A Story of Parametric Trace Slicing, Garbage and Static Analysis
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- Jonah Woods
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1 A Story of Parametric Trace Slicing, Garbage and Static Analysis Giles Reger School of Computer Science, University of Manchester, UK PrePost 2017 Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 1 / 31
2 Context Helped develop the Quantified Event Automata (QEA) language and associated MarQ runtime monitoring tool Have started thinking about typestate-analysis for QEA, wrote about it at ISoLA 2016 This idea grew out of that and I thank Adrian for encouraging me to write the idea down Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 2 / 31
3 Introduction In this talk I will outline some ideas around how we can relate the ideas of Garbage collection at runtime Static identification of object unreachability to improve the performance of runtime monitoring based on parametric trace slicing Note that we are explicitly exclusively in the realms of monitoring Java programs using a monitor that shares the same JVM. These ideas haven t yet been implemented but the intention is to realise them in the MarQ runtime monitoring tool for QEA Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 3 / 31
4 The Idea At a high level: Parametric trace slicing is a runtime monitoring approach that tracks the behaviour of groups of objects By detecting when some of those objects become garbage we can Optimise the monitoring algorithm Potentially detect violations of co-safety properties But there can be a delay before something is recognised as garbage The idea is to statically identify points where an object will become unreachable to insert explicit garbage events Now I will introduce parametric trace slicing and how it can be improved by garbage detection and then discuss how static analysis can play a part Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 4 / 31
5 Overview 1 Parametric Trace Slicing 2 Online Monitoring and Garbage 3 Static Analysis 4 What s Next? Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 5 / 31
6 Parametric Trace Slicing Used first in tracematches but named and extended to total matching in the JavaMOP work. Later adopted by the QEA language (and others) A solution for parametric runtime monitoring concerned with events that carry parameters The philosophy behind the approach is to slice a trace based on the values of parameters and to consider each slice separately I will introduce the idea by example Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 6 / 31
7 HasNext Example HasNext For every iterator object i (instance of java.util.iterator) we only call i.next() if a preceding call of i.hasnext() returned true with no intermediate calls to i.next() or i.hasnext(). 1 hasnextt(i) next(i) 2 hasnextt(i) hasnextf(i) next(i) hasnextf(i) 3 next(i), hasnextt(i) 4 Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 7 / 31
8 HasNext Example hasnextt(i1) next(i1) hasnextt(i1) hasnextf(i2) next(i2) next(i1) 1 hasnextt(i) next(i) 2 hasnextt(i) hasnextf(i) next(i) hasnextf(i) 3 next(i), hasnextt(i) 4 Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 8 / 31
9 HasNext Example hasnextt(i1) next(i1) hasnextt(i1) hasnextf(i2) next(i2) next(i1) [i i1] [i i2] 1 hasnextt(i) next(i) 2 hasnextt(i) hasnextf(i) next(i) hasnextf(i) 3 next(i), hasnextt(i) 4 Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 8 / 31
10 HasNext Example hasnextt(i1) next(i1) hasnextt(i1) hasnextf(i2) next(i2) next(i1) [i i1] hasnextt(i1) [i i2] 1 hasnextt(i) next(i) 2 hasnextt(i) hasnextf(i) next(i) hasnextf(i) 3 next(i), hasnextt(i) 4 Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 8 / 31
11 HasNext Example hasnextt(i1) next(i1) hasnextt(i1) hasnextf(i2) next(i2) next(i1) [i i1] hasnextt(i1) next(i1) [i i2] 1 hasnextt(i) next(i) 2 hasnextt(i) hasnextf(i) next(i) hasnextf(i) 3 next(i), hasnextt(i) 4 Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 8 / 31
12 HasNext Example hasnextt(i1) next(i1) hasnextt(i1) hasnextf(i2) next(i2) next(i1) [i i1] hasnextt(i1) next(i1) hasnextt(i1) [i i2] 1 hasnextt(i) next(i) 2 hasnextt(i) hasnextf(i) next(i) hasnextf(i) 3 next(i), hasnextt(i) 4 Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 8 / 31
