Compilation 2012 Code Generation

Transcription

1 Compilation 2012 Jan Midtgaard Michael I. Schwartzbach Aarhus University

2 Phases Computing resources, such as: layout of data structures offsets register allocation Generating an internal representation of machine code for statements and expressions Optimizing the generated code (ignored for now) Emitting the code to files in assembler format Assembling the emitted code to binary format 2

3 Joos Compute offsets and signatures Generate code for static initializers Generate code for statements and expressions Optimize the generated code (ignored for now) Compute locals and stack limits Emit Jasmin code Assemble Jasmin code to class files 3

4 Computing Offsets Each formal and local variables must have an offset in the stack frame The this object always has offset 0 The naive solution: enumerate all formals and locals The better solution: reuse offsets for locals in disjoint scopes The clever solution: exploit liveness information must still respect the runtime types of locals Each slot in the local array must have a unique type at each location (but not necessarily unique across the whole method) 4

5 Naive Offsets public void m(int p 1, int q 2, Object r 3 ) { 4 int x = 42; int w 5 ; { int z 6 ; z = 87; { boolean a ; Object o 8 ; { boolean b 9 ; int z 10 ; b = true; boolean c 11 ; c = b && (x==87); { int y 12 ; y = x; 7 max = 12 5

6 Better Offsets public void m(int p 1, int q 2, Object r 3 ) { 4 int x = 42; int w 5 ; { int z 6 ; z = 87; { boolean a ; Object o 7 ; { boolean b 8 ; int z 9 ; b = true; boolean c 10 ; c = b && (x==87); { int y 8 ; y = x; 6 max = 10 6

7 Clever Offsets public void m(int p 1, int q 2, Object r 3 ) { 1 int x = 42; int w 2 ; { int z 2 ; z = 87; { boolean a ; Object o 2 ; { boolean b 2 ; int z 3 ; b = true; boolean c 3 ; c = b && (x==87); { int y 2 ; y = x; 2 max = 3 7

8 Computing Signatures (1/2) The function sig(σ) encodes a type: sig(void) = V sig(short) = S sig(char) = C sig(σ[]) = [desc(σ) sig(byte) = B sig(int) = I sig(boolean) = Z sig(c 1.C C k ) = C 1 /C 2 /.../C k desc(void) = V desc(byte) = B desc(short) = S desc(int) = I desc(char) = C desc(boolean) = Z desc(σ[]) = [desc(σ) desc(c 1.C C k ) = LC 1 /C 2 /.../C k ; 8

9 Computing Signatures (2/2) This extends to fields, methods, and constructors The field named x in class C: sig(c)/x The method σ m(σ 1 x 1,..., σ k x k ) in class C: sig(c)/m(desc(σ 1 )...desc(σ k ))desc(σ) The constructor C(σ 1 x 1,..., σ k x k ) in class C: sig(c)/<init>(desc(σ 1 )...desc(σ k ))V 9

10 Static Initializers (1/3) Initialization of static fields is performed when the class is loaded by the JVM All static fields are first given default values The code for initialization is written in a special method with the name <clinit> Fields that are static final and constant valued must then be initialized Finally, all other static fields are initialized 10

11 Static Initializers (2/3) public class A { public static int x = A.y+1; public static final int y = 42; public static void main(string[] args) { System.out.print(A.x); 43 public class A { public static int x = A.y+1; public static int y = 42; public static void main(string[] args) { System.out.print(A.x); 1 public class A { public static int x = A.y+1; public static final int y = A.fortytwo(); public static int fortytwo() { return 42; public static void main(string[] args) { System.out.print(A.x); 1 11

12 Static Initializers (3/3) class A { public static final int y = z; public static final int z = y; public static void main(string args[]) { System.out.println(y+z); Error: Use before definition class A { public static final int y = A.z; public static final int z = A.y; public static void main(string args[]) { System.out.println(y+z); 0

13 Generating Code Each statement and expression generates a sequence of bytecodes A code template shows how to generate bytecodes for a given language construct The template ignores the surrounding context And it ignores uniqueness of label names The given label names are symbolic; you have to make sure they are unique via Instruction.make_label This yields a simple, recursive strategy for the code generation 13

14 Code Template Invariants A statement and a void expression leaves the stack height unchanged A non-void expression increases the stack height by one This is a local property of each template The generated code must be verifiable This is not a local property, since the verifier performs a global static analysis 14

15 Code Templates (1/12) if(e) S 1 else S 2 E ifeq false S 1 goto endif false: S 2 endif: nop if(e) S E ifeq false S false: nop nop not strictly necessary b/c reachability constraints guarantee this is not the last instruction while(e) S goto cond: loop: S cond: E ifne loop while(true) S loop: S goto loop 15

16 Code Templates (2/12) { σ n = E; S E σstore offset(n) S σstore is either istore or astore depending on σ { σ n; S S E; E type(e) = void E; E pop type(e) void throw E; E athrow return E; E σreturn return; return 16

17 Code Templates (3/12) new C(E 1,...,E k δ new sig(c) dup E 1... E k invokespecial sig(δ) this(e 1,...,E k δ aload 0 E 1... E k invokespecial δ indicates that δ is the corresponding resolved declaration 17

18 Code Templates (4/12) super(e 1,...,E k δ aload 0 E 1 The current class contains the non-static field initializations: σ 1 x 1 = I 1 ;... σ n x n = I n ;... E k invokespecial sig(δ) aload 0 I 1 putfield sig(x 1 ) desc(σ 1 )... aload 0 I n putfield sig(x n ) desc(σ n ) 18

