com.ibm.wala.analysis.arraybounds

Class ArrayBoundsGraph

java.lang.Object

com.ibm.wala.analysis.arraybounds.hypergraph.DirectedHyperGraph<java.lang.Integer>

com.ibm.wala.analysis.arraybounds.ArrayBoundsGraph

public class ArrayBoundsGraph
extends DirectedHyperGraph<java.lang.Integer>

Some thoughts about implementation details, not mentioned in [1]:

As it is written The paper describes, that the distance is equal to the shortest hyper path. But what if we don't know anything about a variable (i.e. it is returned by a method)? There will be no path at all, the distance should be unlimited.

Initializing all nodes with -infinity instead of infinity, seems to work at first glance, as we also have hyper edges with more than one source, which cause the maximum to be propagated instead of minimum. However, this will not work, as loops will not get updated properly.

We need to make sure, that only nodes, which are not connected to the source of shortest path computation are set to infinity. To do so, it is enough to set nodes, which don't have a predecessor to infinity. (Nodes in cycles will always have an ancestor, which is not part of the cycle. So all nodes are either connected to the source, or a node with no predecessor.)

In this implementation this is done, by adding an infinity node and connect all lose ends to it (see ArrayBoundsGraphBuilder.bundleDeadEnds(ArrayBoundsGraph)). Note, that array length and the zero node are dead ends, if they are not the source of a shortest path computation. To prevent changing the inequality graph, depending on which source is used, a kind of trap door construct is used (See createSourceVar(Integer)).

There are some variations, but these are minor changes to improve results:

• handling of constants (see addConstant(Integer, Integer))
• pi nodes (see addPhi(Integer))
• array length nodes (see arrayLength)

[1] Bodík, Rastislav, Rajiv Gupta, and Vivek Sarkar. "ABCD: eliminating array bounds checks on demand." ACM SIGPLAN Notices. Vol. 35. No. 5. ACM, 2000.

Field Summary

Fields

Modifier and Type	Field and Description
static java.lang.Integer	UNLIMITED We need a ssa variable representing unlimited (values we don't know anything about).
static java.lang.Integer	ZERO We need a ssa variable representing zero.
static java.lang.Integer	ZERO_HELPER

Constructor Summary

Constructors

Constructor and Description
ArrayBoundsGraph()

Method Summary

Modifier and Type	Method and Description
void	addAdditionEdge(java.lang.Integer src, java.lang.Integer dst, java.lang.Integer value)
void	addArray(java.lang.Integer array)
void	addConstant(java.lang.Integer variable, java.lang.Integer value) Add variable as constant with value value.
void	addEdge(java.lang.Integer src, java.lang.Integer dst)
HyperNode<java.lang.Integer>	addNode(java.lang.Integer value)
void	addPhi(java.lang.Integer dst)
void	addPi(java.lang.Integer dst, java.lang.Integer src1, java.lang.Integer src2)
void	createSourceVar(java.lang.Integer var) Adds var as source var.
java.lang.Integer	generateNewVar()
java.util.HashMap<java.lang.Integer,java.util.Set<java.lang.Integer>>	getArrayAccess()
java.util.HashMap<java.lang.Integer,java.lang.Integer>	getArrayLength()
java.lang.Integer	getArrayNode(java.lang.Integer array)
java.util.HashSet<java.lang.Integer>	getPhis()
Weight	getVariableWeight(java.lang.Integer variable)
void	markAsArrayAccess(java.lang.Integer array, java.lang.Integer index)
void	markAsArrayLength(java.lang.Integer array, java.lang.Integer variable) Mark variable as length for array.
void	markAsDeadEnd(java.lang.Integer variable)
void	postProcessConstants()
void	reset() Resets the weight of all nodes.

Methods inherited from class com.ibm.wala.analysis.arraybounds.hypergraph.DirectedHyperGraph

getEdges, getNodes, toString, updateNodeEdges

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

Field Detail

ZERO

public static final java.lang.Integer ZERO

We need a ssa variable representing zero. So we just use an integer, which is never produced by ssa generation

ZERO_HELPER

public static final java.lang.Integer ZERO_HELPER

UNLIMITED

public static final java.lang.Integer UNLIMITED

We need a ssa variable representing unlimited (values we don't know anything about). So we just use an integer, which is never produced by ssa generation

Constructor Detail

ArrayBoundsGraph

public ArrayBoundsGraph()

Method Detail

postProcessConstants

public void postProcessConstants()

addAdditionEdge

public void addAdditionEdge(java.lang.Integer src,
                            java.lang.Integer dst,
                            java.lang.Integer value)

addArray

public void addArray(java.lang.Integer array)

addConstant

public void addConstant(java.lang.Integer variable,
                        java.lang.Integer value)

Add variable as constant with value value.

This will create the following construct: [zero] -(value)-> [h1] -0- > [variable] -(-value)-> [h2] -0-> [zero].

The bidirectional linking, allows things like

 int[] a = new int[2]();
 a[0] = 1;
 

to work properly. h1, h2 are helper nodes: [zero] and [variable] may have other predecessors, this will cause their in edges to be merged to a single hyper edge with weight zero. The helper nodes are inserted to keep the proper distance from [zero].

addEdge

public void addEdge(java.lang.Integer src,
                    java.lang.Integer dst)

addNode

public HyperNode<java.lang.Integer> addNode(java.lang.Integer value)

addPhi

public void addPhi(java.lang.Integer dst)

addPi

public void addPi(java.lang.Integer dst,
                  java.lang.Integer src1,
                  java.lang.Integer src2)

createSourceVar

public void createSourceVar(java.lang.Integer var)

Adds var as source var. A source var is a variable, which can be used as source for shortest path computation.

This will create the following construct: [unlimited] -> [var] -> [var] -(unlimited)-> [unlimited]

This is a trap door construct: if [var] is not set to 0 it will get the value unlimited, if [var] is set to 0 it will stay 0.

generateNewVar

public java.lang.Integer generateNewVar()

getArrayAccess

public java.util.HashMap<java.lang.Integer,java.util.Set<java.lang.Integer>> getArrayAccess()

getArrayLength

public java.util.HashMap<java.lang.Integer,java.lang.Integer> getArrayLength()

getArrayNode

public java.lang.Integer getArrayNode(java.lang.Integer array)

getPhis

public java.util.HashSet<java.lang.Integer> getPhis()

markAsArrayAccess

public void markAsArrayAccess(java.lang.Integer array,
                              java.lang.Integer index)

markAsArrayLength

public void markAsArrayLength(java.lang.Integer array,
                              java.lang.Integer variable)

Mark variable as length for array.

markAsDeadEnd

public void markAsDeadEnd(java.lang.Integer variable)

getVariableWeight

public Weight getVariableWeight(java.lang.Integer variable)

reset

public void reset()

Description copied from class: DirectedHyperGraph

Resets the weight of all nodes.

Overrides:

reset in class DirectedHyperGraph<java.lang.Integer>