com.google.javascript.jscomp

Class DeadAssignmentsElimination

java.lang.Object

com.google.javascript.jscomp.NodeTraversal.AbstractPostOrderCallback

com.google.javascript.jscomp.DeadAssignmentsElimination

All Implemented Interfaces:

CompilerPass, NodeTraversal.Callback, NodeTraversal.ScopedCallback

class DeadAssignmentsElimination
extends NodeTraversal.AbstractPostOrderCallback
implements CompilerPass, NodeTraversal.ScopedCallback

Removes local variable assignments that are useless based on information from LiveVariablesAnalysis. If there is an assignment to variable x and x is dead after this assignment, we know that the current content of x will not be read and this assignment is useless.

Nested Class Summary

Nested Classes

Modifier and Type	Class and Description
private static class	DeadAssignmentsElimination.VariableLiveness

Field Summary

Fields

Modifier and Type	Field and Description
private AbstractCompiler	compiler
private LiveVariablesAnalysis	liveness
private static com.google.common.base.Predicate<Node>	matchRemovableAssigns

Constructor Summary

Constructors

Constructor and Description
DeadAssignmentsElimination(AbstractCompiler compiler)

Method Summary

Modifier and Type	Method and Description
private DeadAssignmentsElimination.VariableLiveness	checkHookBranchReadBeforeKill(Node trueCase, Node falseCase, java.lang.String variable)
void	enterScope(NodeTraversal t) Called immediately after entering a new scope.
void	exitScope(NodeTraversal t) Called immediately before exiting a scope.
private DeadAssignmentsElimination.VariableLiveness	isVariableReadBeforeKill(Node n, java.lang.String variable) Give an expression and a variable.
private boolean	isVariableStillLiveWithinExpression(Node n, Node exprRoot, java.lang.String variable) Given a variable, node n in the tree and a sub-tree denoted by exprRoot as the root, this function returns true if there exists a read of that variable before a write to that variable that is on the right side of n.
void	process(Node externs, Node root) Process the JS with root node root.
private void	tryRemoveAssignment(NodeTraversal t, Node n, DataFlowAnalysis.FlowState<LiveVariablesAnalysis.LiveVariableLattice> state)
private void	tryRemoveAssignment(NodeTraversal t, Node n, Node exprRoot, DataFlowAnalysis.FlowState<LiveVariablesAnalysis.LiveVariableLattice> state) Determines if any local variables are dead after the instruction n and are assigned within the subtree of n.
private void	tryRemoveDeadAssignments(NodeTraversal t, ControlFlowGraph<Node> cfg) Try to remove useless assignments from a control flow graph that has been annotated with liveness information.
void	visit(NodeTraversal t, Node n, Node parent) Visits a node in postorder (after its children have been visited).

Methods inherited from class com.google.javascript.jscomp.NodeTraversal.AbstractPostOrderCallback

shouldTraverse

Methods inherited from class java.lang.Object

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

Methods inherited from interface com.google.javascript.jscomp.NodeTraversal.Callback

shouldTraverse

Field Detail

compiler

private final AbstractCompiler compiler

liveness

private LiveVariablesAnalysis liveness

matchRemovableAssigns

private static final com.google.common.base.Predicate<Node> matchRemovableAssigns

Constructor Detail

DeadAssignmentsElimination

public DeadAssignmentsElimination(AbstractCompiler compiler)

Method Detail

process

public void process(Node externs,
                    Node root)

Description copied from interface: CompilerPass

Process the JS with root node root. Can modify the contents of each Node tree

Specified by:

process in interface CompilerPass

Parameters:

externs - Top of external JS tree

root - Top of JS tree

enterScope

public void enterScope(NodeTraversal t)

Description copied from interface: NodeTraversal.ScopedCallback

Called immediately after entering a new scope. The new scope can be accessed through t.getScope()

Specified by:

enterScope in interface NodeTraversal.ScopedCallback

exitScope

public void exitScope(NodeTraversal t)

Description copied from interface: NodeTraversal.ScopedCallback

Called immediately before exiting a scope. The ending scope can be accessed through t.getScope()

Specified by:

exitScope in interface NodeTraversal.ScopedCallback

visit

public void visit(NodeTraversal t,
                  Node n,
                  Node parent)

Description copied from interface: NodeTraversal.Callback

Visits a node in postorder (after its children have been visited). A node is visited only if all its parents should be traversed (NodeTraversal.Callback.shouldTraverse(NodeTraversal, Node, Node)).

Implementations can have side effects (e.g. modifying the parse tree).

Specified by:

visit in interface NodeTraversal.Callback

tryRemoveDeadAssignments

private void tryRemoveDeadAssignments(NodeTraversal t,
                                      ControlFlowGraph<Node> cfg)

Try to remove useless assignments from a control flow graph that has been annotated with liveness information.

Parameters:

t - The node traversal.

cfg - The control flow graph of the program annotated with liveness information.

tryRemoveAssignment

private void tryRemoveAssignment(NodeTraversal t,
                                 Node n,
                                 DataFlowAnalysis.FlowState<LiveVariablesAnalysis.LiveVariableLattice> state)

tryRemoveAssignment

private void tryRemoveAssignment(NodeTraversal t,
                                 Node n,
                                 Node exprRoot,
                                 DataFlowAnalysis.FlowState<LiveVariablesAnalysis.LiveVariableLattice> state)

Determines if any local variables are dead after the instruction n and are assigned within the subtree of n. Removes those assignments if there are any.

Parameters:

n - Target instruction.

exprRoot - The CFG node where the liveness information in state is still correct.

state - The liveness information at n.

isVariableStillLiveWithinExpression

private boolean isVariableStillLiveWithinExpression(Node n,
                                                    Node exprRoot,
                                                    java.lang.String variable)

Given a variable, node n in the tree and a sub-tree denoted by exprRoot as the root, this function returns true if there exists a read of that variable before a write to that variable that is on the right side of n. For example, suppose the node is x = 1: y = 1, x = 1; // false, there is no reads at all. y = 1, x = 1, print(x) // true, there is a read right of n. y = 1, x = 1, x = 2, print(x) // false, there is a read right of n but // it is after a write.

Parameters:

n - The current node we should look at.

exprRoot - The node

isVariableReadBeforeKill

private DeadAssignmentsElimination.VariableLiveness isVariableReadBeforeKill(Node n,
                                                                             java.lang.String variable)

Give an expression and a variable. It returns READ, if the first reference of that variable is a read. It returns KILL, if the first reference of that variable is an assignment. It returns MAY_LIVE otherwise.

checkHookBranchReadBeforeKill

private DeadAssignmentsElimination.VariableLiveness checkHookBranchReadBeforeKill(Node trueCase,
                                                                                  Node falseCase,
                                                                                  java.lang.String variable)