19 Code Templates (5/12) E.m(E 1,...,E k δ sig(δ) is on a class E.m(E 1,...,E k δ sig(δ) is on an interface E E 1... E k invokevirtual sig(δ) E E 1... E k invokeinterface sig(δ) C.m(E 1,...,E k δ E 1... E k invokestatic sig(δ) 19

20 Code Templates (6/12) (C)E E checkcast sig(c) (char)e E i2c E instanceof C E instanceof sig(c) 20

21 Code Templates (7/12) this aload 0 n type(n) = σ σload offset(n) E.f E getfield sig(f) desc(type(e.f)) C.f getstatic sig(f) desc(type(c.f)) E 1 [E 2 ] E 1 E 2 σaload type(e 1 [E 2 ]) = σ σaload is either iaload, baload, saload, caload, or aaload depending on σ 21

22 Code Templates (8/12) n = E type(n) = σ E dup σstore offset(n) E 1.f = E 2 E 1 E 2 dup_x1 putfield sig(f) desc(type(e 1.f)) C.f = E E dup putstatic sig(f) desc(type(c.f)) E 1 [E 2 ] = E 3 E 1 E 2 type(e 1 [E 2 ]) = σ E 3 dup_x2 σastore 22

23 Code Templates (9/12) new σ[e] E multianewarray desc(σ ) 1 type(new σ[e]) = σ E.length E arraylength E.clone() E invokevirtual sig(type(e))/clone()ljava/lang/object; 23

24 Code Templates (10/12) 42 ldc_int 42 true ldc_int 1 null aconst_null "abc" ldc_string "abc" 24

25 Code Templates (11/12) E 1 + E 2 E 1 type(e 1 +E 2 ) = int E 2 iadd E 1 + E 2 E 1 E type(e 2 1 +E 2 ) = String invokevirtual S/concat(LS;)LS; S java/lang/string - E E ineg 25

26 Code Templates (12/12) E 1 E 2 E 1 dup ifne firsttrue pop E 2 firsttrue: nop E 1 && E 2 E 1 dup ifeq firstfalse pop E 2 firstfalse: nop nops not strictly necessary b/c reachability constraints guarantee this is not the last instruction 26

27 Stack and Locals Limits The generated code must explicitly state: the maximal number of local and formal offsets the maximal local stack height This is used to determine the size of the frame The locals limit is the maximal offset + 1 The stack limit is computed by a static analysis 27

28 Stack Limit Analysis Consider the control flow graph of the bytecodes succ(s i ) denotes the set of successor bytecodes Δ(S i ) denotes the change in stack height by S i S 0 denotes the first bytecode For every bytecode S i we define the following integer-valued properties: B[[S i ]] denotes the stack height before S i A[[S i ]] denotes the stack height after S i 28

29 Dataflow Constraints B[[S 0 ]] = 0 A[[S i ]] = B[[S i ]] + Δ(S i ) x succ(s i ): A[[S i ]] = B[[x]] A[[S i ]] 0 These constraints must have a solution The stack limit is the largest value of any A[[S i ]] 29

30 Jasmin Class Format (1/3) The overall structure of a Jasmin file is:.source sourcefile.class modifiers name.super sig(superclass).implements sig(interface).field modifiers desc(type) constructors methods 30

31 Jasmin Class Format (2/3) The structure of a constructor is:.method modifiers sig(constructor).throws sig(exception).limit stack stacklimit.limit locals localslimit bytecodes.end method 31

32 Jasmin Class Format (3/3) The structure of a method is:.method modifiers sig(method).throws sig(exception).limit stack stacklimit.limit locals localslimit bytecodes.end method 32

33 A Tiny Class public class Foo { public int y = 42; public Foo(int z) { y = y+z; public String print(int n) { if (n==0) return new Integer(y).toString(); else return new Foo(y).print(n-1); 33

34 The Generated Code.source Foo.java.class public Foo.super java/lang/object.field public "y" I.method public <init>(i)v.limit stack 3.limit locals 2 aload_0 invokespecial java/lang/object/<init>()v aload_0 bipush 42 putfield Foo/y I aload_0 aload_0 getfield Foo/y I iload_1 iadd dup_x1 putfield Foo/y I pop return.end method.method public print(i)ljava/lang/string;.limit stack 3.limit locals 2 iload_1 ifne false0 new java/lang/integer dup aload_0 getfield Foo/y I invokespecial java/lang/integer/<init>(i)v invokevirtual java/lang/integer/tostring()ljava/lang/string; areturn false0: new Foo dup aload_0 getfield Foo/y I invokespecial Foo/<init>(I)V iload_1 iconst_1 isub invokevirtual Foo/print(I)Ljava/lang/String; areturn.end method 34

35 The Binary Class File cafe babe e 001e 0c00 0a a f6c 616e 672f 496e a f6c 616e 672f 4f62 6a c69 6e69 743e c d f6f e 740c c 0a00 1d f a53 6f c a a c f e67 0c00 0f f 6f2e 6a c6a f 6c61 6e67 2f e 673b 0a a 001d 000b c 6a f6c 616e 672f e67 3b f e ab a10 2ab a2a b b 605a b b a e a e1b 039f a bb00 1d59 2ab b700 0cb6 001b b0bb a b400 04b7 001a 1b04 64b b