Data Flow Analysis – Program Analysis and Understanding | CMSC 631, Study notes of Computer Science

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Data Flow Analysis

CMSC 631 — Program Analysis and

Understanding

Fall 2007

CMSC 631

• Source code parsed to produce AST

• AST transformed to CFG

• Data flow analysis operates on control flow graph

(and other intermediate representations)

Compiler Structure

Source Code Abstract Syntax Tree Control Flow Graph Object Code

CMSC 631

• • • • • •

Control-Flow Graph Example

x := a + b y := a * b y > a a := a + 1 x := a + b

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Variations on CFGs

• Usually don’t include declarations (e.g., int x;)

■ But there’s usually something in the implementation

• Useful to have a unique entry and exit node

■ Treat them differently during dataflow analysis

• May group statements into basic blocks

■ A sequence of instructions with no branches into or

out of the block

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• • • • • • • •

Graph Example with Entry and Exit

x := a + b y := a * b y > a a := a + 1 x := a + b

exit

entry

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CFG vs. AST

• CFGs are much simpler than ASTs

■ Fewer forms, less redundancy, simple expressions

• But...AST is a more faithful representation

■ CFGs introduce temporaries

■ Lose block structure of program

• So for AST,

■ Easier to report error + other messages

■ Easier to explain to programmer

■ Easier to unparse to produce readable code

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Available Expressions

CMSC 631

• Expression^ e^ is^ available^ at program point^ p^ if

■ e is computed on every path to p, and

■ the value of e has not changed since the last time e is

computed on p

• Optimization

■ If an expression is available, need not be recomputed

- (At least, if it’s still in a register somewhere)

Available Expressions

CMSC 631

• What is the effect of each

statement on the set of facts?

Gen and Kill

Stmt Gen Kill x := a + b a + b y := a * b a * b a := a + 1 a + 1, a + b, a * b x := a + b y := a * b y > a a := a + 1 x := a + b

exit

entry

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Computing Available Expressions

x := a + b y := a * b y > a a := a + 1 x := a + b

entry

exit

CMSC 631

Computing Available Expressions

{a + b}

x := a + b y := a * b y > a a := a + 1 x := a + b

entry

exit

CMSC 631

Computing Available Expressions

{a + b}

x := a + b y := a * b y > a a := a + 1 x := a + b

entry

exit

CMSC 631

Computing Available Expressions

{a + b}

{a + b, a * b}

x := a + b y := a * b y > a a := a + 1 x := a + b

entry

exit

CMSC 631

Computing Available Expressions

{a + b}

{a + b, a * b}

{a + b, a * b}

x := a + b y := a * b y > a a := a + 1 x := a + b

entry

exit