Alias removal

Alias removal mk Tue, 03/30/2010 - 16:01

The routine optimizer.aliasRemoval() walks through the program looking for simple assignment statements, e.g. V:=W. It replaces all subsequent occurrences of V by W, provided V is assigned a value once and W does not change in the remainder of the code. Special care should be taken for iterator blocks as illustrated in the case below:

	i:=0;
	b:= "done";
barrier go:= true;
	c:=i+1;
	d:="step";
	v:=d;
	io.print(v);
	i:=c;
redo go:= i<2;
exit go;
	io.print(b);
	optimizer.aliasRemoval();

The constant strings are propagated to the

print()

routine, while the initial assigment

i:=0

should be retained. The code block becomes:

	i:=0;
barrier go:= true;
	c:=i+1;
	io.print("step");
	i:=c;
redo go:= i<2;
exit go;
	io.print("done");

A special case is backward propagation of constants. The following snippet is the result of the JITO emptyset. It can be further reduced to avoid useless assignments.

    _53 := sql.bind("sys","_tables","type",0);
    (_54,_56,_58,_60) := bat.partition(_53);
    _53 := nil;
    _67 := _54;
    _54 := nil;
    _75 := _67;
    _67 := nil;
    _83 := _75;
    _75 := nil;