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Iterative Program Analysis

Iterative Program Analysis. Mooly Sagiv http://www.cs.tau.ac.il/~msagiv/courses/pa.html Tel Aviv University 640-6706 Textbook: Principles of Program Analysis Chapter 2.1 +6 (modified) Appendix A. Outline. A gentle introduction constant propagation Mathematical background

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Iterative Program Analysis

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  1. Iterative Program Analysis Mooly Sagiv http://www.cs.tau.ac.il/~msagiv/courses/pa.html Tel Aviv University 640-6706 Textbook: Principles of Program Analysis Chapter 2.1 +6 (modified) Appendix A

  2. Outline • A gentle introduction constant propagation • Mathematical background • Chaotic iterations • Abstract interpretation • More examples • Kill/Gen Problems • Garbage variables • Pointer analysis • Stack • Heap (later)

  3. [x0, y0, z0] [x1, y0, z3] [x1, y0, z3] [x1, y7, z3] [x0, y0, z3] [x1, y7, z3] [x3, y7, z3] [x3, y7, z3] A Simple Example Program z = 3 x = 1 while (x > 0) ( if (x = 1) then y = 7 else y = z + 4 x = 3 print y )

  4. [x0, y0, z0] [x1, y0, z3] [x, y0, z3] [x1, y7, z3] [x1, y7, z3] [x0, y0, z3] [x3, y7, z3] [x3, y7, z3] A Simple Example Program z = 3 x = 1 while (x > 0) ( if (x = 1) then y = 7 else y = z + 4 x = 3 print y )

  5. [x0, y0, z0] [x1, y0, z3] [x, y0, z3] [x, y7, z3] [x1, y7, z3] [x0, y0, z3] [x3, y7, z3] [x3, y7, z3] A Simple Example Program z = 3 x = 1 while (x > 0) ( if (x = 1) then y = 7 else y = z + 4 x = 3 print y )

  6. [x0, y0, z0] [x1, y0, z3] [x, y0, z3] [x, y7, z3] [x, y7,z3] [x0, y0, z3] [x1, y7,z3] [x3, y7, z3] [x3, y7, z3] A Simple Example Program z = 3 x = 1 while (x > 0) ( if (x = 1) then y = 7 else y =z + 4 x = 3 print y )

  7. [x0, y0, z0] [x1, y0, z3] [x, y0, z3] [x1, y7, z3] [x, y7,z3] [x0, y0, z3] [x1, y7,z3] [x, y7,z3] [x3, y7, z3] [x3, y7, z3] A Simple Example Program z = 3 x = 1 while (x > 0) ( if (x = 1) then y = 7 else y =z + 4 x = 3 print y )

  8. [x0, y0, z0] [x1, y0, z3] [x, y0, z3] [x1, y7, z3] [x0, y0, z3] [x, y7,z3] [x, y7,z3] [x3, y7, z3] [x3, y7, z3] A Simple Example Program z = 3 x = 1 while (x > 0) ( if (x = 1) then y = 7 else y =z + 4 x = 3 print y )

  9. Computing Constants • Construct a control flow graph (CFG) • Associate transfer functions with control flow graph edges • Iterate until a solution is found • The solution is unique • But order of evaluation may affect the number of iterations

  10. Constructing CFG z =3 z = 3 x = 1 while (x > 0) ( if (x = 1) then y = 7 else y =z + 4 x = 3 print y ) x =1 while (x>0) if (x=1) y =7 y =z+4 x=3 print y

  11. Associating Transfer Functions z =3 e.e[z3] x =1 e.e[x1] e. if x 0 then e else  while (x>0) e. if x >0 then e else  if (x=1) e. if x 0 then e else  e. e [x1, y , z] y =7 y =z+4 e.e e.e[y7] e.e[ye(z)+4] x=3 e.e[x3] print y

  12. Iterative Computation [x0, y0, z0] z =3 e.e[z3]  x =1 e.e[x1] e. if x 0 then e else   while (x>0) e. if x >0 then e else   if (x=1) e. if x 0 then e else  e. e [x1, y , z]   y =7 y =z+4 e.e e.e[y7] e.e[ye(z)+4]  x=3 e.e[x3]  print y

