CSE2304, feedback on prac-1, 2006

To start compilation of an expression, exp:

comp 0 "=" exp

 

case	action
comp reg "=" x	R reg = x
comp reg op x	R reg op= x
comp reg "=" (exp op exp')	comp reg "=" exp comp reg op exp'
comp reg op (exp op' exp')	comp (reg+1) "=" exp comp (reg+1) op' exp' R(reg) op= R(reg+1)

for all variables, x, expressions exp & exp', and arithmetic operators, op ("+", "-", "*", "/").

 

Note how comp recursively traverses the parse-tree of an expression.

 

Example:

comp 0 "=" x*y+z*w	comp 0 "=" x*y	comp 0 "=" x	R0=x
		comp 0 "*" y	R0*=y
	comp 0 "+" z*w	comp 1 "=" z	R1=z
		comp 1 "*" w	R1*=w
		R0+=R1

 

That is sufficient for the prac. By the way, an optimising compiler would do a lot more, possibly rearranging an expression, provided that the semantics of the programming language allow it to do so.

E.g. x+y*z (2 registers, 4 instructions) ---> y*z+x (1 register, 3 instructions),

but that was not required for the prac.