Program Analysis Systematic Domain Design

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Program AnalysisSystematic Domain Design

• Mooly Sagiv
• http//www.cs.tau.ac.il/msagiv/courses/pa04.html
• Tel Aviv University
• 640-6706
• Textbook Principles of Program Analysis
• Chapter 4, CC79, CC92

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Outline

• Domains with infinite heights
• Systematic construction of Galois connection
• Precision

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Specialized Chaotic Iterations
Chaotic(G(V, E) Graph, s Node, L lattice, ?
L, f E ?(L ?L) ) for each v in V to n do
dfentryv ? Inv ? WL s
while (WL ? ? ) do select and remove an
element u ? WL for each v, such that. (u,
v) ?E do temp f(e)(dfentryu)
new dfentry(v)? temp if
(new ? dfentryv) then
dfentryv new
WL WL ?v
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Widening

• Accelerate the termination of Chaotic iterations
  by computing a more conservative solution
• Can handle lattices of infinite heights

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Specialized Chaotic Iterations ?
Chaotic(G(V, E) Graph, s Node, L lattice, ?
L, f E ?(L ?L) ) for each v in V to n do
dfentryv ? Inv ? WL s
while (WL ? ? ) do select and remove an
element u ? WL for each v, such that. (u,
v) ?E do temp f(e)(dfentryu)
new dfentry(v) ? temp if
(new ? dfentryv) then
dfentryv new
WL WL ?v
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Example Interval Analysis

• Find a lower and an upper bound of the value of a
  variable
• Usages?
• Lattice L (Z?-?, ??Z ?-?, ?, ?, ?, ?, ?,?)
• a, b ? c, d if c ? a and d ? b
• a, b ? c, d min(a, c), max(b, d)
• a, b ? c, d max(a, c), min(b, d)
• ?
• ?
• Galois connection

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Example ProgramInterval Analysis

• x 11 while x ? 10002 do x x
  13

IntEntry(1) minint,maxint IntExit(1) 1,1
IntEntry(2) IntExit(1) ? IntExit(3) IntExit(2)
IntEntry(2)
IntEntry(3) IntExit(2) ? minint,1000 IntExit(3
) IntEntry(3)1,1
IntEntry(4) IntExit(2) ? 1001,maxint IntExit(4
) IntEntry(4)
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Widening for Interval Analysis

• ?? c, d c, d
• a, b ? c, d if a ? c then a else
  -?, if b ? d then b else ?

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Example ProgramInterval Analysis

• x 11 while x ? 10002 do x x
  13

IntEntry(1) -?, ? IntExit(1) 1,1
IntEntry(2) InExit(2) ? (IntExit(1) ?
IntExit(3)) IntExit(2) IntEntry(2)
IntEntry(3) IntExit(2) ? -?,1000 IntExit(3)
IntEntry(3)1,1
IntEntry(4) IntExit(2) ? 1001, ? IntExit(4)
IntEntry(4)
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Requirements on Widening

• For all elements l1 ? l2 ? l1 ? l2
• For all ascending chains l0 ? l1 ? l2 ? the
  following sequence is finite
• y0 l0
• yi1 yi ? li1
• For a monotonic function f L ? Ldefine
• x0 ?
• xi1 xi ? f(xi )
• Theorem
• There exits k such that xk1 xk
• xk ?Red(f) l l ? L, f(l) ? l

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Narrowing

• Improve the result of widening
• y ? x ? y ? (x ?y) ? x
• For all decreasing chains x0 ? x1 ?the
  following sequence is finite
• y0 x0
• yi1 yi ? xi1
• For a monotonic function f L ? L and x ?Red(f)
  l l ? L, f(l) ? ldefine
• y0 x
• yi1 yi ? f(yi )
• Theorem
• There exits k such that yk1 yk
• yk ?Red(f) l l ? L, f(l) ? l

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Narrowing for Interval Analysis

• a, b ? ? a, b
• a, b ? c, d if a -? then
  c else a, if b ? then d else b

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Example ProgramInterval Analysis

• x 11 while x ? 10002 do x x
  13

IntEntry(1) -? , ? IntExit(1) 1,1
IntEntry(2) InExit(2) ?( IntExit(1) ?
IntExit(3)) IntExit(2) IntEntry(2)
IntEntry(3) IntExit(2) ? -?,1000 IntExit(3)
IntEntry(3)1,1
IntEntry(4) IntExit(2) ? 1001, ? IntExit(4)
IntEntry(4)
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Non Montonicity of Widening
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Example Lattice Octagon (Shaham00, Mine02)

• Inequalities between variables
• Constraint graph G(V, E, w)
• V includes a vertex for every variable
• Additional zero node
• weight function w E ? Z
• Constraints
• x ? y w(x, y)
• Lattice
• Abstraction
• Concretization
• Widening
• Relationships to intervals

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Widening and Narrowing Summary

• Very simple but produces impressive precision
• Sometimes non-monotonic
• The McCarthy 91 function
• Also useful in the finite case
• Can be used as a methodological tool
• But not widely accepted

int f(x) -? , ? if x gt 100 then 101, ?
return x -10 91, ?-10 else -?, 100
return f(f(x11)) 91, 91
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Combining Data Flow Analyzes

• Develop new algorithms from old
• If I know how to conservatively represent
• Pointers
• Integers
• Do I know how to handle C programs with integers
  and pointers?