  13. Iterative Computation [x0, y0, z0] z =3 e.e[z3] [x1, y0, z0] x =1 e.e[x1] e. if x 0 then e else   while (x>0) e. if x >0 then e else   if (x=1) e. if x 0 then e else  e. e [x1, y , z]   y =7 y =z+4 e.e e.e[y7] e.e[ye(z)+4]  x=3 e.e[x3]  print y

  14. Iterative Computation [x0, y0, z0] z =3 e.e[z3] [x0, y0, z3] x =1 e.e[x1] e. if x 0 then e else   while (x>0) e. if x >0 then e else   if (x=1) e. if x 0 then e else  e. e [x1, y , z]   y =7 y =z+4 e.e e.e[y7] e.e[ye(z)+4]  x=3 e.e[x3]  print y

  15. Iterative Computation [x0, y0, z0] z =3 e.e[z3] [x0, y0, z3] x =1 e.e[x1] e. if x 0 then e else  [x1, y0, z3] while (x>0) e. if x >0 then e else   if (x=1) e. if x 0 then e else  e. e [x1, y , z]   y =7 y =z+4 e.e e.e[y7] e.e[ye(z)+4]  x=3 e.e[x3]  print y

  16. Iterative Computation [x0, y0, z0] z =3 e.e[z3] [x0, y0, z3] x =1 e.e[x1] e. if x 0 then e else  [x1, y0, z3] while (x>0) e. if x >0 then e else  [x1, y0, z3] if (x=1) e. if x 0 then e else  e. e [x1, y , z]   y =7 y =z+4 e.e e.e[y7] e.e[ye(z)+4]  x=3 e.e[x3]  print y

  17. Iterative Computation [x0, y0, z0] z =3 e.e[z3] [x0, y0, z3] x =1 e.e[x1] e. if x 0 then e else  [x1, y0, z3] while (x>0) e. if x >0 then e else  [x1, y0, z3] if (x=1) e. if x 0 then e else  e. e [x1, y , z]  y =7 y =z+4 [x1, y0, z3] e.e e.e[y7] e.e[ye(z)+4]  x=3 e.e[x3]  print y

  18. Iterative Computation [x0, y0, z0] z =3 e.e[z3] [x0, y0, z3] x =1 e.e[x1] e. if x 0 then e else  [x1, y0, z3] while (x>0) e. if x >0 then e else  [x1, y0, z3] if (x=1) e. if x 0 then e else  e. e [x1, y , z]  y =7 y =z+4 [x1, y0, z3] e.e e.e[y7] e.e[ye(z)+4] [x1, y7, z3] x=3 e.e[x3]  print y

  19. Iterative Computation [x0, y0, z0] z =3 e.e[z3] [x0, y0, z3] x =1 e.e[x1] e. if x 0 then e else  [x1, y0, z3] while (x>0) e. if x >0 then e else  [x1, y0, z3] if (x=1) e. if x 0 then e else  e. e [x1, y , z]  y =7 y =z+4 [x1, y0, z3] e.e e.e[y7] e.e[ye(z)+4] [x1, y7, z3] x=3 e.e[x3] [x3, y7, z3] print y

  20. Iterative Computation [x0, y0, z0] z =3 e.e[z3] [x0, y0, z3] x =1 e.e[x1] e. if x 0 then e else  [x, y0, z3] while (x>0) e. if x >0 then e else  [x1, y0, z3] if (x=1) e. if x 0 then e else  e. e [x1, y , z]  y =7 y =z+4 [x1, y0, z3] e.e e.e[y7] e.e[ye(z)+4] [x1, y7, z3] x=3 e.e[x3] [x3, y7, z3] print y

  21. Iterative Computation [x0, y0, z0] z =3 e.e[z3] [x0, y0, z3] x =1 e.e[x1] e. if x 0 then e else  [x, y0, z3] while (x>0) e. if x >0 then e else  [x, y0, z3] if (x=1) e. if x 0 then e else  e. e [x1, y , z]  y =7 y =z+4 [x1, y0, z3] e.e e.e[y7] e.e[ye(z)+4] [x1, y7, z3] x=3 e.e[x3] [x3, y7, z3] print y

