com.google.gwt.dev.jjs.impl

Class TypeTightener

java.lang.Object

com.google.gwt.dev.jjs.impl.TypeTightener

public class TypeTightener
extends Object

The purpose of this pass is to record "type flow" information and then use the information to infer places where "tighter" (that is, more specific) types can be inferred for locals, fields, parameters, and method return types. We also optimize dynamic casts and instanceof operations. Examples: This declaration of variable foo:

 final List foo = new ArrayList();
 

can be tightened from List to ArrayList because no type other than ArrayList can ever be assigned to foo. The return value of the method bar:

 Collection bar() {
   return new LinkedHashSet;
 }
 

can be tightened from Collection to LinkedHashSet since it will never return any other type. By working in conjunction with MethodCallTightener, Type tightening can eliminate generating run-time dispatch code for polymorphic methods. Type flow occurs automatically in most JExpressions. But locals, fields, parameters, and method return types serve as "way points" where type information is fixed based on the declared type. Type tightening can be done by analyzing the types "flowing" into each way point, and then updating the declared type of the way point to be a more specific type than it had before. Oddly, it's quite possible to tighten a variable to the Null type, which means either the variable was never assigned, or it was only ever assigned null. This is great for two reasons: 1) Once a variable has been tightened to null, it will no longer impact the variables that depend on it. 2) It creates some very interesting opportunities to optimize later, since we know statically that the value of the variable is always null. Open issue: we don't handle recursion where a method passes (some of) its own args to itself or returns its own call result. With our naive analysis, we can't figure out that tightening might occur. Type flow is not supported for primitive types, only reference types.

Nested Class Summary

Nested Classes

Modifier and Type	Class and Description
class	TypeTightener.FixDanglingRefsVisitor Replaces dangling null references with dummy calls.
class	TypeTightener.TightenTypesVisitor Wherever possible, use the type flow information recorded by RecordVisitor to change the declared type of a field, local, parameter, or method to a more specific type.

Method Summary

Methods

Modifier and Type	Method and Description
static OptimizerStats	exec(JProgram program)

Methods inherited from class java.lang.Object

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

Method Detail

exec

public static OptimizerStats exec(JProgram program)