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Combining Data Flow Analyzes

• Develop new algorithms from old
• If I know how to conservatively represent
• Pointers
• Integers
• Do I know how to handle C programs with integers
  and pointers?
• Improve the precision of an analysis
• Obtain a more efficient analysis

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Combining Data Flow Analyzers

• Lattice constructors
• L1 ? L2
• S ? L1
• Galois connection constructors
• Constructing the abstract effect of elementary
  statements
• Model the relevant parts of the program
• Abstract irrelevant parts of the program

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Galois Connections

• For
• A complete lattice (L1, ?1) (L1, ?, ?1, ?1,
  ?1, ?1)
• A complete lattice (L2, ?2) (, ?, ?2, ?2, ?2,
  ?2)
• ?L1?L2
• ? L2?L1
• We say that (L1, ?, ?, L2) is a Galois
  connection
• ? and ? are monotone
• For all c ? L1 ?(?(c)) ? c
• For all a? L2 ?(?(a)) ? a

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Cartesian Products

• A complete lattice (L1, ?1) (L1, ?, ?1, ?1,
  ?1, ?1)
• A complete lattice (L2, ?2) (, ?, ?2, ?2, ?2,
  ?2)
• Define a Poset L (L1 ? L2 ,? ) where
• (x1, x2) ? (y1, y2) if
• x1 ? y1 and
• x2 ? y2
• L is a complete lattice
• But what does an element in L represent?

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Cartesian Products (cont)

• A complete lattice (L1, ?1) (L1, ?, ?1, ?1,
  ?1, ?1)
• A complete lattice (L2, ?2) (, ?, ?2, ?2, ?2,
  ?2)
• Complete lattice L (L1 ? L2 ,? )
• A concrete lattice C (usually a powerset)
• A Galois connection (C, ?1 , ?1, L1)
• A Galois connection (C, ?2 , ?2, L2)
• Define ?C? L1 ? L2 and ? L1 ? L2 ? C ?
• Example Parity ? Sign

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Cartesian Products (cont)

• A Galois connection (C, ?1 , ?1, L1)
• A Galois connection (C, ?2 , ?2, L2)
• A Galois connection (C, ? , ?, L1 ? L2 )
• ?(c) lt?1(c), ?2(c)gt
• ?(lta1, a2gt) ?1(a1) ? ?2(a2)
• Define
• L1?st? L1? L1
• L2?st? L2? L2
• How to define L1 ? L2 ?st? L1 ? L2 ? L1 ? L2
• Preserve soundness
• Preserve relative optimality (induced)
• Example Parity ? Sign

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Component-wise combinations

• Combine several analyses into a single analysis
• Cartesian products (Direct product)
• Independent attribute method
• Relational attribute method
• Total function space
• Monotone function space
• Direct tensor product

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Independent Attribute Method

• A Galois connection (C1, ?1 , ?1, L1)
• A Galois connection (C2, ?2 , ?2, L2)
• A Galois connection (C1?C2, ? , ?, L1 ? L2 )
• ?(ltc1, c2gt) lt?1(c1), ?2(c2)gt
• ?(lta1, a2gt) lt?1(a1) , ?2(a2)gt
• Define
• L1?st? L1? L1
• L2?st? L2? L2
• How to define L1 ? L2 ?st? L1 ? L2 ? L1 ? L2
• Preserve soundness
• Preserve relative optimality (induced)

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Relational Attribute Method

• A Galois connection (P(C1), ?1 , ?1, P(L1))
  where ?1 C1?L1
• ?1 (X) ??1(c) c ? X
• A Galois connection (P(C2), ?2 , ?2, P(L2))
  where ?2 C2?L2
• ?2 (X) ??2(c) c ? X
• A Galois connection (P(C1?C2), ? , ?, P(L1 ? L2))
• ?(ltX1, X2gt) lt?1(c1), ?2(c2)gt c1 ? X1, c2 ?
  X2
• ?(ltY1,Y2gt) ltc1 , c2gt ?1(c1) ? Y1 ?2(c2)
  ? Y2
• But how about transformers?
  

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Conclusions(1)

• Good static analysis
• Precise enough (for the client)
• Efficient enough
• Good static analysis
• Good domain
• Abstract non-important details
• Represent relevant concrete information
• Precise and efficient abstract meaning of
  abstract interpreters
• Efficient join implementation
• Small height or widening

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Conclusions(2)

• The Theory of Static Analysis is well founded
• Abstraction
• Soundness
• Chaotic iterations
• Elimination methods
• Modular methods
• Weak Parts
• Transformations
• Predictable approximations
• System