Commits on Aug 3, 2023

1. Add new simplification rule for invariant loop parameters. …

   This was suggested by Cosmin to address some of the code produced by
   AD.
   
   The idea is that for a loop of the form
   
   loop p = x ...
     ...stms...
     in res
   
   we construct and simplify the body
   
     let p = x
     ...stms...
     in res
   
   and if that simplifies to 'x', then we conclude that the loop
   parameter 'p' must be invariant to the loop and simply bind it (and
   the loop result) to 'x'.
   
   Complication: for multi-parameter loops, we must also check that
   the *original* computation of 'res' does *only* depends on other
   invariant loop parameters.
   
   Currently we do this only for loops that have a constant as one of
   their initial loop parameter values.
   
   The main downside of this rule is that doing recursive simplification
   is quite expensive.  Especially after sequentialisation, pretty much
   every 'reduce' will have been turned into a loop that triggers this
   rule (although the rule itself will fail in most cases, after doing
   the simplification).  Therefore I'm a bit hesitant to enable it as is.
   Sure, the Futhark compiler is slow and it was never meant to be fast,
   but it is still quite easy for the compiler to become *uselessly slow*
   if we are not careful.  E.g. on OptionPricing, this rule itself makes
   compilation 10% slower (and does not actually optimise anything - this
   is purely the cost of failing checks).

   

   athas committed Aug 3, 2023
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