  22. Iterative Computation [x0, y0, z0] z =3 e.e[z3] [x0, y0, z3] x =1 e.e[x1] e. if x 0 then e else  [x, y0, z3] while (x>0) e. if x >0 then e else  [x, y0, z3] if (x=1) e. if x 0 then e else  e. e [x1, y , z]  y =7 y =z+4 [x, y0, z3] e.e e.e[y7] e.e[ye(z)+4] [x1, y7, z3] x=3 e.e[x3] [x3, y7, z3] print y

  23. Iterative Computation [x0, y0, z0] z =3 e.e[z3] [x0, y0, z3] x =1 e.e[x1] e. if x 0 then e else  [x, y0, z3] while (x>0) e. if x >0 then e else  [x, y0, z3] if (x=1) e. if x 0 then e else  e. e [x1, y , z] [x, y0, z3] y =7 y =z+4 [x, y0, z3] e.e e.e[y7] e.e[ye(z)+4] [x1, y7, z3] x=3 e.e[x3] [x3, y7, z3] print y

  24. Iterative Computation [x0, y0, z0] z =3 e.e[z3] [x0, y0, z3] x =1 e.e[x1] e. if x 0 then e else  [x, y0, z3] while (x>0) e. if x >0 then e else  [x, y0, z3] if (x=1) e. if x 0 then e else  e. e [x1, y , z] [x, y0, z3] y =7 y =z+4 [x1, y0, z3] e.e e.e[y7] e.e[ye(z)+4] [x, y7, z3] x=3 e.e[x3] [x3, y7, z3] print y

  25. Order of evaluation • The solution is unique • Different orders may converge faster • Example order: Depth First Order

  26. Mathematical Background • Declaratively define • The result of the analysis • The exact solution • Allow comparison • Prove that the algorithm is sound by proving that the handling of atomic statements is sound

  27. Posets • A partial ordering is a binary relation : L  L  {false, true} • For all l  L : l  l (Reflexive) • For all l1, l2, l3 L : l1  l2, l2  l3  l1  l3 (Transitive) • For all l1, l2 L : l1  l2, l2  l1  l1 = l2 (Anti-Symmetric) • Denoted by (L,  ) • In program analysis • l1 l2 l1 is more precise than ll1 represents fewer concrete states than l2

  28. Example Posets • Total orders (N, ) • Powersets (P(S), ) • Powersets (P(S), ) • Constant propagation

  29. Posets • More notations • l1 l2  l2 l1 • l1 l2  l1 l2  l1 l2 • l1 l2  l2 l1

  30. Upper and Lower Bounds • Consider a poset (L,  ) • A subset L’  L has a lower boundl L if for all l’  L’ : l l’ • A subset L’  L has an upper boundu L if for all l’  L’ : l’  u • A greatest lower bound of a subset L’  L is a lower bound l0 L such that l  l0 for any lower bound l of L’ • A lowest upper bound of a subset L’  L is an upper bound u0 L such that u0  u for any upper bound u of L’ • For every subset L’  L: • The greatest lower bound of L’ is unique if at all exists • L’ (meet) a b = {a, b} • The lowest upper bound of L’ is unique if at all exists • L’ (join) ab =  {a, b}

  31. Complete Lattices • A poset (L,  ) is a complete lattice if every subset has least and upper bounds • L = (L, ) = (L, , , , , ) •  =   =  L •  =  L =   • Examples • Total orders (N, ) • Powersets (P(S), ) • Powersets (P(S), ) • Constant propagation

  32. Complete Lattices • Lemma For every poset (L,  ) the following conditions are equivalent • L is a complete lattice • Every subset of L has a least upper bound • Every subset of L has a greatest lower bound

  33. 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

  34. Finite Maps • A complete lattice (L1, 1) = (L1, , 1, 1, 1, 1) • A finite set V • Define a Poset L = (VL1 ,) where • e1  e2 if for all v  V • e1v  e2v • L is a complete lattice

  35. Chains • A subset Y  L in a poset (L,  ) is a chain if every two elements in Y are ordered • For all l1, l2  Y: l1  l2 or l2  l1 • An ascending chain is a sequence of values • l1  l2  l3  … • A strictly ascending chain is a sequence of values • l1  l2  l3… • A descending chain is a sequence of values • l1  l2  l3  … • A strictly descending chain is a sequence of values • l1  l2  l3  … • L has a finite height if every chain in L is finite • Lemma A poset (L,  ) has finite height if and only if every strictly decreasing and strictly increasing chains are finite

