com.google.javascript.jscomp

Class LiveVariablesAnalysis

java.lang.Object

com.google.javascript.jscomp.DataFlowAnalysis<Node,LiveVariablesAnalysis.LiveVariableLattice>

com.google.javascript.jscomp.LiveVariablesAnalysis

class LiveVariablesAnalysis
extends DataFlowAnalysis<Node,LiveVariablesAnalysis.LiveVariableLattice>

Compute the "liveness" of all local variables. A variable is "live" at a point of a program if the value it is currently holding might be read later. Otherwise, the variable is considered "dead" if we know for sure that it will no longer be read. Dead variables are candidates for dead assignment elimination and variable name sharing. The worst case safe assumption is to assume that all variables are live. In that case, we will have no opportunity for optimizations. This is especially the case within a TRY block when an assignment is not guaranteed to take place. We bail out by assuming that all variables are live.

Due to the possibility of inner functions and closures, certain "local" variables can escape the function. These variables will be considered as global and they can be retrieved with getEscapedLocals().

Nested Class Summary

Nested Classes

Modifier and Type	Class and Description
private static class	LiveVariablesAnalysis.LiveVariableJoinOp
(package private) static class	LiveVariablesAnalysis.LiveVariableLattice The lattice that stores the liveness of all local variables at a given point in the program.

Nested classes/interfaces inherited from class com.google.javascript.jscomp.DataFlowAnalysis

DataFlowAnalysis.BranchedFlowState<L extends LatticeElement>, DataFlowAnalysis.BranchedForwardDataFlowAnalysis<N,L extends LatticeElement>, DataFlowAnalysis.FlowState<L extends LatticeElement>, DataFlowAnalysis.MaxIterationsExceededException

Field Summary

Fields

Modifier and Type	Field and Description
static java.lang.String	ARGUMENT_ARRAY_ALIAS
private java.util.Set<Var>	escaped
private Scope	jsScope
(package private) static int	MAX_VARIABLES_TO_ANALYZE

Fields inherited from class com.google.javascript.jscomp.DataFlowAnalysis

joinOp, MAX_STEPS, orderedWorkSet

Constructor Summary

Constructors

Constructor and Description
LiveVariablesAnalysis(ControlFlowGraph<Node> cfg, Scope jsScope, AbstractCompiler compiler)

Method Summary

Modifier and Type	Method and Description
private void	addToSetIfLocal(Node node, BitSet set)
private void	computeGenKill(Node n, BitSet gen, BitSet kill, boolean conditional) Computes the GEN and KILL set.
(package private) LiveVariablesAnalysis.LiveVariableLattice	createEntryLattice() Gets the incoming state of the entry node.
(package private) LiveVariablesAnalysis.LiveVariableLattice	createInitialEstimateLattice() Gets the state of the initial estimation at each node.
(package private) LiveVariablesAnalysis.LiveVariableLattice	flowThrough(Node node, LiveVariablesAnalysis.LiveVariableLattice input) Computes the output state for a given node and input state.
java.util.Set<? extends Var>	getEscapedLocals()
int	getVarIndex(java.lang.String var)
private boolean	isArgumentsName(Node n)
(package private) boolean	isForward() Checks whether the analysis is a forward flow analysis or backward flow analysis.
(package private) void	markAllParametersEscaped() Give up computing liveness of formal parameter by putting all the parameter names in the escaped set.

Methods inherited from class com.google.javascript.jscomp.DataFlowAnalysis

analyze, analyze, computeEscaped, flow, getCfg, getExitLatticeElement, initialize, join, joinInputs

Methods inherited from class java.lang.Object

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

Field Detail

MAX_VARIABLES_TO_ANALYZE

static final int MAX_VARIABLES_TO_ANALYZE

See Also:

Constant Field Values

ARGUMENT_ARRAY_ALIAS

public static final java.lang.String ARGUMENT_ARRAY_ALIAS

See Also:

Constant Field Values

jsScope

private final Scope jsScope

escaped

private final java.util.Set<Var> escaped

Constructor Detail

LiveVariablesAnalysis

LiveVariablesAnalysis(ControlFlowGraph<Node> cfg,
                      Scope jsScope,
                      AbstractCompiler compiler)

Method Detail

getEscapedLocals

public java.util.Set<? extends Var> getEscapedLocals()

getVarIndex

public int getVarIndex(java.lang.String var)

isForward

boolean isForward()

Description copied from class: DataFlowAnalysis

Checks whether the analysis is a forward flow analysis or backward flow analysis.

Specified by:

isForward in class DataFlowAnalysis<Node,LiveVariablesAnalysis.LiveVariableLattice>

Returns:

true if it is a forward analysis.

createEntryLattice

LiveVariablesAnalysis.LiveVariableLattice createEntryLattice()

Description copied from class: DataFlowAnalysis

Gets the incoming state of the entry node.

Specified by:

createEntryLattice in class DataFlowAnalysis<Node,LiveVariablesAnalysis.LiveVariableLattice>

Returns:

Entry state.

createInitialEstimateLattice

LiveVariablesAnalysis.LiveVariableLattice createInitialEstimateLattice()

Description copied from class: DataFlowAnalysis

Gets the state of the initial estimation at each node.

Specified by:

createInitialEstimateLattice in class DataFlowAnalysis<Node,LiveVariablesAnalysis.LiveVariableLattice>

Returns:

Initial state.

flowThrough

LiveVariablesAnalysis.LiveVariableLattice flowThrough(Node node,
                                                      LiveVariablesAnalysis.LiveVariableLattice input)

Description copied from class: DataFlowAnalysis

Computes the output state for a given node and input state.

Specified by:

flowThrough in class DataFlowAnalysis<Node,LiveVariablesAnalysis.LiveVariableLattice>

Parameters:

node - The node.

input - Input lattice that should be read-only.

Returns:

Output lattice.

computeGenKill

private void computeGenKill(Node n,
                            BitSet gen,
                            BitSet kill,
                            boolean conditional)

Computes the GEN and KILL set.

Parameters:

n - Root node.

gen - Local variables that are live because of the instruction at n will be added to this set.

kill - Local variables that are killed because of the instruction at n will be added to this set.

conditional - true if any assignments encountered are conditionally executed. These assignments might not kill a variable.

addToSetIfLocal

private void addToSetIfLocal(Node node,
                             BitSet set)

markAllParametersEscaped

void markAllParametersEscaped()

Give up computing liveness of formal parameter by putting all the parameter names in the escaped set.

isArgumentsName

private boolean isArgumentsName(Node n)