13 HasNext Example hasnextt(i1) next(i1) hasnextt(i1) hasnextf(i2) next(i2) next(i1) [i i1] hasnextt(i1) next(i1) hasnextt(i1) [i i2] hasnextf(i2) 1 hasnextt(i) next(i) 2 hasnextt(i) hasnextf(i) next(i) hasnextf(i) 3 next(i), hasnextt(i) 4 Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 8 / 31
14 HasNext Example hasnextt(i1) next(i1) hasnextt(i1) hasnextf(i2) next(i2) next(i1) [i i1] hasnextt(i1) next(i1) hasnextt(i1) [i i2] hasnextf(i2) next(i2) 1 hasnextt(i) next(i) 2 hasnextt(i) hasnextf(i) next(i) hasnextf(i) 3 next(i), hasnextt(i) 4 Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 8 / 31
15 HasNext Example hasnextt(i1) next(i1) hasnextt(i1) hasnextf(i2) next(i2) next(i1) [i i1] hasnextt(i1) next(i1) hasnextt(i1) next(i1) [i i2] hasnextf(i2) next(i2) 1 hasnextt(i) next(i) 2 hasnextt(i) hasnextf(i) next(i) hasnextf(i) 3 next(i), hasnextt(i) 4 Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 8 / 31
16 HasNext Example hasnextt(i1) next(i1) hasnextt(i1) hasnextf(i2) next(i2) next(i1) [i i1] hasnextt(i1) next(i1) hasnextt(i1) next(i1) [i i2] hasnextf(i2) next(i2) 1 hasnextt(i) next(i) 2 hasnextt(i) hasnextf(i) next(i) hasnextf(i) 3 next(i), hasnextt(i) 4 Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 8 / 31
17 HasNext Example hasnextt(i1) next(i1) hasnextt(i1) hasnextf(i2) next(i2) next(i1) [i i1] hasnextt(i1) next(i1) hasnextt(i1) next(i1) [i i2] hasnextf(i2) next(i2) 1 hasnextt(i) next(i) 2 hasnextt(i) hasnextf(i) next(i) hasnextf(i) 3 next(i), hasnextt(i) 4 Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 8 / 31
18 HasNext Example hasnextt(i1) next(i1) hasnextt(i1) hasnextf(i2) next(i2) next(i1) [i i1] hasnextt(i1) next(i1) hasnextt(i1) next(i1) [i i2] hasnextf(i2) next(i2) 1 hasnextt(i) next(i) 2 hasnextt(i) hasnextf(i) next(i) hasnextf(i) 3 next(i), hasnextt(i) 4 Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 8 / 31
19 HasNext Example hasnextt(i1) next(i1) hasnextt(i1) hasnextf(i2) next(i2) next(i1) [i i1] hasnextt(i1) next(i1) hasnextt(i1) next(i1) [i i2] hasnextf(i2) next(i2) 1 hasnextt(i) next(i) 2 hasnextt(i) hasnextf(i) next(i) hasnextf(i) 3 next(i), hasnextt(i) 4 Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 8 / 31
20 HasNext Example hasnextt(i1) next(i1) hasnextt(i1) hasnextf(i2) next(i2) next(i1) [i i1] hasnextt(i1) next(i1) hasnextt(i1) next(i1) [i i2] hasnextf(i2) next(i2) 1 hasnextt(i) next(i) 2 hasnextt(i) hasnextf(i) next(i) hasnextf(i) 3 next(i), hasnextt(i) 4 Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 8 / 31
21 HasNext Example hasnextt(i1) next(i1) hasnextt(i1) hasnextf(i2) next(i2) next(i1) [i i1] hasnextt(i1) next(i1) hasnextt(i1) next(i1) [i i2] hasnextf(i2) next(i2) X 1 hasnextt(i) next(i) 2 hasnextt(i) hasnextf(i) next(i) hasnextf(i) 3 next(i), hasnextt(i) 4 Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 8 / 31
22 UnsafeIter Example UnsafeIter For every collection c and iterator object i created from c, the iterator i is not used (e.g. by calls to i.next()) after c has been updated. use(i) update(c) create(c, i) update(c) use(i) Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 9 / 31
23 UnsafeIter Example create(a, i1) use(i1) create(a, i2) use(i2) update(a) use(i1) use(i) update(c) create(c, i) update(c) use(i) Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 10 / 31
24 UnsafeIter Example create(a, i1) use(i1) create(a, i2) use(i2) update(a) use(i1) [c A, i i1] [c A, i i2] use(i) update(c) create(c, i) update(c) use(i) Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 10 / 31
25 UnsafeIter Example create(a, i1) use(i1) create(a, i2) use(i2) update(a) use(i1) [c A, i i1] create(a, i1) [c A, i i2] use(i) update(c) create(c, i) update(c) use(i) Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 10 / 31
26 UnsafeIter Example create(a, i1) use(i1) create(a, i2) use(i2) update(a) use(i1) [c A, i i1] create(a, i1) use(i1) [c A, i i2] use(i) update(c) create(c, i) update(c) use(i) Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 10 / 31
27 UnsafeIter Example create(a, i1) use(i1) create(a, i2) use(i2) update(a) use(i1) [c A, i i1] create(a, i1) use(i1) [c A, i i2] create(a, i2) use(i) update(c) create(c, i) update(c) use(i) Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 10 / 31
28 UnsafeIter Example create(a, i1) use(i1) create(a, i2) use(i2) update(a) use(i1) [c A, i i1] create(a, i1) use(i1) [c A, i i2] create(a, i2) use(i2) use(i) update(c) create(c, i) update(c) use(i) Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 10 / 31
