(tm)
>> >> >> >> >Opcode alu. This RISC86(tm) operation is explained on page 24 as
>> >> >> >> >`alu - either of the integer execution units`.
>> >> >> >> >
>> >> >> >> >Whereas in chapter 3 on page 12, this document lists some (all?)
>> >> operations
>> >> >> >> >which can be performed in the Integer Y execution unit. In the
>> list of
>> >> >> >> >operations '(ADD, AND, CMP, OR, SUB and XOR)' there is none of
the
>> >> SHIFT's
>> >> >> >> >mentioned.
>> >> >> >> >
>> >> >> >> >By trying it out (I think) I found that chapter 3 is right and
the
>> >> table
>> >> >> >> >in chapter 4 has typos.
>> >> >> >> >
>> >> >> >> >My question: Is there any updated version of this document
>> available or
>> >> >> >> >do I have to try out all the other opcodes not listed in chapter
>> 3, but
>> >> >> >> >marked as 'alu' in the table in chapter 4 (like mov, movzx)?
>> >> >> >> >
>> >> >> >> >Thank you
>> >> >> >>
>> >> >> >> Hi,
>> >> >> >>
>> >> >> >> the latest version of the document is on the our webside.
>> >> >> >
>> >> >> >so, it still seems to have different information on the same
>> >> instruction :-(
>> >> >> >
>> >> >> >Is there any additional information available, not shown on your web
>> page?
>> >> >> >
>> >> >> >Thanks again!
>> >> >> >
>> >> >> >>
>> >> >> >> Kind regards
>> >> >> >> Bernard Likefett
>> >> >> >> AMD Technical Support
>> >> >> >
>> >> >> >
>> >> >> Bernard Likefett
>> >> >> AMD Technical Support
>> >> >> 
>> >> >> Please included all previous emails
>> >> >> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>> >> >> Advanced Micro Devices _______
>> >> >> AMD House \____ | Advanced
>> >> >> Frimley Business Park /| | | Micro
>> >> >> Frimley, Camberley | |___| | Devices
>> >> >> Surrey |____/ \|
>> >> >> GU16 5SL
>> >> >> United Kingdom
>> >> >> 
>> >> >> EMail id euro DOT tech AT amd DOT com Our Web site is http://www.amd.com
>> >> >> Phone +44 (0)1276 803299 Fax +44 (0)1276 803298
>> >> >> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>> >> >
>> >> >Another thing in that manual is the nice table labeled 'Instruction
>> >> >Dispatch and Execution Timing' starting at page 35. Just a few
>> >> >questions:
>> >> >How many internal cycles do all these vector operations take?
>> >> >What internal execution units are used?
>> >> >
>> >> >Well, there is no answer, so you have to try them out. The only thing
>> >> >you can be sure of, is that you should always use opcodes which can get
>> >> >decoded in parallel, these are the ones marked with 'short' since it
>> >> >seems that the bottleneck of that CPU is the decoder.
>> >> >
>> >> >The next thing is the nice tables in the chapter labeled 'Code Sample
>> >> >Analysis'. Did you really understand them? I tried to optimize some
>> >> >real code and took these tables as input, but I failed :-( My processor
>> >> >seems to behave very different. I did not find out what was wrong.
>> >> >So it seems to me, that a lot of information in this document is
>> >> >only for marketing purposes, there are too few details and too many
>> >> >wrong informations to really help to optimize the code.
>> >> >
>> >> >Wolfgang
>> >> >
>> >> >
>> >
>> >-- 
>> >                       OK. Lets make a signature file.
>>
>+-------------------------------------------------------------------------+
>> >|        Jan Hubicka (Jan Hubi\v{c}ka in TeX) hubicka AT freesoft DOT cz
  |
>> >|         Czech free software foundation: http://www.freesoft.cz
  |
>> >|AA project - the new way for computer graphics -
http://www.ta.jcu.cz/aa |
>> >|  homepage: http://www.paru.cas.cz/~hubicka/, games koules, Xonix,
fast  |
>> >|  fractal zoomer XaoS, index of Czech GNU/Linux/UN*X documentation
etc.  | 
>>
>+-------------------------------------------------------------------------+
>> >
>> >
>
>-- 
>                       OK. Lets make a signature file.
>+-------------------------------------------------------------------------+
>|        Jan Hubicka (Jan Hubi\v{c}ka in TeX) hubicka AT freesoft DOT cz         |
>|         Czech free software foundation: http://www.freesoft.cz          |
>|AA project - the new way for computer graphics - http://www.ta.jcu.cz/aa |
>|  homepage: http://www.paru.cas.cz/~hubicka/, games koules, Xonix, fast  |
>|  fractal zoomer XaoS, index of Czech GNU/Linux/UN*X documentation etc.  | 
>+-------------------------------------------------------------------------+
>
>