Runtime Support for OOLs Part II Comp 412

Transcription

1 COMP 412 FALL 2017 Runtime Support for OOLs Part II Comp 412 soure IR Front End Optimizer Bak End IR target Copright 2017, Keith D. Cooper & Linda Torzon, all rights reserved. Students enrolled in Comp 412 at Rie Universit have epliit permission to make opies of these materials for their personal use. Fault from other eduational institutions ma use these materials for nonprofit eduational purposes, provided this opright notie is preserved. Chapter 6 & 7 in EaC2e

2 The Java Name Spae Class Point { publi int, ; publi void draw(); Class ColorPoint etends Point { Color ; publi void draw() { publi void test() { = ; draw(); Class C { int, ; publi void m() { Point p = new ColorPoint(); = p.; p.draw(); The i eample for disussing inheritane. We will use & etend it. // inherits,, & draw() from Point // loal data // override (hide) Point s draw // loal // independent of Point & ColorPoint // loal data // loal // uses ColorPoint, and, b inheritane // the definitions from Point Point is alled a base and ColorPoint a derived Point is ColorPoint s super and Colorpoint is a sub of Point COMP 412, Fall

3 Runtime Strutures for OOLs From Conept to Implementation Members of a should share members Repliation would waste spae Repliation would be a nightmare This disussion is generi rather than Java-speifi. fee() fie() foe() ount The Conept ount data 1 data 2 fee() fie() The Implementation Single op of the for eah method Move pointer into objet for faster aess One more slot in the OR ount Realit foe() One less indiretion in eah all COMP 412, Fall

4 The Basis of Inheritane Inheritane is the primar mehanism for reuse in OOLs Class derives definitions of its members from is a sub of and is a super of Can have multi-level inheritane Sub has all members of super Inludes transitive superes (super-super, ) Sub an add new members Sub an override inherited definitions ColorPoint inherits & from Point, adds m, and overrides draw() w diretion of inheritane Some languages allow a to have two or more superes This feature is alled multiple inheritane Can introdue oneptual problems with name resolution Languages use different rules to disambiguate these situations COMP 412, Fall

5 Representing Inheritane at Runtime The hierarh must be represented with runtime objets Class super Point super _Point.draw ColorPoint super _.CP.draw _.CP.test objets of LR is a Point UL is a ColorPoint LR UL plain old objets Eah is, it, an objet Class, super & vetor pointers are data members of LR.draw() is found as LR. Point. + offset LR.draw 1 Hierarh must be built (statiall, at initialization, or at load) 1 Code vetor ould eist in eah objet, whih would make method dispath muh faster at the ost COMP of inreased 412, Fall spae 2017 in eah objet. Muh of the engineering on OOLs was done in small memor das. 4

6 OR Laout Assume single inheritane: a has onl one super Inheritane ditates OR Laout OR needs slots for eah delared or inherited data member Eah objet needs an instane variable that points to its Eah needs an instane variable that points to its super Can use prefiing of storage to la out the rest of the OR Use the standard trik to avoid padding Bak to Our Java Eample Class Point Class Point { publi int, ; Class ColorPoint etends Point { Color ; OR for a Point OR for a ColorPoint What happens if we ast a ColorPoint to a Point? COMP A Point 412, to Fall a ColorPoint? 2017 Take the word etends literall. 5

7 Open World versus Closed World Compile time prefiing assumes that the struture is known when laout is performed. Two ommon ases our in OO language design. Closed-World Assumption Class struture is known and losed prior to runtime Can la out ORs in the ompiler and/or the linker (Compile time) Open-World Assumption Class struture an hange at runtime Cannot la out ORs until the are alloated Walk hierarh at alloation Need the ompile-time data strutures at runtime (Interpreter or JIT) C++ has a losed struture. Java as an open struture. Closed-world languages lose some dnamism. COMP 412, Fall 2017 C++ virtual funtions help to reapture it. 6

8 Wait! Eplain Virtual Funtion A virtual funtion is epliitl resolved at runtime Wh does this matter? Assume: Point is ColorPoint s super & both es provide draw() Program reates a ColorPoint p Program assigns p to a Point q Program invokes q.draw() Whih draw() is invoked? Languages use ombinations of virtual funtion resolution and abstrat funtions to implement abstration mehanisms suh as abstrat es and interfaes. It depends on when the funtion name is resolved, or bound Compile-time binding would use q s tpe and invoke Point s draw() Runtime binding would use the p s tpe & invoke ColorPoint s draw In C++, the virtual keword fores runtime resolution of that name Virtual delaration in super holds for all subes Fores a runtime searh through the objet s pointer Too COMP man 412, uses Fall 2017 of abstrat 7