29 UnsafeIter Example create(a, i1) use(i1) create(a, i2) use(i2) update(a) use(i1) [c A, i i1] create(a, i1) use(i1) update(a) [c A, i i2] create(a, i2) use(i2) update(a) use(i) update(c) create(c, i) update(c) use(i) Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 10 / 31
30 UnsafeIter Example create(a, i1) use(i1) create(a, i2) use(i2) update(a) use(i1) [c A, i i1] create(a, i1) use(i1) update(a) use(i1) [c A, i i2] create(a, i2) use(i2) update(a) use(i) update(c) create(c, i) update(c) use(i) Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 10 / 31
31 UnsafeIter Example create(a, i1) use(i1) create(a, i2) use(i2) update(a) use(i1) [c A, i i1] create(a, i1) use(i1) update(a) use(i1) [c A, i i2] create(a, i2) use(i2) update(a) use(i) update(c) create(c, i) update(c) use(i) Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 10 / 31
32 UnsafeIter Example create(a, i1) use(i1) create(a, i2) use(i2) update(a) use(i1) [c A, i i1] create(a, i1) use(i1) update(a) use(i1) [c A, i i2] create(a, i2) use(i2) update(a) use(i) update(c) create(c, i) update(c) use(i) Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 10 / 31
33 UnsafeIter Example create(a, i1) use(i1) create(a, i2) use(i2) update(a) use(i1) [c A, i i1] create(a, i1) use(i1) update(a) use(i1) [c A, i i2] create(a, i2) use(i2) update(a) use(i) update(c) create(c, i) update(c) use(i) Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 10 / 31
34 UnsafeIter Example create(a, i1) use(i1) create(a, i2) use(i2) update(a) use(i1) [c A, i i1] create(a, i1) use(i1) update(a) use(i1) X [c A, i i2] create(a, i2) use(i2) update(a) use(i) update(c) create(c, i) update(c) use(i) Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 10 / 31
35 UnsafeIter Example create(a, i1) use(i1) create(a, i2) use(i2) update(a) use(i1) [c A, i i1] create(a, i1) use(i1) update(a) use(i1) X [c A, i i2] create(a, i2) use(i2) update(a) use(i) update(c) create(c, i) update(c) use(i) Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 10 / 31
36 UnsafeIter Example create(a, i1) use(i1) create(a, i2) use(i2) update(a) use(i1) [c A, i i1] create(a, i1) use(i1) update(a) use(i1) X [c A, i i2] create(a, i2) use(i2) update(a) use(i) update(c) create(c, i) update(c) use(i) Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 10 / 31
37 UnsafeIter Example create(a, i1) use(i1) create(a, i2) use(i2) update(a) use(i1) [c A, i i1] create(a, i1) use(i1) update(a) use(i1) X [c A, i i2] create(a, i2) use(i2) update(a) use(i) update(c) create(c, i) update(c) use(i) Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 10 / 31
38 OpenClose Example OpenClose For every file object f, the file cannot be written to or closed if not opened, cannot be opened once already open, and must eventually be closed once opened. open(f) close(f), write(f) 1 2 close(f) write(f) 3 open(f) Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 11 / 31
39 OpenClose Example open(a) open(b) write(a) write(b) close(b) open(f) close(f), write(f) 1 2 close(f) write(f) 3 open(f) Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 12 / 31
40 OpenClose Example open(a) open(b) write(a) write(b) close(b) [f A] [f B] open(f) close(f), write(f) 1 2 close(f) write(f) 3 open(f) Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 12 / 31
41 OpenClose Example open(a) open(b) write(a) write(b) close(b) [f A] open(a) [f B] open(f) close(f), write(f) 1 2 close(f) write(f) 3 open(f) Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 12 / 31
42 OpenClose Example open(a) open(b) write(a) write(b) close(b) [f A] open(a) [f B] open(b) open(f) close(f), write(f) 1 2 close(f) write(f) 3 open(f) Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 12 / 31
43 OpenClose Example open(a) open(b) write(a) write(b) close(b) [f A] open(a) write(a) [f B] open(b) open(f) close(f), write(f) 1 2 close(f) write(f) 3 open(f) Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 12 / 31
44 OpenClose Example open(a) open(b) write(a) write(b) close(b) [f A] open(a) write(a) [f B] open(b) write(b) open(f) close(f), write(f) 1 2 close(f) write(f) 3 open(f) Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 12 / 31
45 OpenClose Example open(a) open(b) write(a) write(b) close(b) [f A] open(a) write(a) [f B] open(b) write(b) close(b) open(f) close(f), write(f) 1 2 close(f) write(f) 3 open(f) Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 12 / 31
