The Java Virtual Machine. Compiler construction The structure of a frame. JVM stacks. Lecture 2

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1 Compile constuction 2009 Lectue 2 Code geneation 1: Geneating code The Java Vitual Machine Data types Pimitive types, including intege and floating-point types of vaious sizes and the boolean type. The suppot fo boolean is vey limited; Java (and Javalette) boolean expessions ae compiled to int values. Javalette needs only int and double. Refeence types, used fo efeences to objects; not needed by basic Javalette. Data aeas Local vaiables and paametes ae stoed on the JVM stack (since Javalette is single-theaded thee is only one stack). Objects (including Java aays) ae stoed in the heap. The heap is not used by basic Javalette. JVM stacks Fames A JVM stack consists of fames. A new stack fame is allocated fo each method invocation. Diffeent JVM invocation instuctions: invokestatic fo static methods; use this fo Javalette functions. invokevitual fo instance methods; not needed fo basic Javalette. invokespecial fo special cases, e.g. initialization. Use in class file heade. JVM handles bueaucacy of method invocation: Allocating and deallocating fames. making paametes available to invoked method. making etun value available to invoking method. The stuctue of a fame Local vaiables aay An aay of wods containing local vaiables and paametes. An int value equies one wod, a double value two wods. Paametes occupy the fist pat of the aay, locally declaed vaiables the second pat. All vaiables ae efeed to using thei index in this aay. The size of the aay is specified in the class file. Opeand stack A stack of wods fo tempoay stoage. Many JVM opeations manipulate this stack. Each enty can hold a value of any JVM type (also double). Results fom method invocations ae left on top of stack. Maximal size of stack is specified in the class file.

2 A simple example, 1 A simple example, 2 static void main(sting[] ags){ int = f(3,5); System.out.pintln(); int = x+y+2; etun ; Snapshot: in main, befoe invokestatic f: ags 3 5 static void main(sting[] ags){ int = f(3,5); System.out.pintln(); int = x+y+2; etun ; Snapshot: in f, befoe fist instuction ags x 3 y 5 A simple example, 3 A simple example, 4 static void main(sting[] ags){ int = f(3,5); System.out.pintln(); int = x+y+2; etun ; Snapshot: in f, befoe etun: ags x 3 y static void main(sting[] ags){ int = f(3,5); System.out.pintln(); int = x+y+2; etun ; Snapshot: in f, afte invokestatic f: ags 10

3 A simple example, 5 Java bytecode static void main(sting[] ags){ int = f(3,5); System.out.pintln(); int = x+y+2; etun ; Snapshot: in main, befoe invoking pintln: ags 10 Geneal popeties Instuctions to push values on the opeand stack, stoe stack top in a vaiable, do aithmetic on opeands on the opeand stack, jump (conditionally) to othe instuctions, invoke and etun fom methods,... One byte opcodes; ca 250 diffeent instuctions. Instuctions may have aguments: (small) constants, indices to pool of (bigge) constants, local vaiable indices. Compact fomat, suitable fo mobile code. Binay fomat, no official assembly fom. assemble Download it! (Link on couse web site) Unzipped diectoy contains jasmin.ja. To assemble file myfile.j containing assemble code, you un > java -ja jasmin.ja myfile.j This poduces myfile.class, which can be un by java intepete. Note that classpath must be set so jasmin.ja is found. Option -d path wites the class file in diectoy path Disassembling > javap -c myfile.class pints an assemble vesion on stdout in (almost) syntax. instuctions 1 Aithmetic Push intege/sting constant c: ldc c Push double constant c: ldc2 w c Pefom binay opeation on integes: iadd isub imul idiv iem iand io Pefom binay opeation on doubles: dadd dsub dmul ddiv Booleans ae teated as integes: false = 0, tue = 1. Typed opeations Diffeent opeations depending on type: e.g. iadd and dadd. Also load/stoe opeations ae typed. Consequence: You will need to know fo all subexpessions which type they have. You compute this duing type-checking; we now see the benefit of saving this infomation.

