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From: Paul Rubin <no.email@nospam.invalid>
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Subject: Re: Stack Operator Symbols
Date: Sat, 29 Oct 2016 14:21:54 -0700
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Andrew Haley <andrew29@littlepinkcloud.invalid> writes:
> first I'd try to understand how I got into a situation which needed 3DUP.

I implemented the Tak function which is a widely used Lisp benchmark:

  https://en.wikipedia.org/wiki/Tak_(function)

I wrote a Gforth version with and without locals:

  : tak-locals { x y z -- n}
      y x >= if z else
          x 1- y z recurse
          y 1- z x recurse
          z 1- x y recurse
          recurse then ; 

  : 3dup ( a b c -- a b c a b c ) >r 2dup r@ -rot r> ;
  : tak ( x y z -- n ) 
      -rot 2dup <=
      if
          2drop ( z ) 
      else ( z x y )
          3dup rot 1- -rot recurse >r     \ tak (1- z) x y
          3dup 1- -rot recurse >r         \ tak (1- y) z x
          -rot 1- -rot recurse            \ tak (1- x) y z
          r> r> recurse
      then ;

  : run 18 12 6 tak cr . cr  ;

Using 3dup seems natural here.  Writing an asm version defeats the idea
of benchmarking Forth with this function.