com.google.gwt.dev.jjs.impl

Class DeadCodeElimination.DeadCodeVisitor

java.lang.Object

com.google.gwt.dev.jjs.ast.JVisitor

com.google.gwt.dev.jjs.ast.JModVisitor

com.google.gwt.dev.jjs.impl.JChangeTrackingVisitor

com.google.gwt.dev.jjs.impl.DeadCodeElimination.DeadCodeVisitor

Enclosing class:

DeadCodeElimination

public class DeadCodeElimination.DeadCodeVisitor
extends JChangeTrackingVisitor

Eliminates dead or unreachable code when possible, and makes local simplifications like changing "x || true" to "x". This visitor should perform all of its optimizations in a single pass. Except in rare cases, running this pass multiple times should produce no changes after the first pass. The only currently known exception to this rule is in endVisit(JNewInstance, Context), where the target constructor may be non-empty at the beginning of DCE and become empty during the run, which potentially unlocks optimizations at call sites. TODO: leverage ignoring expression output more to remove intermediary operations in favor of pure side effects. TODO: move more simplifications into methods like simplifyCast, so that more simplifications can be made on a single pass through a tree.

Field Summary

Fields inherited from class com.google.gwt.dev.jjs.ast.JVisitor

LVALUE_CONTEXT, UNMODIFIABLE_CONTEXT

Constructor Summary

Constructors

Constructor and Description
DeadCodeVisitor(OptimizerContext optimizerCtx)

Method Summary

Modifier and Type	Method and Description
void	endVisit(JBinaryOperation x, Context ctx) Short circuit binary operations.
void	endVisit(JBlock x, Context ctx) Prune dead statements and empty blocks.
void	endVisit(JCastOperation x, Context ctx)
void	endVisit(JConditional x, Context ctx)
void	endVisit(JDeclarationStatement x, Context ctx)
void	endVisit(JDoStatement x, Context ctx) Convert do { } while (false); into a block.
void	endVisit(JExpressionStatement x, Context ctx)
void	endVisit(JFieldRef x, Context ctx) NOTE: not called from JsniFieldRef.
void	endVisit(JForStatement x, Context ctx) Prune for (X; false; Y) statements, but make sure X is run.
void	endVisit(JIfStatement x, Context ctx) Simplify if statements.
void	endVisit(JInstanceOf x, Context ctx) Simplify JInstanceOf expression whose output is ignored.
void	endVisit(JLocalRef x, Context ctx)
void	endVisit(JMethodCall x, Context ctx) Resolve method calls that can be computed statically.
void	endVisit(JMultiExpression x, Context ctx) Remove any parts of JMultiExpression that have no side-effect.
void	endVisit(JNewInstance x, Context ctx)
void	endVisit(JParameterRef x, Context ctx)
void	endVisit(JPostfixOperation x, Context ctx) Replace post-inc/dec with pre-inc/dec if the result doesn't matter.
void	endVisit(JPrefixOperation x, Context ctx) Simplify the ! operator if possible.
void	endVisit(JSwitchStatement x, Context ctx) Optimize switch statements.
void	endVisit(JTryStatement x, Context ctx) 1) Remove catch blocks whose exception type is not instantiable.
void	endVisit(JWhileStatement x, Context ctx) Prune while (false) statements.
boolean	visit(JBinaryOperation x, Context ctx)
boolean	visit(JClassType x, Context ctx)
boolean	visit(JDeclarationStatement x, Context ctx)
boolean	visit(JExpressionStatement x, Context ctx)
boolean	visit(JMethodCall x, Context ctx) NOTE: not called from JsniMethodRef.
boolean	visit(JMultiExpression x, Context ctx)
boolean	visit(JPostfixOperation x, Context ctx)
boolean	visit(JPrefixOperation x, Context ctx)
boolean	visit(JSwitchStatement x, Context ctx)

Methods inherited from class com.google.gwt.dev.jjs.impl.JChangeTrackingVisitor

endVisit, endVisit, endVisit, endVisit, enter, enter, enter, enter, exit, exit, exit, exit, fieldsWereRemoved, getCurrentField, getCurrentMethod, madeChanges, methodsWereRemoved, visit, visit, visit, visit, wasRemoved, wasRemoved

Methods inherited from class com.google.gwt.dev.jjs.ast.JModVisitor

accept, accept, accept, acceptImmutable, acceptLvalue, acceptWithInsertRemove, acceptWithInsertRemoveImmutable, checkReplacement, didChange, getNumMods, traverse

Methods inherited from class com.google.gwt.dev.jjs.ast.JVisitor

accept, accept, accept, endVisit, endVisit, endVisit, endVisit, endVisit, endVisit, endVisit, endVisit, endVisit, endVisit, endVisit, endVisit, endVisit, endVisit, endVisit, endVisit, endVisit, endVisit, endVisit, endVisit, endVisit, endVisit, endVisit, endVisit, endVisit, endVisit, endVisit, endVisit, endVisit, endVisit, endVisit, endVisit, endVisit, endVisit, endVisit, endVisit, endVisit, endVisit, endVisit, endVisit, endVisit, endVisit, endVisit, endVisit, endVisit, endVisit, endVisit, endVisit, endVisit, endVisit, endVisit, endVisit, endVisit, translateException, visit, visit, visit, visit, visit, visit, visit, visit, visit, visit, visit, visit, visit, visit, visit, visit, visit, visit, visit, visit, visit, visit, visit, visit, visit, visit, visit, visit, visit, visit, visit, visit, visit, visit, visit, visit, visit, visit, visit, visit, visit, visit, visit, visit, visit, visit, visit, visit, visit, visit, visit, visit, visit, visit, visit, visit, visit, visit, visit, visit, visit, visit, visit, visit, visit