4 Pushing values on the stack Intege constants Small values: iconst 1 pushes intege 1. Similaly fo -1, 0, 2, 3, 4, 5. A little bigge: bipush n pushes n, fo 128 n 127. Even bigge: sipush n pushes n, fo n Abitay: ldc n pushes n. Value of n stoed in constant pool, index in the instuction. jasmin handles constant pool; you can wite constants. To conside You will need a datatype of instuctions (in Haskell) o a class hieachy (in Java/C++). But will you need all fou foms of push instuctions? Simila consideations fo loading/stoing local vaiables. instuctions 2 Loading local vaiable to stack Load (push) intege vaiable n: iload n. If n = 0,1,2,3, thee ae one-byte vaiants iload 0, etc. Load (push) double vaiable n: dload n. If n = 0,1,2,3, thee ae one-byte vaiants dload 0, etc. Stoing vaiable to memoy Stoe (and pop) intege vaiable n: istoe n. If n = 0,1,2,3, thee ae one-byte vaiants istoe 0, etc. Stoe (and pop) double vaiable n: dstoe n. If n = 0,1,2,3, thee ae one-byte vaiants dstoe 0, etc. Incement of a vaiable can be done without loading and stoing: iinc Java instuctions 3 Labels The label L itself is an instuction: L: Make sue that all labels in a function ae distinct! In the JVM bytecode, code is stoed in an aay of bytes and the label is just the index. Jumps Jump (unconditionally) to a label: goto L Jump if compaison holds between the topmost two integes on stack: if icmpeq L, if icmplt L, etc Jump if compaison holds between the topmost intege and zeo: ifeq L, iflt L, etc. Fo doubles, the situation is diffeent: dcmpl, dcmpg compae the two doubles and etuns an intege -1, 0, o 1. Geneating code fo expessions The poblem Input: A type-annotated AST fo an expession. Output: A sequence of instuctions with net effect to push value of exp on the stack. The solution: Syntax-diected tanslation Constants: Push on stack. Aithmetic expessions: Geneate (ecusively) and concatenate code fo left and ight opeand; last instuction is aithmetic instuction. Function call: Geneate and concatenate code fo all aguments; last instuction is suitable invoke instuction. Vaiables: Load vaiable, using its numbe; how do you know this?

5 Additional input: The code geneato s state. The poblem As we saw on the pevious slide, the AST is not enough; we need to know the index (addess) of each vaiable. These will be computed by the compile itself, when geneating code fo function defs and vaiable declaations. Moe geneally: Needed state infomation Vaiable numbe/index fo each local vaiable (incl paametes). Type fo each function. Index to use fo next vaiable. Numbe to use fo next label. Cuent stack depth. Maximal stack depth. Code emitted so fa. A bette way to pesent tanslation Compilation schemes Pseudocode notation, simila to Haskell, using opeations on the state in monadic style. Example Defining codegenexp :: Exp -> Result (), by patten matching on the abstact syntax (pesented hee in concete syntax) codegenexp(exp1 + exp2 : int) = codegenexp exp1 codegenexp exp2 put [IAdd] -- add to code incstack (-1) -- decease cuent depth -- othe cases simila Define a suitable type fo the state Geneating code fo statements In Java/C++ Define a class with methods fo accessing and updating the vaious state components; don t use public instance vaiables! Make use of suitable collection classes. In Haskell Use a state monad; also hee, define suitable monadic functions fo accessing and updating the state. With suitable abstactions you will be able to modify you code easily if you ealy decisions need to be changed. Again: syntax-diected tanslation Assignment: Geneate code fo RHS; stoe in vaiable (state gives index). Declaation: Get next vaiable index fom state and update state with vaiable/index. Retun: Geneate code fo expession; add etun instuction. Block: concatenate code fom statements.... Some difficulties How does block stuctued scope affect tanslation? How to handle contol stuctues that lead to jumps (if and while)?