46 OpenClose Example open(a) open(b) write(a) write(b) close(b) [f A] open(a) write(a) [f B] open(b) write(b) close(b) open(f) 1 2 close(f) close(f), write(f) write(f) 3 open(f) Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 12 / 31
47 OpenClose Example open(a) open(b) write(a) write(b) close(b) [f A] open(a) write(a) X [f B] open(b) write(b) close(b) open(f) 1 2 close(f) close(f), write(f) write(f) 3 open(f) Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 12 / 31
48 OpenClose Example open(a) open(b) write(a) write(b) close(b) [f A] open(a) write(a) X [f B] open(b) write(b) close(b) open(f) 1 2 close(f) close(f), write(f) write(f) 3 open(f) Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 12 / 31
49 OpenClose Example open(a) open(b) write(a) write(b) close(b) [f A] open(a) write(a) X [f B] open(b) write(b) close(b) open(f) 1 2 close(f) close(f), write(f) write(f) 3 open(f) Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 12 / 31
50 OpenClose Example open(a) open(b) write(a) write(b) close(b) [f A] open(a) write(a) X [f B] open(b) write(b) close(b) open(f) 1 2 close(f) close(f), write(f) write(f) 3 open(f) Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 12 / 31
51 Overview 1 Parametric Trace Slicing 2 Online Monitoring and Garbage 3 Static Analysis 4 What s Next? Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 13 / 31
52 The (basic) online monitoring algorithm Not particularly important - but notice it depends on the size of Lookup, which is dependent on the number of objects being monitored. 1: Let Lookup be a map from valuations to states initial mapping the empty valuation to the initial state 2: for event e(θ) τ do 3: for θ in dom(lookup) from biggest to smallest do 4: if θ is consistent with θ then 5: if θ θ then 6: Update Lookup(θ ) using e 7: else if θ θ is not in dom(lookup) then 8: Add θ θ to Lookup using Lookup(θ ) updated using e 9: if an entry in Lookup is in a non-accepting state then Fail 10: else Accept Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 14 / 31
53 Typical Monitoring Setup Events are generated by AspectJ and references to monitored objects are passed directly to the monitor The monitor stores bindings of these objects associated with the current state of the associated automaton and searches these for each new event So the monitor holds direct references into the memory of the monitored program Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 15 / 31
54 Garbage-Related Issues Monitoring Overhead Overhead is dependent on number of monitored objects There are optimisations that reduce the dependency but it still exists Keeping objects that no longer contribute is inefficient Memory leaks Keeping objects alive after they should die is a memory leak and can significantly change the behaviour of the monitored program Anticipation If we remove an object we need to ensure that no associated slices are in a non-accepting state where acceptance is now unreachable Conversely, we have the chance of detecting such cases before the end of the program Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 16 / 31
55 Weak Reference Solution This is the typical approach (taken by tracematches, JavaMOP, RuleR, optionally in MarQ, and other tools as well) Wrap every monitored object in a java.lang.ref.weakreference In some cases can use implicitly collected objects such as java.util.weakhashmap (or more likely custom-variants) But in other cases, explicit clearing of such objects is required In either case it is sometimes necessary to detect when an object becomes garbage in case further action is required (e.g. if file A became garbage in the OpenClose example). Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 17 / 31
56 Explicit Garbage Event Solution Optional in MarQ Idea: Separate identification of garbage from how it is handled in the monitor Implicitly extend QEA with so-called garbage events Generate garbage events whenever garbage is observed To generate garbage events, create a special object that is only referenced by the monitored object via a collection such that its collection triggers an event We can think of this as a monitor that only detects garbage and whose verdicts are those objects that become garbage Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 18 / 31
57 Explicitly Adding Garbage Events A state is a failure state if no accepting state can be reached. A state is a success state if no non-accepting state can be reached. Add a garbage event to each state to either a failure or success state open(f) 1 2 close(f) close(f), write(f) open(f) 3 write(f) Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 19 / 31