9 Often alled dispath Runtime Resolution for Code Members How does the running program find the for a given method invoation? Closed Class Struture Mapping of names to methods is stati and known (C++) Fied offsets & indiret alls ( vetor funtion pointer) Virtual funtions fore runtime resolution Class Point ColorPoint super super super _.Point.draw _.CP.draw _.CP.test LR is a Point UL is a ColorPoint LR UL Complete vetor in eah COMP UL finds 412, draw Fall at 2017 offset 0 in ColorPoint s vetor 8

10 Runtime Resolution for Code Members How does the running program find the for a given method invoation? Closed Class Struture Mapping of names to methods is stati and known (C++) Fied offsets & indiret alls ( vetor funtion pointer) Virtual funtions fore runtime resolution If ColorPoint inherited draw from Point, its vetor would refer to Point s draw. Class super Point super _.Point.draw ColorPoint super _.Point.draw _.CP.test LR is a Point UL is a ColorPoint LR UL Complete vetor in eah COMP UL finds 412, draw Fall at 2017 offset 0 in ColorPoint s vetor 9

11 Runtime Resolution for Code Members How does the running program find the for a given method invoation? Closed Class Struture And, of ourse, for a little more Mapping of names to methods is stati spae and in known eah OR, (C++) we ould eliminate the indiretion through Fied offsets & indiret alls ( vetor funtion pointer) the OR. Virtual funtions fore runtime resolution (Keep a vetor in eah OR) Class super Point super _.Point.draw ColorPoint super _.Point.draw _.CP.test LR is a Point UL is a ColorPoint LR UL Complete vetor in eah COMP UL finds 412, draw Fall at 2017 offset 0 in ColorPoint s vetor 10

12 Runtime Resolution for Code Members How does the running program find the for a given method invoation? Open Class Struture Dnami mapping, unknown until runtime In restrited ases, an build omplete vetors at eah hange Must lookup offsets, but an then referene into vetor at a fied offset Class Point ColorPoint super super super _.Point.draw _.CP.draw _.CP.test LR UL Super methods found b walking hierarh COMP UL finds 412, draw Fall at 2017 offset 0 in ColorPoint s vetor 11

13 Runtime Resolution for Code Members How does the running program find the for a given method invoation? Open Class Struture Dnami mapping, unknown until runtime In general ase, need runtime representation of hierarh Lookup b tetual name in table of methods (trul slow!) Class Point ColorPoint super super super draw _.Point.draw draw test _.CP.draw _.CP.test LR UL Super methods found b walking hierarh COMP 412, Fall

14 Runtime Resolution for Code Members How does the running program find the for a given method invoation? Open Class Struture Dnami mapping, unknown until runtime In general ase, need runtime representation of hierarh Lookup b tetual name in table of methods (trul slow!) If ColorPoint inherited draw from Point, its vetor would lak a draw. It would find draw at offset 0 in Point s vetor. Class super Point super draw ColorPoint _.Point.draw super test _.CP.test LR UL Super methods found b walking hierarh COMP 412, Fall

15 Runtime Resolution for Code Members And, in an open struture, the vetor in eah OR design reates man, man updates when a hange in the How does struture the running hanges program the find the for a given method invoation? vetor(s). (effiien?) General Open problem: Class Struture updating ORs of instantiated objets when their hanges Dnami (language mapping, design unknown issue) until runtime In general ase, need runtime representation of hierarh Lookup b tetual name in table of methods (trul slow!) Class super Point super draw ColorPoint _.Point.draw super test _.CP.test LR UL Super methods found b walking hierarh COMP 412, Fall

16 Closed World versus Open World Closed Class Struture Compile-time or link-time laout (ORs, es, vetors) If known at ompile time, an generate stati No vetor, no indiretion, effiient implementation Loate the old fashioned (& fast) wa C++ virtual funtions use runtime resolution, as if open world Open Class Struture Cannot la out ORs until hierarh is known With infrequent hange, an hange vetors at eah hange Single-level vetors, fied offsets, indiret alls Instantiated objets of hanged es are still a problem With frequent hange, ma need full runtime hierarh Searh for method in hierarh (w/tag) & ahe result Muh more epensive, on eah all or on first all Smalltalk 80 COMP 412, Fall

