SWEN90010 – High Integrity
Systems Engineering Hoare Logic

DMD 8.17 (Level 8, Doug McDonell Bldg)
http://people.eng.unimelb.edu.au/tobym @tobycmurray

HOARE LOGIC (PROVING PROGRAMS CORRECT)

If I prove my program is correct, does that guarantee it will have no bugs?
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If I prove my program is correct, does that guarantee it will have no bugs?
Incorrect specification. (you proved the wrong thing)
Program differs from what was proved. (proofs are always over mathematical models)

Misconception
Proving is hard.
(Not if you understand programming carefully.)
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Programming Language
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“main” program:
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Proof Structure
Follows the program structure, top-down
To prove something about X,
we reason about X’s internal components.
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Hoare Triples
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Hoare Triples
precondition
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Hoare Triples
precondition
postcondition
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Hoare Triples
precondition
{ x = 2 } x := x + 1 { x = 3 }
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postcondition

Hoare Triples
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precondition
{ x = 2 } x := x + 1 { x = 3 }
postcondition
(strongest postcondition)

Hoare Triples
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precondition
{ x = 2 } x := x + 1 { x = 3 }
{ x = 0 } x := x + 1 { x = 1 }
postcondition
(strongest postcondition)

Hoare Triples
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precondition
{ x = 2 } x := x + 1 { x = 3 }
(weakest precondition)
{ x = 0 } x := x + 1 { x = 1 }
postcondition
(strongest postcondition)

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Logic (Refresher)
We use standard propositional symbols:
“not”, “negation”
“and”, “conjunction”
“or”, “disjunction”
if A is true then B is also true A is not true
A is true and B is true
at least one of A or B is true
the proposition that is always true
the proposition that is never true

Inference Rules
Some trivial examples:
modus ponens
A structural rule:
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Assignment Statements
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Assignment Statements
{ x = 2 } x := x + 1 { x = 3 }
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Assignment Statements
{ x = 2 } x := x + 1 { x = 3 }
{ x ≥ 0 } x := x + 1 { x – 1 ≥ 0 }
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Assignment Statements
{ x = 2 } x := x + 1 { x = 3 }
{ x ≥ 0 } x := x + 1 { x – 1 ≥ 0 }
{ x ≥ 0 } x := sha256(x) { ? }
11 Copyright University of Melbourne 2016, provided under Creative Commons Attribution License

Assignment Statements
{ x = 2 } x := x + 1 { x = 3 }
{ x ≥ 0 } x := x + 1 { x – 1 ≥ 0 }
{ x ≥ 0 } x := sha256(x) { ? }
{ x = 0 } x := x + 1 { x = 1 }
11 Copyright University of Melbourne 2016, provided under Creative Commons Attribution License

Assignment Statements
{ x = 2 } x := x + 1 { x = 3 }
{ x ≥ 0 } x := x + 1 { x – 1 ≥ 0 }
{ x ≥ 0 } x := sha256(x) { ? }
{ x = 0 } x := x + 1 { x = 1 }
{ x + 1 = 1 } x := x + 1 { x = 1 }
11 Copyright University of Melbourne 2016, provided under Creative Commons Attribution License

Assignment Statements
{ x = 2 } x := x + 1 { x = 3 }
{ x ≥ 0 } x := x + 1 { x – 1 ≥ 0 }
{ x ≥ 0 } x := sha256(x) { ? }
{ x = 0 } x := x + 1 { x = 1 }
{ x + 1 = 1 } x := x + 1 { x = 1 }
{ x + 1 ≥ 0 } x := x + 1 { x ≥ 0 }
11 Copyright University of Melbourne 2016, provided under Creative Commons Attribution License