58 Explicitly Adding Garbage Events A state is a failure state if no accepting state can be reached. A state is a success state if no non-accepting state can be reached. Add a garbage event to each state to either a failure or success state Σ 4 open(f) garbage(f) 1 close(f) 2 write(f) close(f), write(f) 3 open(f), garbage(f) Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 19 / 31
59 Overview 1 Parametric Trace Slicing 2 Online Monitoring and Garbage 3 Static Analysis 4 What s Next? Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 20 / 31
60 Where Static Analysis fits in We assume that events relate to program points, usually method calls (e.g. via AspectJ) We will 1. Consider ways to statically determine pairs of program points A and B where objects created at point A will become unreachable at point B 2. Consider various ways in which this information can improve runtime monitoring based on parametric trace slicing Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 21 / 31
61 Small Example Program p u b l i c s t a t i c void w r i t e T o F i l e ( S t r i n g filename, C o l l e c t i o n r e c o r d s ){ F i l e f i l e = new F i l e ( f i l e N a m e ) ; f i l e. open ( ) ; I t e r a t o r i t e r a t o r = r e c o r d s. i t e r a t o r ( ) ; while ( i t e r a t o r. hasnext ( ) ) { f i l e. w r i t e ( i t e r a t o r. next ( ) ) ; } r e c o r d s. r e m o v e A l l ( ) ; } A points where an object is introduced new File(fileName) records.iterator() (factory method) B points where an object becomes unreachable End of loop e.g. after last usage Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 22 / 31
62 Small Example Program p u b l i c s t a t i c void w r i t e T o F i l e ( S t r i n g filename, C o l l e c t i o n r e c o r d s ){ F i l e f i l e = new F i l e ( f i l e N a m e ) ; f i l e. open ( ) ; I t e r a t o r i t e r a t o r = r e c o r d s. i t e r a t o r ( ) ; while ( i t e r a t o r. hasnext ( ) ) { f i l e. w r i t e ( i t e r a t o r. next ( ) ) ; } r e c o r d s. r e m o v e A l l ( ) ; } statically satisfies HasNext as iterator is local assuming we identify iterator() as a factory method statically satisfies UnsafeIter for this iterator but need to track collection as it escapes statically violates OpenClose as the local file is not closed Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 23 / 31
63 Escape Analysis Determines if an object escapes a method Uses pointer-analysis to track abstract objects Typically flow-insensitive and intraprocedural File file = new File ( filename ); file. open (); file. write ( iterator. next ()); Iterator iterator = records. iterator (); while ( iterator. hasnext ()) file. write ( iterator. next ()); Objects only accessed, so file and iterator do not escape. Requires us to identify iterator as a factory method Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 24 / 31
64 Free-me Analysis Works on the call-flow graph of a program. Designed for explicit freeing. Flow insensitive pointer analysis to identify abstract objects Start with set of assignments Propagate via assignments, accesses etc Represent globally reachable objects as one Method summaries Summarise a method by how it treats its input variables An input variable is either returned, becomes globally reachable, or becomes reachable from another input parameter Can also identify pure and factory methods Liveness analysis Backwards flow-sensitive analysis to detect reachability Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 25 / 31
65 Statically Generating Garbage Events Once we have points A and B we can insert explicit garbage events at B points. Unlike free-me analysis, we can organise things so that it does not matter if we create multiple garbage events for the same object. This allows Earlier generation of garbage events Earlier anticipation of failure However, this is limited to shortly lived objects (i.e. that become locally unreachable) and such objects are often garbage collected reasonably quickly. In the extreme case, we could use this information to inline monitoring and make it stack-based. However, in such cases static techniques would hopefully be able to statically check the property. Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 26 / 31
