Provide info about how a program segment manipulates data
Analysis must be conservative and aggressive
Collect information for optimization
Reaching definition
Available expression
Live variable
Busy expression
Reaching Definition : A definition d reaches a point p if there is a path from d to p and d is not killed on the path Available Expression : An expression X+Y is available at point p if every path to p evaluates X+Y and after the last such evaluation no assignment to X or Y Live Variable : For a variable X and point p whether value of X at p can be used along some path starting from p. If yes X is live at p else X is dead at p Busy Expression : An expression B op C is busy at point p if along every path from p we come to computation B op C before any definition of B or C
Typical Equation
out(S) = gen(S) [ in(S) − kill(S)
Gen : definitions generated Kill : definitions killed In : input definitions Out : output definitions
Reaching Definitions
Unambiguous Definitions :
Assignments
Ambiguous Definitions :
–procedure call with
X as var parameter
–procedure that can
access X
–pointer *q = y