  36. Monotone Functions • A poset (L,  ) • A function f: L  L is monotoneif for every l1, l2 L: • l1  l2 f(l1 )  f(l2)

  37. Lemma 1 Consider a lattice L. f: L  L is monotone iff for all X  L: {f(z) | z X }  f({z | z X })

  38. Distributive (additive functions) Consider a lattice L. f: L  L is distributive if for all X  L: {f(z) | z X } = f({z | z X })

  39. f() f2() Fix(f) Red(f) f2() Ext(f) f()  Fixed Points • A monotone function f: L  L where (L, , , , , ) is a complete lattice • Fix(f) = { l: l  L, f(l) = l} • Red(f) = {l: l  L, f(l)  l} • Ext(f) = {l: l  L, l  f(l)} • l1  l2 f(l1 )  f(l2) • Tarski’s Theorem 1955: if f is monotone then: • lfp(f) =  Fix(f) =  Red(f)  Fix(f) • gfp(f) =  Fix(f) =  Ext(f)  Fix(f) gfp(f) lfp(f)

  40. Computing lfp(f) x =  while f(x)x do x = f(x)

  41. Chaotic Iterations • A lattice L = (L, , , , , ) with finite strictly increasing chains • Ln = L  L  …  L • A monotone function f: LnLn • Compute lfp(f) • The simultaneous least fixed of the system {x[i] = fi(x) : 1  i n } for i :=1 to n do x[i] =  WL = {1, 2, …, n} while (WL  ) do select and remove an element i  WL new := fi(x) if (new  x[i]) then x[i] := new; Add all the indexes that directly depends on i to WL x := (, , …, ) while (f(x)  x ) do x := f(x)

  42. 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 dfentry[v] :=  df[v] =  WL = {1, 2, … n} while (WL  ) do select and remove an element u  WL for each v, such that. (u, v) E do temp = f(e)(dfentry[u]) new := dfentry(v) temp if (new  dfentry[v]) then dfentry[v] := new; WL := WL {v}

  43. [x0, y0, z0] 1 z =3 e.e[z3] 2 x =1 e.e[x1] e. if x 0 then e else  3 while (x>0) e. if x >0 then e else  4 if (x=1) e. if x 0 then e else  e. e [x1, y , z] 5 6  y =7 y =z+4 e.e e.e[y7] e.e[ye(z)+4] 7 x=3 e.e[x3] 8 print y

  44. Specialized Chaotic IterationsSystem of Equations S = dfentry[s] =  dfentry[v] = {f(u, v) (dfentry[u]) | (u, v)  E } FS:LnLn FS(X)[s] =  FS(X)[v]= {f(u, v)(X[u]) | (u, v)  E } lfp(S) = lfp(FS)

  45. Complexity of Chaotic Iterations • Parameters: • n the number of CFG nodes • k is the maximum outdegree of edges • A lattice of height h • c is the maximum cost of • applying f(e) •  • L comparisons • ComplexityO(n * h * c * k)

  46. Soundness • Every detected constant is indeed such • Every error will be detected • The least fixed points represents all occurring runtime states

  47. Completeness • Every constant is indeed detected as such • Every detected error is real • Every state represented by the least fixed is reachable for some input

  48. The Abstract Interpretation Technique • The foundation of program analysis • Goals • Establish soundness of (find faults in) a given program analysis algorithm • Design new program analysis algorithms • The main ideas: • Relate each step in the algorithm to a step in a structural operational semantics • Establish global correctness using a general theorem • Not limited to a particular form of analysis

  49. Galois Connections • Lattices Cand A and functions : C A and : AC • The pair of functions (, ) form Galois connectionif •  and  are monotone •  a  A • ( (a)) a •  c  C • c   ((C)) • Alternatively if: c  C  a A (c)  a iff c  (a) •  and  uniquely determine each other

  50.  Abstract  Descriptors of sets of stores Galois Connections  Concrete Sets of stores

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