66 Supporting Offline Monitoring Where else can this idea help? In Offline monitoring it is necessary to record the identity of objects. Typically this is done using IdentityHashCode but this is not unique across garbage collections. Idea: record garbage events to allow to replay garbage collection offline. This now becomes a point of correctness rather than efficiency Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 27 / 31
67 Minimally Monitoring Abstract Objects If an object O is created in method M and O does not escape M then we can enumerate the N paths O can take through M and once we have observed all N paths we can stop monitoring M. Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 28 / 31
68 Minimally Monitoring Abstract Objects If an object O is created in method M and O does not escape M then we can enumerate the N paths O can take through M and once we have observed all N paths we can stop monitoring M. The requirement for O to escape M can be relaxed such that we stop monitoring an object if it takes a path that has already been monitored, allowing for some paths to always require monitoring e.g. if O escapes. Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 28 / 31
69 Minimally Monitoring Abstract Objects If an object O is created in method M and O does not escape M then we can enumerate the N paths O can take through M and once we have observed all N paths we can stop monitoring M. The requirement for O to escape M can be relaxed such that we stop monitoring an object if it takes a path that has already been monitored, allowing for some paths to always require monitoring e.g. if O escapes. One can also restrict this to path prefixes Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 28 / 31
70 Minimally Monitoring Abstract Objects If an object O is created in method M and O does not escape M then we can enumerate the N paths O can take through M and once we have observed all N paths we can stop monitoring M. The requirement for O to escape M can be relaxed such that we stop monitoring an object if it takes a path that has already been monitored, allowing for some paths to always require monitoring e.g. if O escapes. One can also restrict this to path prefixes This is similar to earlier work that attempted to detect loops where only a constant number of iterations of that loop required monitoring. Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 28 / 31
71 Minimally Monitoring Abstract Objects If an object O is created in method M and O does not escape M then we can enumerate the N paths O can take through M and once we have observed all N paths we can stop monitoring M. The requirement for O to escape M can be relaxed such that we stop monitoring an object if it takes a path that has already been monitored, allowing for some paths to always require monitoring e.g. if O escapes. One can also restrict this to path prefixes This is similar to earlier work that attempted to detect loops where only a constant number of iterations of that loop required monitoring. This extends the idea of explicitly adding garbage events to the idea of statically noticing redundant objects i.e. those whose behaviour has been necessarily monitored previously. Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 28 / 31
72 Overview 1 Parametric Trace Slicing 2 Online Monitoring and Garbage 3 Static Analysis 4 What s Next? Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 29 / 31
73 Implement it Plan to implement ideas in an analysis agnostic way i.e. using a set of pairs of program points make use of existing implementations for static analysis to suggest such pairs integrate into the MarQ monitoring tool Missing QEA features Free variables: reachability can be over-aproximated in analysis Existential quantification: unclear if anything can be done Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 30 / 31
74 Risks, Limitations and Conclusions Risks and Limitations As mentioned, mostly applies to short-lived objects that are garbage collected quickly anyway as very difficult to lift to an inter-procedural analysis However, in most cases an under-approximation of unreachable objects can be useful Cases where it can be applied might also be able to be fully statically verified using typestate analysis Conclusions Need to try it out and see Giles Reger A Story of Parametric Trace Slicing, Garbage and Static Analysis 31 / 31
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