Computer Science 313 – Advanced Programming Topics

Handling Common Subroutines  Must perform set of actions in many places

Handling Common Subroutines  Must perform set of actions in many places Paste code into each different method Write 1 method called everywhere

Why Write a Separate Method  Fewer errors from not updating 1 copy  Code is much simpler to debug  Optimizing & refactoring code far simpler  Makes code more reusable

Why Copy-and-Paste Code  Calls expensive relative to most code  Calls will take instructions each  Violates assumption of next instruction executed  Compiler may be prevented from optimizing  Unadulterated stupidity making these comments  In many situations, costs do not matter  Only on critical path should we even think about this  Remember Amdahl’s law: 10% slowdown to 20% of code = 0.97 slowdown

Method Inlining  From Michael Hind, a leading IBM researcher  Best solution looks like copy-and-paste code  Easiest way to tell if someone a poseur

What is Inlining?  Replaces method call with actual code  Acts as if written using copy-and-paste  But does not suck when maintaining code public int add(int a, int b) { if (b == 3) { return add3(a); } return a + b; } public int add3(int x) { return x + 3; }

What is Inlining?  Replaces method call with actual code  Acts as if written using copy-and-paste  But does not suck when maintaining code public int add(int a, int b) { if (b == 3) { return add3(a); } return a + b; } public int add3(int x) { return x + 3; }

What is Inlining?  Replaces method call with actual code  Acts as if written using copy-and-paste  But does not suck when maintaining code public int add(int a, int b) { if (b == 3) { return a + 3; } return a + b; } public int add3(int x) { return x + 3; }

C/C++ Inlining Options  Suggest to compiler with inline keyword  Allows programmer to hint at function to inline  Compiler may override if not a good idea inline int add3(int x) { return x + 3; }  Rewrite function as macro using #define  Global search-and-replace performed on code  Compiler has no choice; function forced inline  Create absolute nightmare  Create absolute nightmare for maintenance

Inlining Support in Java  Automatically inlines certain small methods  But only if and when exact definition known  Subclasses cannot change definition later staticfinalprivate  Method must be static, final, or private  Code must be written to use this skill  Rare for people to know details of this  Simple poseur detector from not knowing this

Inlining Support in Java  Automatically inlines certain small methods  But only if and when exact definition known  Subclasses cannot change definition later staticfinalprivate  Method must be static, final, or private  Code must be written to use this skill  Rare for people to know details of this  Simple poseur detector from not knowing this How not to look like an idiot:

Do the Right (Coding) Thing  Breakup code & write only small methods  Small ones private final to get inlined  Methods far easier to write, debug, & reuse  All benefits and no slowdown!

CSC212/213 Professor Fun!  Make methods static when possible  Code becomes easier to read and follow  Ensures everything can be inlined  Make fields private & use get methods  Make accessor final since it cannot be changed  Guess what, the getter code gets inlined

Bad Methods; No Inline public int three() { return 3; } public int two() { return 2; } public int five() { return three() + two(); } public int addToX(int y) { return x + y; } public int getAndUpdateX(int z) { x = x + z; return x; }

Now We Can Inline public STATIC int three() { return 3; } public STATIC int two() { return 2; } public STATIC int five() {return three()+two();} PRIVATE int addToX(int y) { return x + y; } public FINAL int getAndUpdateX(int z) { x = ADD T O X( Z ); return x; }

Simple Java Inlining Results

JIT Advantage  So far, inlining based on static analysis  Information available when program compiled  Only inlines small methods, since they are safe  But calling larger methods also takes time  Java has JIT compiler it can use  Information gathered as program runs  Frequent calls (hot paths) will be found

Next Step To Worry About  Can inline methods on critical path  Even when larger than otherwise allowed  Even if not final, if calls are not polymorphic  JVM does this automatically  So may not hurt to use patterns on critical path!

Inlining Even When We Cannot

For Next Class  Lab #5 on Angel & due week from Fri.  Read pages in book  Command pattern will be focus of reading  Great for when we need to use remote control  How do we use Command pattern?  How does it fit in with other patterns?  Where should this be used?  Is the pattern behavioral, creational, or structural?