Assignment Statements
{ x = 2 } x := x + 1 { x = 3 }
{ x ≥ 0 } x := x + 1 { x – 1 ≥ 0 }
{ x ≥ 0 } x := sha256(x) { ? }
{ x = 0 } x := x + 1 { x = 1 }
{ x + 1 = 1 } x := x + 1 { x = 1 }
{ x + 1 ≥ 0 } x := x + 1 { x ≥ 0 }
{ sha256(x) ≥ 0 } x := sha256(x) { x ≥ 0 }
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Rule of Assignment
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Rule of Assignment
{ (x = 1)[x + 1/x] } x := x + 1 { x = 1 }
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Rule of Assignment
{ (x = 1)[x + 1/x] } x := x + 1 { x = 1 } { x + 1 = 1 } x := x + 1 { x = 1 }
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Rule of Assignment
{ (x = 1)[x + 1/x] } x := x + 1 { x = 1 } { x + 1 = 1 } x := x + 1 { x = 1 } { x = 0 } x := x + 1 { x = 1 }
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Rule of Assignment
{ (x = 1)[x + 1/x] } x := x + 1 { x = 1 } { x + 1 = 1 } x := x + 1 { x = 1 } { x = 0 } x := x + 1 { x = 1 }
{ (x ≥ 0)[x + 1/x] } x := x + 1 { x ≥ 0 }
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Rule of Assignment
{ (x = 1)[x + 1/x] } x := x + 1 { x = 1 } { x + 1 = 1 } x := x + 1 { x = 1 } { x = 0 } x := x + 1 { x = 1 }
{ (x ≥ 0)[x + 1/x] } x := x + 1 { x ≥ 0 } { x + 1 ≥ 0 } x := x + 1 { x ≥ 0 }
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Rule of Consequence
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Rule of Consequence
{ x = 0 } x := x + 1 { x = 1 }
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Rule of Consequence
{ x = 0 } x := x + 1 { x = 1 } { x = 0 } x := x + 1 { x > 0 }
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Rule of Consequence
{ x = 0 } x := x + 1 { x = 1 } { x = 0 } x := x + 1 { x > 0 }
{ x ≥ 0 } x := x + 1 { x > 0 }
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Rule of Consequence
{ x = 0 } x := x + 1 { x = 1 } { x = 0 } x := x + 1 { x > 0 }
{ x ≥ 0 } x := x + 1 { x > 0 } { x = 0 } x := x + 1 { x > 0 }
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Combining the Two Rules
Assignment axiom Consequence rule
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Combining the Two Rules
Assignment axiom Consequence rule
Suppose we wish to prove:
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Combining the Two Rules
Assignment axiom Consequence rule
Suppose we wish to prove:
{ x = 0 } x := x + 1 { x > 0 }
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Combining the Two Rules
Assignment axiom Consequence rule
Apply consequence rule first
Suppose we wish to prove:
{ x = 0 } x := x + 1 { x > 0 }
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Combining the Two Rules
Assignment axiom Consequence rule
{ x = 0 } x := x + 1 { x > 0 }
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Combining the Two Rules
Assignment axiom Consequence rule
{ ?P } x := x + 1 { x > 0 }
{ x = 0 } x := x + 1 { x > 0 }
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Combining the Two Rules
Assignment axiom Consequence rule
x = 0 ⇒ ?P { ?P } x := x + 1 { x > 0 }
{ x = 0 } x := x + 1 { x > 0 }
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Combining the Two Rules
Assignment axiom Consequence rule
Now apply the assignment axiom
x = 0 ⇒ ?P { ?P } x := x + 1 { x > 0 }
{ x = 0 } x := x + 1 { x > 0 }
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Combining the Two Rules
Assignment axiom Consequence rule
x = 0 ⇒ x + 1 > 0 { x + 1 > 0 } x := x + 1 { x > 0 }
{ x = 0 } x := x + 1 { x > 0 }
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Combining the Two Rules
Assignment axiom Consequence rule
This is a general pattern:
apply consequence rule first to generalise precondition, then apply other rules
x = 0 ⇒ x + 1 > 0 { x + 1 > 0 } x := x + 1 { x > 0 }
{ x = 0 } x := x + 1 { x > 0 }
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Assignment Rule as Weakest Precondition Rule
Given some concrete postcondition P,
the assignment rule says how to calculate
the weakest precondition to guarantee P holds afterwards:
The rules we will see for the other kinds of statements (if, while, skip, sequencing etc.) will also be of this form, allowing us to calculate weakest preconditions from
a concrete postcondition.
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Weakest Precondition Reasoning
Given concrete preconditions P’ and Q and some program S, to prove:
{P′} S {Q}
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Weakest Precondition Reasoning
Given concrete preconditions P’ and Q and some program S, to prove:
{P′} S {Q}
First apply the consequence rule to generalise the
precondition, giving us two goals to prove:
{?P} S {Q}
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Weakest Precondition Reasoning
Given concrete preconditions P’ and Q and some program S, to prove:
{P′} S {Q}
First apply the consequence rule to generalise the
precondition, giving us two goals to prove:
Copyright University of Melbourne 2016, provided under Creative Commons Attribution License
{?P} S {Q}
Then, calculate what ?P is by applying the weakest precondition rules