17 Method Calls Given the runtime struture, how does a all work? Compiled does not ontain the allee s address Referene is relative to reeiver Follow the hain: OR vetor (or OR vetor) Call overhead is higher, in the best ase Rest of pre-all, post-return, prolog, & epilog are as in an ALL In the general ase, ma need dnami dispath Map member to a searh ke Perform runtime searh through hierarh This proess is epensive, potentiall ver epensive Use a method ahe to speed the searh Cahe holds <searh ke,, method pointer > General ase: Virtual funtions where hoie of method depends on runtime tpe of atual reeiver bound late or at runtime. In Smalltalk 80, all method dispath worked this wa. COMP 412, Fall

18 Method Calls Improvements are possible in speial ases If has no subes, an generate diret alls Class struture must be stati or must be FINAL If struture is stati Can generate omplete method table for eah Use prefied objet reords and omplete vetors Indiretion through the pointer or pointer in eah objet reord Keeps overhead low If struture hanges infrequentl Build omplete method tables at initialization & when struture hanges If running program an reate new es Well, not all things an be effiient See Deutsh & Shiffman, POPL 1984 Language design Behavior Language design COMP 412, Fall

19 What is this JIT thing? Just-in-time ompilers JITs were developed to speed the eeution of OOLs Eeution model is to interpret until enough is known, & then ompile JITs apitalize on ontet that is disovered at runtime Compiler runs during program eeution Compiler must, it, be quite fast Compiled must be fast enough to over ost of JIT eeution Creates an interesting set of design tradeoffs for the JIT JITs have sine been applied to man situations other than OOL eeution First well known JIT was for Smalltalk 80 Allowed $10,000 SUN 2 to ompete with $180,000 Dorado Most Java sstems now inlude a JIT HotSpot Server Compiler is an eellent eample Dnamo was a bolt-on JIT for ordinar eeutables (es, 36 ears ago) Deutsh & Shiffman Clik, Palezn, & Vik Bala, Duesterwald, & Banerjia Enough overs a lot of ground, from the struture of the hierarh & STOP COMP the tpes 412, of Fall objets 2017to knowledge of whih methods are frequentl invoked. 18

20 What About Multiple Inheritane? The Idea Let C be a sub of both A and B C draws some, but not all, methods from A C draws some, but not all, methods from B Need a linguisti mehanism to speif partial inheritane Problems arise when C inherits from both A & B C s OR an etend A or B, but not both C s vetor an etend A or B, but not both Some D might inherit from either A or B or both Need onsisten in OR laout & vetor laout Both A & B might provide fum() whih is seen in C? C++ produes a snta error when fum() is used Java 8 adds a limited form of multiple inheritane with default methods. Need a better wa to sa inherit COMP 412, Fall

21 Modified eample OR Laout with Multiple Inheritane We an tr to use prefiing again Class Point { publi int, ; publi void draw(); publi Point add(); Class CThing { Color ; publi void olorize(); How do asts work? If we ast a CPoint as a Point, & are at the desired offsets If we ast a CPoint as a CThing, is at the wrong offset - Etra field is the ke - Cast bumps to point at 2 nd field Class CPoint etends Point and CThing { publi void draw(); COMP 412, Fall Class that ours out of single inheritane prefi order needs additional information field.

22 Assume 64 bit slots in the ORs OR Laout with Multiple Inheritane Need to prefi both OR & vetor Class Point { publi int, ; publi void draw(); publi Point add(); draw add _.Point.draw _.Point.add Class CThing { Color ; publi void olorize(); olorize _.CThing.olorize Class CPoint etends Point and CThing { publi void draw(); draw add olorize _.CPoint.draw But, where does CPoint s COMP 412, Fall 2017 olorize find? 21

23 OR Laout with Multiple Inheritane Solution: Prefi both storage & vetor Class Point { publi int, ; publi void draw(); publi Point add(); draw add _.Point.draw _.Point.add Class CThing { Color ; publi void olorize(); is at offset 8 in a CThing olorize _.CThing.olorize Class CPoint etends Point and CThing { publi void draw(); draw add olorize COMP 412, Fall is at offset 32 in a CPoint _.CPoint.draw

24 OR Laout with Multiple Inheritane To fi CPoint s offset for, use a trampoline funtion Class Point { publi int, ; publi void draw(); publi Point add(); draw add _.Point.draw _.Point.add Class CThing { Color ; publi void olorize(); olorize _.CThing.olorize Class CPoint etends Point and CThing { publi void draw(); draw add += 24 COMP 412, Fall _.CPoint.draw