[ast-optimizer-branch] Simplification of Simple grammar

[Sebastian Pop <m1sp at csc dot liv dot ac dot uk>]

Hi, 

I thought a little about what Simple representation can do 
for simplifying the work on goto elimination : and I think 
that it could be possible to simplify a little more the 
Simple grammar.

What I need is just transform all switches in nested if statements. 
This way switch_BREAK_STMTs are eliminated and there are no more 
problems when the GOTO_STMT is in a SWITCH_STMT.

Another simplification is to allow a single loop structure :
for the moment I treat all loop structures but that multiplies by 3 
the amount of code needed to simplify correctly goto, breaks and continues.

A last change could be to have the ELSE_CLAUSE by default on all 
IF_STMTs.  This way there's no need to call again the function 
tree_build_scope everytime we're on an IF_STMT node and we want to be sure 
that this node has an ELSE_CLAUSE...

I think that the scope of Simple is to simplify the analysis and transformations
we will write on Simple.  Since we don't lose information by transforming
all switches in nested IF_STMTs, or transforming all loop structures 
into DO_STMTs, I propose to simplify Simple grammar a little. 


Original grammar from tree-simple.c 

      stmt
	      : compstmt
	      | expr ';'
	      | IF '(' condexpr ')' stmt
	      | IF '(' condexpr ')' stmt ELSE stmt
	      | WHILE '(' condexpr ')' stmt
	      | DO stmt WHILE '(' condexpr ')'
	      | FOR '('exprseq ';' condexpr ';'exprseq ')' stmt
	      | SWITCH '(' val ')' casestmts
	      | ';'

Simplified grammar :

      stmt
	      : compstmt
	      | expr ';'
	      | IF '(' condexpr ')' stmt ELSE stmt
	      | DO stmt WHILE '(' condexpr ')'
	      | ';'


Seb.