1.6 Behavioral Equivalence

2 Two very important concepts in the study and analysis of programs –Equivalence between programs –Congruence between statements –Replacing statements and programs

3 Consider the two programs: P1::[ out x:integer where x=0 l0: x:=1 :l0’ ] P2::[ out x:integer where x=0 local t:integer where t=0 l0: t:=1 :l0’ l1: x:=t :l1’ ]

4 Computation generated by P1 –,,, … Computation generated by P2 –,,,,… Computations contain too much distinguishing information, irrelevant to the correctness of the program, like –Control variable –Local variables

5 Observable variables: O a subset of state variables –Usually input or output variables –Control variables are never observable Label renaming =>equivalent programs

6 We define the observable state corresponding to s, denoted by s|O, to be the restriction of s to just the observable variables O. Thus, s|O is an interpretation of O that coincides with s on all the variables in O.

7 Given a computation σ :s0, s1, … We define the observable behavior corresponding to َ to be the sequence َ σ o : s0 |O, s1 |O, …

8 For P1 and P2, and O={x}, observable behaviors: σ 1 O :,,, … σ 2 O :,,,, …

9 Reduced behavior The reduced behavior σ r –relative to O, –corresponding to a computation σ, is the sequence obtained from σ by the following transformations : Replace each state s i by its observable part s i |O Omit from the sequence each observable state that is identical to its predecessor but not identical to all of its successors. –Not to delete the infinite suffix.

10 Applying these transformations to the computations σ 1 and σ 2 or just the second transformation to σ 2 O σ 1 r :,,, … σ 2 r :,,, …

11 Equivalence of transition systems For a basic transition system P, we denote by R(P) the set of all reduced behaviors generated by P. Let P1 and P2 be two basic transition systems and O subsetof Π1 intersect Π2 be a set of variables (observable variables for both systems). The systems P1 and P2 are defined to be equivalent (relative to O), denoted by P1~P2, if R(P1)=R(P2).

12 Which is equivalent to which? Q1::[out x: integer where x=0; x:=2] Q2::[out x: integer where x=0; x:=1; x:=x+1] Q3::[out x: integer where x=0; [local t: integer; t:=1; x:=t+1]] Observable set?

13 Congruence between statements –To explain the meaning of a statement S by another more familiar statement S’, that is congruent to S (perform the same task as S), but may be more efficient.

14 Congruence of statements Consider the two statements: T1::[x:=1;x:=2] T2::[x:=1;x:=x+1] Viewing them as the bodies of programs, they are equivalent: P1::[out x: integer where x=0;T1] P2::[out x: integer where x=0;T2]

15 Our expectation about equivalent statements is that they are completely interchangeable: the behavior of a program containing T1 will not change when we replace an occurrence of T1 with T2.

16 Consider Q1 and Q2: Q1:: [out x: integer where x=0;[T1 || x:=0]] Q2:: [out x: integer where x=0;[T2 || x:=0]] Are they equivalent? Obtain the set of reduced behaviors of Q1 and Q2.

17 Let P[S] be a program context, which is a program in which statement variable S appears as one of the statements. For example: Q[S]:: [out x: integer where x=0;[S|| x:=0]] Let programs P[S1] and P[S2] be the programs obtained by replacing statement variable S with the concrete statements S1 and S2, respectively. Statements S1 and S2 are defined to be congruent, denoted by S1~S2, if P[S1]~P[S2] for every program context P[S].

18 examples Commutativity –Selection and cooperation constructions are commutative. [S1 or S2] ~ [S2 or S1] [S1 || S2] ~[S2 || S1] Associativity –Concatenation, selection, and cooperation constructions are all associative. [S1;[S2;S3]] ~ [[S1;S2];S3]~[S1;S2;S3] For or and ||

19 S~ [S; skip] What about: S1 :: [await x] S2 :: [skip; m: await x] ? Consider: P[S]:: [out x: boolean where x=F l0: [S or [await !x]]; l1: x:=T :l1’]

20 P[S2] may deadlock, while P[S1] may not.

21 await c ~ while !c do skip Implementing await by busy waiting Problem 1.3

22 Implementation versus emulation Replacement of two programs may be desirable, for example in the case that one is expressed in terms of high-level constructs that are not directly available on a considered machine. There are two possible relations; –Emulation –implementation

23 P2 emulates P1 if they are equivalent, i.e., if their sets of reduced behaviors are equal (a symmetric relation). P2 implements P1 if the set of reduced behaviors of P2 is a subset of the set of reduced behaviors of P1.

24 Example: P1::[ out x, y: integer where x=0, y=0 loop forever do [x:=x+1 or y:=y+1]] P2::[ out x, y: integer where x=0, y=0 loop forever do [x:=x+1 ; y:=y+1]]

25 Emulation and implementation relations between statements: –The statement S2 emulates statement S1 if P[S2] emulates P[S1] for every program context P[S]. –S2 emulates S1 iff S2 is congruent to S1. –The statement S2 implements statement S1 if P[S2] implements P[S1] for every program context P[S].

26 What are the relations? –While !c do skip ?? await c –x:=x+1 ?? [[x:=x+1] or [y:=y+1]] –S2= await x ?? S1=[await x] or [await y] –S3=await (x or y) ?? S1=[await x] or [await y]

27 An example to compare S1 and S2 and S3: [local x,y : boolean where x=F, y=T out z: integer where z=0 S; z:=1]

Grouped Statements In our text language, an atomic step (corresponding to a single transition taken in a computation), consists of the execution of at most one statement of the program.

29 We define a class of statements as elementary statements. These statements can be grouped together. The elementary statements: –Skip, assignment, and await statements –If S, S1, …, Sk are elementary statements, then so are: When c do S If c then S1 else S2 [S1 or … or Sk] [S1; …; Sk] –Any statement containing: cooperation or a while statement is not elementary.

30 If S is an elementary statement, then is a grouped statement. Example: Execution of this grouped statement calls for the uninterrupted and successful execution of the three statements participating in the group in succession. This grouped statement is congruent to the statement await y=1 This interpretation implies that execution of a grouped statement cannot be started unless its successful termination is guaranteed.

31 The transition associated with a grouped statement Product of transitions –Let and be two transitions. –Product of and, denoted by o, is s”

Semaphore Statement

Region statement

Mode 3: Message-Passing Text

Model 4: Petri Nets