[antlr-interest] simple tree transformation

[mzukowski at yci.com]

I made an error in this example.  There is always the possibility of an if
without an else.  So this:
>         |       #(! ifst:"if"
>                     e:expr s1:statement  
>                     ( el:"else" s2:statement )?
>                  ) {##=#(ifst, e, s2, el, s1);}

Should have the action in the else closure:

      |       #(! ifst:"if"
                  e:expr s1:statement  
                  ( el:"else" s2:statement {##=#(ifst, e, s2, el, s1);})?
               ) 

Monty

www.codetransform.com

> -----Original Message-----
> From: mzukowski at yci.com [mailto:mzukowski at yci.com]
> Sent: Tuesday, May 07, 2002 8:13 AM
> To: antlr-interest at yahoogroups.com
> Subject: [antlr-interest] simple tree transformation
> 
> 
> > From: Sebastian Danicic [mailto:mas01sd at gold.ac.uk]
> > Would it be possible for you to show me a little example of a simple
> > program transformation?
> > 
> > A good one would be something that converts:
> > 
> > if (B) s1 else s2 => if (B) s2 else s1
> > 
> > or something equally simple. Just so I understand how to 
> > manipulate the AST.
> 
> I would make an antlr "grammar subclass" of GnuCTreeGrammar 
> with just this
> method in it, with the "if" section modified as below.  The 
> subclass must be
> marked with the "buildAST=true" option.  
> 
> The entire statementBody method must be here, you can't just 
> subclass an
> alternative.  Of course if you had a need to, you could refactor
> GnuCTreeGrammar into one more suited to your needs.  For instance
> statementBody could reference a few rules, one for plain 
> statements, one for
> iterations, one for jumps, one for labeled statments, one for 
> selection
> statements.  So if you were doing lots of different passes 
> that just looked
> at iteration statements you would only need to subclass the
> iterationStatement rule.  
> 
> For the "if" alternative I turned off the automatic tree 
> building since I
> want to construct the tree in a different order.  If I were 
> simply modifying
> the nodes in place (changing their text) then I would let 
> antlr build the
> tree.  Once a tree is built, however, it is hard to swap 
> those nodes around
> because nodes contain within them links to their firstChild 
> and nextSibling.
> A common mistake is to let antlr build the tree and then swap 
> two nodes
> accidently creating a circular structure.
> 
> Then I labeled all the elements I want for my tree.  Note 
> that antlr does
> not have any positional notation for nodes--you can't say #1, #2, etc.
> Finally I construct the tree as I want it, in this case with 
> the statements
> switched. 
> 
> statementBody
>         :       SEMI                    // Empty statements
> 
>         |       compoundStatement       // Group of statements
> 
>         |       #(NStatementExpr expr)                    // 
> Expressions
> 
> // Iteration statements:
> 
>         |       #( "while" expr statement )
>         |       #( "do" statement expr )
>         |       #( "for"
>                 expr expr expr
>                 statement
>                 )
> 
> 
> // Jump statements:
> 
>         |       #( "goto" expr )
>         |       "continue" 
>         |       "break"
>         |       #( "return" ( expr )? )
> 
> 
> // Labeled statements:
>         |       #( NLabel ID (statement)? )
>         |       #( "case" expr (statement)? )
>         |       #( "default" (statement)? )
> 
> 
> 
> // Selection statements:
> 
>         |       #(! ifst:"if"
>                     e:expr s1:statement  
>                     ( el:"else" s2:statement )?
>                  ) {##=#(ifst, e, s2, el, s1);}
>         |       #( "switch" expr statement )
> 
> 
> 
>         ;
> 
> Monty
> www.codetransform.com
> 
> > -----Original Message-----
> > From: Sebastian Danicic [mailto:mas01sd at gold.ac.uk]
> > Sent: Tuesday, May 07, 2002 1:53 AM
> > To: mzukowski at bco.com
> > Subject: cgram
> > 
> > 
> > Dear Monty,
> > 
> > I'm a member of the VASTT (http://www.brunel.ac.uk/~csstmmh2/vast/).
> > We would like to write a slicer and other program 
> > transformations for C
> > using your system.
> > 
> > Would it be possible for you to show me a little example of a simple
> > program transformation.
> > 
> > A good one would be something that converts:
> > 
> > if (B) s1 else s2 => if (B) s2 else s1
> > 
> > or something equally simple. Just so I understand how to 
> > manipulate the
> > AST.
> > 
> > Thank you very much in advance for your help.
> > 
> > Sebastian Danicic (http://www.mcs.gold.ac.uk/~mas01sd/)
> > 
> > 
> 
>  
> 
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