Click above to get retro games delivered to your door ever month!

Quality of the recovered code


 The code Rescue5 recovers is similar to a formatted version of
 the .PPO code emitted when you specify the /p switch with the
 CA-Clipper compiler, except that the code recovered has
 undergone compiler optimisation.

 This means that the code recovered is procedure instead of
 command based (all CA-Clipper commands, except flow-control
 statements, are pre-processed to procedure calls).  This has
 the following implications:

 1) Some CA-Clipper "internals" may be referenced
    directly in the source code.  For example, an @ .. GET
    command becomes a sequence of function calls including a
    call to _GET_(), the QUIT command becomes a call the _QUIT()
    function etc.  This has no effect on the compatibility of
    the code with the original system.

 2) Manifest constants are lost (K_F2 becomes -1 etc.).

 3) The CA-Clipper optimiser will have removed some redundant
    code.  In most cases this makes statements more logical:

    Original PRG code
    -----------------
    USE clients

    PPO output (CA-Clipper /p switch)
    ---------------------------------
    dbusearea(.f.,,"clients",,if(.f. .or. .f., !.f., nil), .f.)

    The code Rescue5 recovered
    --------------------------
    dbusearea(.f.,NIL,"clients",NIL,NIL,.f.)

    In the above example, if(.f. .or. .f., !.f., nil) has been
    resolved by the CA-Clipper optimiser to the constant nil.


 In addition to the above, the following code statements may be
 recovered in a slightly different form to that originally
 written.  In all cases this is because the CA-Clipper compiler
 has produced p- code that can be decompiled in more than one
 way.  Note that the code recovered by Rescue5 is functionally
 identical to the original:

 1) DO CASE may become IF ELSEIF

    Under some circumstances it is impossible to
    differentiate between IF..ELSEIF..ENDIF and DO
    CASE..ENDCASE, since both sometimes compile to the same
    p-code.  Rescue5 generates an IF construct.  Since both
    constructs have exactly the same effect this does not change
    the functionality of the program.

 2) Unary increment and decrement

    Where a unary increment or decrement operator appears on a
    line by itself, CA-Clipper produces p-code for the "pre"
    version, even if the "post" version was used in the original
    code (nCount++ on a line by itself will become ++nCount).
    Outside compound statements the two expressions are
    arithmetically equivalent.

 3) Declared but unreferenced static variables become 'file-wide'

    Rescue5 determines whether a static variable is file-wide
    or function-scoped according to its actual usage.  Therefore
    a function that declared a static variable but never
    actually references it cannot 'claim ownership' of the
    variable.

 4) Logically equivalent IF statements

    CA-Clipper produces the same p-code for:

      if x    
        if y  
         :    
        endif 
      endif   

    and

      if x .and. y
         :        
      endif       

    Since the two are functionally equivalent, Rescue5 generates
    the second, less verbose version.

 5) Code grouping

    A technique that is widely used, code grouping is the
    placing of a comma-separated list of expressions within
    parentheses:

      cName := (cust->(dbsetorder(2), dbseek('123')), cust->name)

    The expressions are evaluated from left to right, the result
    of the entire group being the value of the last expression.
    Thus the expression above is the equivalent of:

      cust->(dbsetorder(2)) 
      cust->(dbseek('123')) 
      cName = cust->name    

    Under rare circumstances Rescue5 may encounter p-code that
    is ambiguous to the extent that the original code might have
    been grouped or might not.  In these cases Rescue5 will
    not group the recovered code.

    It is possible (although very unusual) for Rescue not to group
    code that was originally grouped.  The /c switch forces Rescue5
    to try to group ambiguous code, eliminating errors introduced by
    the default grouping strategy.  /c should be used with caution,
    since it can introduce other problems by causing Rescue to attempt
    to group code that was not originally grouped.

 6) Macro in a code block becomes a macro-ed code block

    Under some circumstances the CA-Clipper compiler 'converts'
    a code block into a macro:

       eval({|| memvar->&"str"})

    Becomes:

       eval(&"{|| memvar->" + chr(34) + "str" + chr(34) + "}")

    The two expressions are functionally identical.

 7) 'Phantom' procedures

    If the original system was compiled without the /n switch,
    CA-Clipper will have automatically declared a procedure for
    each of the source files.

    These are emitted by Rescue5 as procedures with no body (no
    code between the procedure declaration and the return
    statement).

    It is possible for CA-Clipper to create procedures with
    illegal names in this manner (CA- Clipper has created a
    procedure 'internally' that it could not normally compile).