Statements that alter the flow of control

The goto statement alters the flow of control. If we implement goto statements then we need to define a LABEL for a statement. A production can be added for this purpose:

1. S →     LABEL : S  
2.     LABEL →     id  

In this production system, semantic action is attached to record the LABEL and its value in the symbol table.

Following grammar used to incorporate structure flow-of-control constructs:

1. S →  if E then S  
2.      S  →   if E then S else S  
3.      S →    while E do S  
4.      S →    begin L end  
5.      S→     A  
6.      L→    L ; S  
7.      L →   S  

Here, S is a statement, L is a statement-list, A is an assignment statement and E is a Boolean-valued expression.

Translation scheme for statement that alters flow of control

• We introduce the marker non-terminal M as in case of grammar for Boolean expression.
• This M is put before statement in both if then else. In case of while-do, we need to put M before E as we need to come back to it after executing S.
• In case of if-then-else, if we evaluate E to be true, first S will be executed.
• After this we should ensure that instead of second S, the code after the if-then else will be executed. Then we place another non-terminal marker N after first S.

The grammar is as follows:

1. S →   if E then M S  
2.      S →    if E then M S else M S  
3.      S →    while M E do M S  
4.      S →    begin L end  
5.      S →    A  
6.      L→    L ; M S  
7.      L →   S  
8.      M →    ∈  
9.      N →    ∈  

The translation scheme for this grammar is as follows:

Production rule	Semantic actions
S → if E then M S1	BACKPATCH (E.TRUE, M.QUAD) S.NEXT = MERGE (E.FALSE, S1.NEXT)
S → if E then M1 S1 else M2 S2	BACKPATCH (E.TRUE, M1.QUAD) BACKPATCH (E.FALSE, M2.QUAD) S.NEXT = MERGE (S1.NEXT, N.NEXT, S2.NEXT)
S → while M1 E do M2 S1	BACKPATCH (S1,NEXT, M1.QUAD) BACKPATCH (E.TRUE, M2.QUAD) S.NEXT = E.FALSE GEN (goto M1.QUAD)
S → begin L end	S.NEXT = L.NEXT
S → A	S.NEXT = MAKELIST ()
L → L ; M S	BACKPATHCH (L1.NEXT, M.QUAD) L.NEXT = S.NEXT
L → S	L.NEXT = S.NEXT
M → ∈	M.QUAD = NEXTQUAD
N→ ∈	N.NEXT = MAKELIST (NEXTQUAD) GEN (goto_)