Re: Bit Swizzling

From	Chris <xxx.syseng.yyy@gfsys.co.uk>
Newsgroups	comp.arch, comp.compilers
Subject	Re: Bit Swizzling
Date	2020-09-06 17:48 +0100
Organization	Aioe.org NNTP Server
Message-ID	<20-09-019@comp.compilers> (permalink)
References	<riumcj$3j9$1@dont-email.me> <rivvah$1neg$2@gioia.aioe.org> <20-09-014@comp.compilers> <20-09-016@comp.compilers>

Cross-posted to 2 groups.

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On 09/05/20 19:50, John Levine wrote:
> In article<rivvah$1neg$2@gioia.aioe.org>,
>>> -----
>>> Are there any algorithms which take a known-at-compile-time sequence
>>> of bitwise operations on an 8-bit to 64-bit quantity, and optimize
>>> them down to their minimal set of operations?
>
>> Why not just use a lookup table ?. Minimum ops and fast...
>
> Assuming you're looking for something you can implement in logic
> rather than by table lookup, it sounds like a set of Karnaugh maps.

Unless this is just an intellectual exercise for the fun of it, an
engineer would choose the minimal design at lowest cost to
satisfy the requirements. Table methods don't have to be in
software as a single eprom or gate array could do it in hardware.
8 inputs to address lines, then 8 bits of output, scale as required,
so why make life more difficult than necessary ?.

Old project here, where a programmer spent nearly 5 pages of 6800
asm to translate an input connect pin layout to that required for
the internal functions. Code was impenetrable, so substituted a
256 byte lookup table. Less space that the code and easily
modified for new requirements...

Chris
[I think the question is whether there is a way to mechanically
generate a version of the opaque assembler. -John]

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Bit swizzling "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2020-09-05 12:05 -0400
  Re: Bit swizzling Hans-Peter Diettrich <DrDiettrich1@netscape.net> - 2020-09-05 19:38 +0200
  Re: Bit Swizzling "John Levine" <johnl@taugh.com> - 2020-09-05 18:50 +0000
    Re: Bit Swizzling Chris <xxx.syseng.yyy@gfsys.co.uk> - 2020-09-06 17:48 +0100
  Re: Bit swizzling Kaz Kylheku <793-849-0957@kylheku.com> - 2020-09-05 20:15 +0000
  Re: Bit swizzling "davidl...@gmail.com" <davidlovemore@gmail.com> - 2020-09-06 00:31 -0700
  Re: Bit swizzling Martin Ward <martin@gkc.org.uk> - 2020-09-07 12:08 +0100
    Re: Bit swizzling Tom Crick <tomcrick@gmail.com> - 2020-09-08 09:35 +0100
  Re: Bit swizzling "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2020-09-07 10:55 -0400
    Re: Bit swizzling Hans-Peter Diettrich <DrDiettrich1@netscape.net> - 2020-09-08 01:45 +0200
  Re: Bit swizzling gah4 <gah4@u.washington.edu> - 2020-09-10 10:34 -0700
    Re: Bit swizzling "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2020-09-10 17:13 -0400

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