COMS W4117
Compilers and Translators:
Software Verification Tools
Lecture 6: Foundations of Data-Flow Analysis
September 20, 2007

Lecture Outline

1. Review
2. Available expressions
3. Basic questions about the iterative algorithm
4. Lattices and semilattices
5. Transfer functions
6. The iterative algorithm for general frameworks
7. Meaning of a data-flow solution
8. Reading

1. Review

1. Introduction to data-flow analysis
2. Data-flow schemas
3. Reaching definitions
4. Live-variable analysis

2. Available Expressions

• Available expressions is a forward data-flow analysis problem similar to reaching definitions.
• We say an expression x + y is available at a point p in a flow graph if every path from ENTRY to p evaluates x + y and between the last such evaluation and p there are no assignments tox or y.
• A block kills x + y if it assigns x or y and does not subsequently recompute x + y.
• A block generates x + y if it evaluates x + y and does not subsequently define x or y.
• We can compute the set of generated expressions for each point in a block working from the beginning of the block to the end.
• Transfer equations and control-flow equations for available expressions: pp. 610-614.
• Iterative algorithm for available expressions: Alg. 9.17, p. 614.

3. Basic Questions about the Iterative Algorithm

• Basic questions
• When is the iterative algorithm correct?
• How precise is the solution?
• Under what conditions does the iterative algorithm converge?
• What is the meaning of the solution to the data-flow equations?

4. Lattices and Semilattices

• The following definitions are fundamental (Sect. 9.3.1, pp. 618-623).
• Lattice vs. semilattice.
• Partial order defined by the meet operator of a semilattice.
• Greatest lower bound, least upper bound.
• Maximum fixed point.
• Lattice diagram (c.f., DAG, Hasse diagram).
• Product lattices.
• Height of a lattice.

5. Data-Flow Analysis Frameworks

• Definition of a data-flow analysis framework.
• Transfer functions.
• Monotone frameworks.
• Distributive frameworks.

6. The Iterative Algorithm for General Frameworks

• The iterative algorithm takes as input:
• A data-flow graph with an ENTRY and EXIT node.
• A direction, forward or backward.
• A set of values V for IN and OUT.
• A meet operator ∧ on V such that (V, ∧) forms a semilattice.
• A set of transfer functions for the blocks of the data-flow graph.
• A constant value for the boundary condition.
• The iterative algorithm produces as output the sets IN[B] and OUT[B] for each block B in the data-flow graph.
• The algorithm is given in Fig. 9.23, p. 627.
• Important properties of the algorithm:
• If the algorithm converges, the result is a solution to the data-flow equations.
• If the framework is monotone, the solution is the MFP of the data-flow equations.
• If the semilattice of the framework is monotone and of finite height, the algorithm is guaranteed to converge.

7. Meaning of a Data-Flow Solution

• For a forward data-flow framework, the IDEAL[B] solution for a block B is the meet over all feasible paths P of f_P(v_ENTRY).
• Any answer greater than IDEAL is incorrect.
• Any answer smaller than or equal to IDEAL is safe.
• MOP[B], the meet over all paths solution, assumes that every path in the data-flow graph can be taken, but some of these paths may not be feasible. Thus, MOP[B] ≤ IDEAL[B] for all blocks B.
• The MFP solution is always smaller than or equal to the MOP solution. Therefore, MFP ≤ MOP ≤ IDEAL.

8. Reading

• ALSU: section 9.3