com.google.javascript.jscomp

Class CoalesceVariableNames

java.lang.Object

com.google.javascript.jscomp.NodeTraversal.AbstractPostOrderCallback

com.google.javascript.jscomp.CoalesceVariableNames

All Implemented Interfaces:

CompilerPass, NodeTraversal.Callback, NodeTraversal.ScopedCallback

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

Reuse variable names if possible.

For example, from var x = 1; print(x); var y = 2; print(y); to var x = 1; print(x); x = 2; print(x). The benefits are slightly shorter code because of the removed var declaration, less unique variables in hope for better renaming, and finally better gzip compression.

The pass operates similar to a typical register allocator found in an optimizing compiler by first computing live ranges with LiveVariablesAnalysis and a variable interference graph. Then it uses graph coloring in GraphColoring to determine which two variables can be merge together safely.

Nested Class Summary

Nested Classes

Modifier and Type	Class and Description
private static class	CoalesceVariableNames.CombinedLiveRangeChecker A simple wrapper calls to call two AbstractCfgNodeTraversalCallback callback during the same traversal.
private static class	CoalesceVariableNames.LiveRangeChecker

Field Summary

Fields

Modifier and Type	Field and Description
private java.util.Deque<GraphColoring<Var,java.lang.Void>>	colorings
private static java.util.Comparator<Var>	coloringTieBreaker
private AbstractCompiler	compiler
private boolean	usePseudoNames

Constructor Summary

Constructors

Constructor and Description
CoalesceVariableNames(AbstractCompiler compiler, boolean usePseudoNames)

Method Summary

Modifier and Type	Method and Description
private UndiGraph<Var,java.lang.Void>	computeVariableNamesInterferenceGraph(NodeTraversal t, ControlFlowGraph<Node> cfg, java.util.Set<Var> escaped)
void	enterScope(NodeTraversal t) Called immediately after entering a new scope.
void	exitScope(NodeTraversal t) Called immediately before exiting a scope.
void	process(Node externs, Node root) Process the JS with root node root.
private static void	removeVarDeclaration(Node name) Tries to remove variable declaration if the variable has been coalesced with another variable that has already been declared.
private static boolean	shouldOptimizeScope(Scope scope)
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

colorings

private final java.util.Deque<GraphColoring<Var,java.lang.Void>> colorings

usePseudoNames

private final boolean usePseudoNames

coloringTieBreaker

private static final java.util.Comparator<Var> coloringTieBreaker

Constructor Detail

CoalesceVariableNames

CoalesceVariableNames(AbstractCompiler compiler,
                      boolean usePseudoNames)

Parameters:

usePseudoNames - For debug purposes, when merging variable foo and bar to foo, rename both variable to foo_bar.

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

shouldOptimizeScope

private static boolean shouldOptimizeScope(Scope scope)

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

computeVariableNamesInterferenceGraph

private UndiGraph<Var,java.lang.Void> computeVariableNamesInterferenceGraph(NodeTraversal t,
                                                                            ControlFlowGraph<Node> cfg,
                                                                            java.util.Set<Var> escaped)

removeVarDeclaration

private static void removeVarDeclaration(Node name)

Tries to remove variable declaration if the variable has been coalesced with another variable that has already been declared.