1. Evidence-Based Automated Program Fixing
Yu Pei, Yi Wei, Carlo Furia, Martin Nordio, Bertrand Meyer
Chair of Software Engineering, ETH Zürich

2. Program Fixing Automated Evidence-based Program fixing
Automated fixing
Evidence-based fixing
Find a fault
Analyze the fault
Where
What
Fix the fault
How
Validate the fix
Test case generation
Fault localization
Design-by-contract
Dynamic/static analysis
Fix synthesis
By enumeration
Regression testing

3. Evidence-based Fixing: Example Faults1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
move_item (v: G)
-- Move `v' to the left of cursor.
require v /= Void ; has (v)
local idx: INTEGER ; found: BOOLEAN do
idx := index
from start until found or after loop
found := (v = item)
if not found then forth end
end
check found and not after end
remove
go_i_th (idx)
put_left (v)
-- idx <= count + 1
-- not before 16 17
go_i_th (i: INTEGER) require 0 <= i and then i <= count + 1 18 19
put_left (v: G) require not before
before: BOOLEAN do Result := (index = 0) end

4. Evidence-based Fixing: Fixes1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
move_item (v: G)
-- Move `v' to the left of cursor.
require v /= Void ; has (v)
local idx: INTEGER ; found: BOOLEAN do
idx := index
from start until found or after loop
found := (v = item)
if not found then forth end
end
check found and not after end
remove
go_i_th (idx)
put_left (v)
if index < idx then idx := idx – 1 end
if before then start end

5. Evidence in Program State Components
<p, l, v>
p : state predicate
l : program location
v : value of predicate p at location l
Design-by-contract
Faults that cause contract violations
A fault can by characterized by cf = <pf , lf , F>
Helps us narrow down faulty code to routine level
Allows us to inspect program states in more detail

6. Evidence-based Techniques

7. Dynamic Analysis Exercise the original program using test cases
Each test case contributes to the score of a state component
The contribution
from failing test cases cannot be canceled out completely
from the first a few test cases is crucial
from one failing test case is stronger than the counter-part from one passing test case
αi for each i-th failing test case, and β∙αi for each i-th passing test case (0 < α, β < 1)

8. Static Analysis Given a state component <p, l, v>
Expression dependence
the syntactic similarity between pf and p
Control dependence
the distance, in terms of number of instructions, between lf and l

9. Combining Static and Dynamic Analysis
Suspiciousness score fixme as the harmonic mean of dyn, edep, and cdep
cf = < idx<=count+1 , l-13 , F>, #f(cf )=15, #p(cf )=0
cdep = 1-0/9 = 1
edep = 3/3 = 1
dyn ≈
fixme ≈ 1.125
Program state components with higher suspiciousness scores are more likely to reflect the fault cause
< index<idx , l-13 , T>
<not before, l-14, F> 12 13 14 15
remove
go_i_th (idx)
put_left (v)
end
-- idx <= count + 1

10. Fixing Actions
Given a state component <p, l, v>, how the state should be changed to avoid that?
Try with applicable operations
Fixing with state modification
Fixing with expression substitution
-- <index<idx, l-13, T>
go_i_th (idx)
idx := idx – 1
-- <index<idx, l-13, T>
go_i_th (idx)
go_i_th (idx - 1)

11. Fix Candidate Generation and Validation
Fix schemas and fix candidates
failing_condition
Predicate of the state component
Potential state invariant only observed in failing runs
Fix validation
if fail_condition then
fixing_action
end
original_instruction
if fail_condition then
fixing_action
else
original_instruction
end

12. Evidence-based Fixing: Experiments
15 faults in 7 classes from EiffelBase
Classes supposed to have high quality
Routine size, in number of loc, varies from 1 to 30
11 precondition violations and 4 postcondition violations
Generated proper fixes for 12 faults
5 faults in 4 classes from a library manipulating text documents
Classes from student project
Routine size varies from 20 to 40
All precondition violations
Generated proper fixes for 3 faults
7.6 min on average per fault

13. Summary Evidence-based automatic program fixing Design-by-contract
Fault localization: dynamic and static analysis
Fix generation: operation enumeration
Future work
More empirical evaluation
User study
Multiple faults
Programs with weak/no/incorrect contracts