Dead Variable Elimination

The dead-var-elim component of the Stratego optimizer eliminates variables from a strategy expression if they are not used in a build. For example, in the expression

  { x, y, z :
    ?F(x, y)
    ; !y
    ; ?G(z)
    ; !H(z, z)
  }

the variable z is used in the final build (not dead), and y is used in the build needed for the match with G (not dead). However, x is dead since it does not contribute to the final result. Dead variable elimination removes dead variables by

• eliminating them from the scope, and
• replacing occurrences in matches with a wildcard

Thus, the example reduces to

  { y, z :
    ?F(_, y)
    ; !y
    ; ?G(z)
    ; !H(z, z)
  }

Variable only matches (?x) with dead variables are turned into wildcard matches (?_) and can be cleaned up by a subsequent simplification.

-- EelcoVisser - 17 Aug 2003