Classes. Code Generation for Objects. Compiling Methods. Dynamic Dispatch. The Need for Dispatching CS412/CS413

Transcription

1 Classes CS4/CS43 Introduction to Comilers Tim Teitelbaum Lecture : Imlementing Objects 8 March 5 Comonents ields/instance variables values ma dier rom object to object usuall mutable methods values shared b all objects o a class usuall immutable comonent visibilit: ublic/rivate/rotected CS 4/43 Sring 5 Introduction to Comilers CS 4/43 Sring 5 Introduction to Comilers Code Generation or Objects Methods Generating method Generating method calls (disatching) Constructors and destructors Fields Memor laout Generating to access ields Field alignment Comiling Methods Methods look like unctions, are te-checked like unctions what is dierent? Argument list: imlicit receiver argument Calling sequence: use disatch vector instead o juming to absolute address CS 4/43 Sring 5 Introduction to Comilers 3 CS 4/43 Sring 5 Introduction to Comilers 4 The Need or Disatching Eamle: interace Point { int (); int (); loat (); class ColoredPoint imlements Point { loat () { return sqrt(*+*); class imlements Point { loat () { return sqrt(*+*+z*z); Point ; i (cond) = new ColoredPoint(); else = new (); int n =.(); Comiler can t tell what to run when method is called! Dnamic Disatch Solution: disatch vector (disatch table, selector table ) Entries in the table are ointers to method Pointers are comuted dnamicall! I T <: S, then vector or objects o te S is a rei o vector or objects o te T object reerence object laout disatch vector method CS 4/43 Sring 5 Introduction to Comilers 5 CS 4/43 Sring 5 Introduction to Comilers 6

2 Wh It Works I S <: T and is a method o an object o te T, then Objects o te S inherit ; can be overridden b S Pointer to has same inde in the or te T and S! Staticall generate to look u ointer to method Pointer values determined dnamicall Point reerence laout z vector getz CS 4/43 Sring 5 Introduction to Comilers 7 Disatch Vector Looku Ever method has its own small integer inde Inde is used to look u method in disatch vector interace A { C <: B <: A void (); (); A class B imlements A { void () () { B,g,h void g() g() { void h() h() { C,g,h,e class C etends B { void e() e() { 3 CS 4/43 Sring 5 Introduction to Comilers 8 Disatch Vector Laouts Inde o is the same in an object o te T <: A Methods ma have multile imlementations For subclasses with unrelated tes I subclass overrides method To eecute a method i: Looku entr i in vector Eecute ointed to b entr value A B C g h g h e 3 Code Generation: Disatch Vectors Allocate one disatch vector er class Objects o same class eecute same method Staticall allocate disatch vectors.data LenList:.long _LenList_irst.long _LenList_rest.long _LenList_length CS 4/43 Sring 5 Introduction to Comilers 9 CS 4/43 Sring 5 Introduction to Comilers Interaces, Abstract Classes Classes deine a te and some values (methods) Interaces are ure object tes : no imlementation no disatch vector: onl a laout Abstract classes are halwa: deine some methods leave others unimlemented no objects (instances) o abstract class needed onl or concrete classes Method Arguments Methods have a secial variable (Java, C++: this) called the receiver object Historicall (Smalltalk): method calls thought o as messages sent to receivers Receiver object is (imlicit) argument to method class A { int int(int,, int int) ) { comile as int int(a this, int,int) ) { CS 4/43 Sring 5 Introduction to Comilers CS 4/43 Sring 5 Introduction to Comilers

3 Static Methods In Java, can declare methods static the have no receiver object Called eactl like al unctions don t need to enter into disatch vector don t need imlicit etra argument or receiver Treated as methods as wa o getting unctions inside the class scoe (access to module internals or semantic analsis) Not reall methods Code Generation: Method Calls Code or unction calls: re-call + ost-call Pre-unction-call : Save registers Push arameters Pre-method call: Save registers Push arameters Push receiver object reerence Looku method in disatch vector CS 4/43 Sring 5 Introduction to Comilers 3 CS 4/43 Sring 5 Introduction to Comilers 4 o.oo(,3); ea eb [eb+4] Eamle oo oo (object) () () ush $3 $3 ush $ $ ush %ea mov (%ea), %eb call call *4(%eb) add $, %es Object Laout Object consists o: Methods Fields Object laout consists o: Pointer to, which contains ointers to methods Fields laout (static data) () CS 4/43 Sring 5 Introduction to Comilers 5 CS 4/43 Sring 5 Introduction to Comilers 6 Allocation o Objects Objects can be stack- or hea-allocated Stack allocation: (C++) Point ; (stack) (static data) Inheritance and Object Laout Method coied down rom suerclass i not overridden b subclass Fields also inherited (needed b inherited in general) Hea: (C++) Point * = new Point; (Java) Point = new Point(); (stack) (hea) (static data) Inheritance: add ields, methods Etend laout Etend disatch vector A suerte object can be used whenever a subte object can be used CS 4/43 Sring 5 Introduction to Comilers 7 CS 4/43 Sring 5 Introduction to Comilers 8 3