Weakest Precondition Reasoning
Given concrete preconditions P’ and Q and some program S, to prove:
{P′} S {Q}
First apply the consequence rule to generalise the
precondition, giving us two goals to prove:
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{?P} S {Q}
Then, calculate what ?P is by applying the weakest precondition rules
Once ?P has been calculated, finally check this implication holds

NEXT LECTURE BEGINS

Rule of Sequencing
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Rule of Sequencing
Theorem: {x = 0} x := x+1; x := x+1 {x=2}
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Rule of Sequencing
Theorem: {x = 0} x := x+1; x := x+1 {x=2} Proof:
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Rule of Sequencing
Theorem: {x = 0} x := x+1; x := x+1 {x=2} Proof:
{x = 0} x := x+1; x := x+1 {x=2}
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Rule of Sequencing
Apply consequence rule first
Theorem: {x = 0} x := x+1; x := x+1 {x=2}
{x = 0} x := x+1; x := x+1 {x=2}
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Rule of Sequencing
Apply consequence rule first
Theorem: {x = 0} x := x+1; x := x+1 {x=2} Proof:
{x = 0} x := x+1; x := x+1 {x=2}
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Rule of Sequencing
Apply consequence rule first
Theorem: {x = 0} x := x+1; x := x+1 {x=2} Proof:
x = 0 ⇒ ?P {?P} x := x+1; x := x+1 {x=2}
{x = 0} x := x+1; x := x+1 {x=2}
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Rule of Sequencing
Theorem: {x = 0} x := x+1; x := x+1 {x=2} Proof:
x = 0 ⇒ ?P {?P} x := x+1; x := x+1 {x=2}
{x = 0} x := x+1; x := x+1 {x=2}
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Rule of Sequencing
Theorem: {x = 0} x := x+1; x := x+1 {x=2} Proof:
{?P} x := x+1 {?R} {?R} x := x+1 {x=2} x = 0 ⇒ ?P {?P} x := x+1; x := x+1 {x=2}
{x = 0} x := x+1; x := x+1 {x=2}
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Rule of Sequencing
Theorem: {x = 0} x := x+1; x := x+1 {x=2} Proof:
{?P} x := x+1 {x+1=2} {x+1=2} x := x+1 {x=2} x = 0 ⇒ ?P {?P} x := x+1; x := x+1 {x=2}
{x = 0} x := x+1; x := x+1 {x=2}
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Rule of Sequencing
Theorem: {x = 0} x := x+1; x := x+1 {x=2} Proof:
{x+2=2} x := x+1 {x+1=2} {x+1=2} x := x+1 {x= x=0 ⇒ x+2=2 {x+2=2} x := x+1; x :=

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