6 What about block stuctue? Result of peceding slide Each local vaiable in a method gets its own index. Example Example code int f (int x) { int a, b; {int a, c; {int x, y; Naive handling of while statemens Questions How big must the local vas aay be? How can we avoid making it bigge? Boolean expessions Booleans as integes Booleans ae teated as integes, tanslating false to 0, tue to 1. Thee ae no JVM opeations coesponding to the elational opeatos; we need to use jumps. codegenexp(exp1 > (exp2 : int)) = codegenexp exp1 codegenexp exp2 lab1, lab2 <- getlabel -- get fesh labels put [if_cmpgt lab1, iconst_0, goto lab2, label lab1, iconst_1, label lab2] An example: while (i > 6) i-- ; while scheme codegenstm (while (exp) stm) = lab1, lab2 <- getlabel put [label lab1] codegenexp exp -- push value of exp put [ifeq lab2] -- if false, fall though incstack (-1) -- the test popped exp codegenstm stm put [goto lab1, -- test again label lab2] -- label fo next stmt Check stack effect! Geneated code lab1: iload 1 bipush 6 if icmpgt lab3 iconst 0 goto lab4 lab3: iconst 1 lab4: ifeq lab2 iinc i (-1) goto lab1 lab2: Bette code lab1: iload 1 bipush 6 if_icmple lab2 iinc i (-1) goto lab1 lab2: The poblem The good code is not compositional, i.e. not built by combining code fom the immediate subtees.

7 Example, continued Recall souce code while (i > 6) i--; Even bette code goto lab2 lab1: iinc 1 (-1) lab2: iload 1 bipush 6 if_icmpgt lab1 Comments Saves one JVM instuction pe loop ound You can get (almost) this code compositionally by changing while scheme (fo you to do!) and changing teatment of Boolean expessions (next slide) Note: Naive codegen is enough to pass! Boolean expessions evisited Used in two diffeent ways As test expessions in contol stuctues (while, if). As ight hand sides in assignments to boolean vaiables o actual (boolean) paametes in function calls. Do code geneation diffeently in these two cases! Test expession Define a scheme that takes as aguments the test expession and two labels to jump to when value is tue and false, espectively. Called fom while and if schemes. To compute Boolean value Geneate code as we indicated befoe, teating Booleans as integes. Called fom assignment and function call schemes. May use function to the left when code with jumps needed. Tanslating function definitions Pedefined methods: output fundef scheme codegendef (typ f (paams) stms) = foall paams (ty x): addva ty x foall stms: codegenstm mx <- getmaxstack locs <- getlocals nm = jvmname f typ paams code <- getcode -- now we can build and etun the abstact syntax -- fo the function using these fou values Using Java libay methods All the pint functions call System.out.pintln. In, pintln gets out as a fist agument: getstatic java/lang/system/out Ljava/io/PintSteam; bipush 77 invokevitual java/io/pintsteam/pintln(i)v The fist instuction pushes a efeence to out on the stack. Then we push the value we want to pint. Then pintln is invoked and gets these two aguments.

8 Pedefined methods: input Input in Javalette vs. Java To ead e.g. an intege in Javalette is simple: Avoiding tivial poblems int main () { pintint (7 * eadint ()) ; In Java, much moe is needed: impot java.io.* ; class Read { public static void main (Sting [] ags) thows IOException { BuffeedReade b = new BuffeedReade (new InputSteamReade(System.in)) ; System.out.pintln(7 * Intege.paseInt(b.eadLine())) ; The Runtime class Instead of geneating lots of instuctions, you can define all pedefined methods in a Runtime class. Just wite it in Java and compile into a.class file! To think about fo input methods: only add thows IOException to functions that eally need it. only ceate one new BuffeedReade pe pogam un.