4 Inheritance and Object Laout Code Sharing Shae class class Shae { Point LL, LL, UR; UR; void void (int which, Point ); ); class class ColoredRect etends Shae { int intcolor; void voidsetcolor(int col); col); color: int setcolor ColoredRect color: int setcolor Machine or Shae. Don t actuall have to co! Works with searate comilation: can inherit without suerclass source CS 4/43 Sring 5 Introduction to Comilers 9 CS 4/43 Sring 5 Introduction to Comilers Field Osets Osets o ields rom beginning o object known staticall, same or all subclasses Eamle: class Shae { Point LL LL /* /* 4 */ */,, UR; /* /* 8 */ */ void (int which, Point ); ); class ColoredRect etends Shae { Color c; c; /* /* */ */ void setcolor(color c_); Osets known or stack and hea allocated objects Field Alignment In man rocessors, a 3-bit load must be to an address divisible b 4, address o 64-bit load must be divisible b 8 In rest (e.g., Pentium), loads are aster i aligned -- avoids etra load Fields should be aligned c struct { int ; char c; int ; char d; int z; double e; d z e CS 4/43 Sring 5 Introduction to Comilers CS 4/43 Sring 5 Introduction to Comilers Accessing Fields Access ields o current object Access equivalent to this. Current method has this as argument Access ields o other objects Access o the orm o. In both cases: Use ointer to object Add oset to the ield Access o. deends on the kind o allocation o o Stack allocation: stack access (%eb + stack oset) Hea allocation: stack access + dereerence CS 4/43 Sring 5 Introduction to Comilers 3 Code Generation: Allocation Hea allocation: o = new C() Allocate hea sace or object Store ointer to disatch vector Stack allocation: Push object on stack Pointer to on stack ush $6 # 3 ields+ call call _GC_malloc mov $LenList_, (%ea) add $4, $4, %es sub sub $6, %es # 3 ields+ mov $LenList_, -4(%eb) CS 4/43 Sring 5 Introduction to Comilers 4 4

5 Constructors Java, C++: classes can declare object constructors that create new objects: new C(,, z) Other languages (Modula-3): objects constructed b new C ; no initialization class LenList { int len; Cell head, tail; LenList() { len = ; Need to know when objects are constructed Hea:new statement Stack:at the beginning o their scoe (blocks or locals, rocedures or arguments, rogram or globals) Comiling Constructors Comiled like methods: seudo-variable this assed to constructor return value is this l = new LenList(); LenList() { len = ; ush $6 $6 # 3 ields+ call call _GC_malloc mov $LenList_, (%ea) add add $4, $4, %es ush %ea call call LenList$constructor add add $4, $4, %es LenList$constructor: ush %eb mov %es,%eb mov 8(%eb), ea ea mov $, $, 4(%ea) mov %eb,%es o o %eb ret ret CS 4/43 Sring 5 Introduction to Comilers 5 CS 4/43 Sring 5 Introduction to Comilers 6 Destructors Analsis and Otimizations In some languages (e.g., C++), objects can also declare to eecute when objects are destructed Hea: when invoking delete (elicit de-allocation) Stack: when scoe o variables ends End o blocks or local variables End o rogram or global variables End o rocedure or unction arguments Datalow analsis reasons about variables and values Records (objects) consist o a collection o variables (ields) analsis must searatel kee track o individual ields Diicult analsis or hea-allocated objects Object lietime outlives rocedure lietime Need to erorm inter-rocedural analsis Constructors/destructors: must take their eects into account CS 4/43 Sring 5 Introduction to Comilers 7 CS 4/43 Sring 5 Introduction to Comilers 8 Class Hierarch Analsis Method calls = dnamic, via disatch vectors Overhead o going through Prohibits unction inlining Makes other inter-rocedural analses less recise Static analsis o dnamic method calls Determine ossible methods invoked at each call site Need to determine rincial tes o objects at each rogram oint (Class Hierarch Analsis) I analsis determines object o is alwas o te T (not subte), then it recisel knows the or o.oo() Otimizations: transorm dnamic method calls into static calls, inline method calls Summar Method disatch accomlished using disatch vector, imlicit method receiver argument No disatch o static methods needed Inheritance causes etension o ields as well as methods; can be shared Field alignment: declaration order matters! Each real class has a single disatch vector in data segment: installed at object creation or constructor Analsis more diicult in the resence o objects Class hierarch analsis = recisel determine object class CS 4/43 Sring 5 Introduction to Comilers 9 CS 4/43 Sring 5 Introduction to Comilers 3 5