Re: Bit Swizzling

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From	Chris <xxx.syseng.yyy@gfsys.co.uk>
Newsgroups	comp.arch, comp.compilers
Subject	Re: Bit Swizzling
Date	Sun, 06 Sep 2020 17:48:58 +0100
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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@nospicedham.gmail.com> - 2020-09-04 20:33 -0400
  Re: Bit Swizzling MitchAlsup <MitchAlsup@aol.com> - 2020-09-04 18:26 -0700
    Re: Bit Swizzling MitchAlsup <MitchAlsup@aol.com> - 2020-09-04 18:27 -0700
  Re: Bit Swizzling Joshua Landau <joshua.landau.ws@gmail.com> - 2020-09-04 20:32 -0700
    Re: Bit Swizzling "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2020-09-05 09:16 -0400
  Re: Bit Swizzling Bart <bc@nospicedham.freeuk.com> - 2020-09-05 12:33 +0100
  Re: Bit Swizzling Chris <xxx.syseng.yyy@gfsys.co.uk> - 2020-09-05 13:12 +0100
    Re: Bit Swizzling "John Levine" <johnl@taugh.com> - 2020-09-05 18:50 +0000
      Re: Bit Swizzling "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2020-09-06 11:01 -0400
      Re: Bit Swizzling Chris <xxx.syseng.yyy@gfsys.co.uk> - 2020-09-06 17:48 +0100
  Re: Bit Swizzling olcott <NoOne@nospicedham.NoWhere.com> - 2020-09-08 11:25 -0500
    Re: Bit Swizzling olcott <NoOne@nospicedham.NoWhere.com> - 2020-09-08 11:50 -0500

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