1. Live Variable Analysis
Determines for each “point” in a program what (variable’s) values are live at that point.
Program “points” are defined before and after each statement of a basic block.
A value is “live” iff
It has been written (defined)
It might be read (used) on some path later in the control flow graph

2. Statement Dataflow Consider a statement, S,
dest = src1 operator src2, OR
dest = operator(src1, src2, … srcN)
Keep two sets for each statement
Written (defined)
Read (used)

3. Live Variables – Basic Block
For each block, BB, compute
Written, read, readB4written, for BB
Initialize each BB
Live-in = readB4written, Live-out = Ø
Repeat until neither live-in, live-out change
For each BB
Live-out(BB) U= U Live-in(Successor)
Live-in(BB) U= Live-out(BB) – written(BB)

4. Live Analysis – Statements

5. Whole Algorithm Traverse each statement, S, in function
Compute S->written, S->read
For each basic block, BB, compute BB->live-in and BB->live-out (See Live Variables – Basic Block)
For each basic block, BB
Live = BB->live-out
Traverse each statement, S, of BB, in reverse
Live = Live – S->written
S->live = Live
Live = Live ∨ S->read