Methods inherited from class java.lang.Object

equals, finalize, getClass, hashCode, toString

Constructor Detail

DeadCodeVisitor

public DeadCodeVisitor(OptimizerContext optimizerCtx)

Method Detail

endVisit

public void endVisit(JBinaryOperation x,
                     Context ctx)

Short circuit binary operations.

Overrides:

endVisit in class JVisitor

endVisit

public void endVisit(JBlock x,
                     Context ctx)

Prune dead statements and empty blocks.

Overrides:

endVisit in class JVisitor

endVisit

public void endVisit(JCastOperation x,
                     Context ctx)

Overrides:

endVisit in class JVisitor

endVisit

public void endVisit(JConditional x,
                     Context ctx)

Overrides:

endVisit in class JVisitor

endVisit

public void endVisit(JDeclarationStatement x,
                     Context ctx)

Overrides:

endVisit in class JVisitor

endVisit

public void endVisit(JDoStatement x,
                     Context ctx)

Convert do { } while (false); into a block.

Overrides:

endVisit in class JVisitor

endVisit

public void endVisit(JExpressionStatement x,
                     Context ctx)

Overrides:

endVisit in class JVisitor

endVisit

public void endVisit(JFieldRef x,
                     Context ctx)

Description copied from class: JVisitor

NOTE: not called from JsniFieldRef.

Overrides:

endVisit in class JVisitor

endVisit

public void endVisit(JForStatement x,
                     Context ctx)

Prune for (X; false; Y) statements, but make sure X is run.

Overrides:

endVisit in class JVisitor

endVisit

public void endVisit(JIfStatement x,
                     Context ctx)

Simplify if statements.

Overrides:

endVisit in class JVisitor

endVisit

public void endVisit(JInstanceOf x,
                     Context ctx)

Simplify JInstanceOf expression whose output is ignored.

Overrides:

endVisit in class JVisitor

endVisit

public void endVisit(JLocalRef x,
                     Context ctx)

Overrides:

endVisit in class JVisitor

endVisit

public void endVisit(JMethodCall x,
                     Context ctx)

Resolve method calls that can be computed statically.

Overrides:

endVisit in class JVisitor

endVisit

public void endVisit(JMultiExpression x,
                     Context ctx)

Remove any parts of JMultiExpression that have no side-effect.

Overrides:

endVisit in class JVisitor

endVisit

public void endVisit(JNewInstance x,
                     Context ctx)

Overrides:

endVisit in class JVisitor

endVisit

public void endVisit(JParameterRef x,
                     Context ctx)

Overrides:

endVisit in class JVisitor

endVisit

public void endVisit(JPostfixOperation x,
                     Context ctx)

Replace post-inc/dec with pre-inc/dec if the result doesn't matter.

Overrides:

endVisit in class JVisitor

endVisit

public void endVisit(JPrefixOperation x,
                     Context ctx)

Simplify the ! operator if possible.

Overrides:

endVisit in class JVisitor

endVisit

public void endVisit(JSwitchStatement x,
                     Context ctx)

Optimize switch statements.

Overrides:

endVisit in class JVisitor

endVisit

public void endVisit(JTryStatement x,
                     Context ctx)

1) Remove catch blocks whose exception type is not instantiable. 2) Prune try statements with no body. 3) Hoist up try statements with no catches and an empty finally.

Overrides:

endVisit in class JVisitor

endVisit

public void endVisit(JWhileStatement x,
                     Context ctx)

Prune while (false) statements.

Overrides:

endVisit in class JVisitor

visit

public boolean visit(JBinaryOperation x,
                     Context ctx)

Overrides:

visit in class JVisitor

visit

public boolean visit(JClassType x,
                     Context ctx)

Overrides:

visit in class JVisitor

visit

public boolean visit(JDeclarationStatement x,
                     Context ctx)

Overrides:

visit in class JVisitor

visit

public boolean visit(JExpressionStatement x,
                     Context ctx)

Overrides:

visit in class JVisitor

visit

public boolean visit(JMethodCall x,
                     Context ctx)

Description copied from class: JVisitor

NOTE: not called from JsniMethodRef.

Overrides:

visit in class JVisitor

visit

public boolean visit(JMultiExpression x,
                     Context ctx)

Overrides:

visit in class JVisitor

visit

public boolean visit(JPostfixOperation x,
                     Context ctx)

Overrides:

visit in class JVisitor

visit

public boolean visit(JPrefixOperation x,
                     Context ctx)

Overrides:

visit in class JVisitor

visit

public boolean visit(JSwitchStatement x,
                     Context ctx)

Overrides:

visit in class JVisitor