Activation Records

Activation Records Modern Compiler Implementation in JavaAppel Chapter 6 CMSC 432 Shon Vick

Local Variables, Instantiations Ex: function f(x:int) : int = let var y := x + x in if y < 10 then f(y) else y – 1 end Many (recursive) f calls -> Many x’s and y’s  Runtime Stack

Higher-order Function nested function + higher-order function -> not in stack-mode In Pseudo-C int (*)() f(int x) { int g(int y) {return x+y;} return g; } int (*h)() = f(3); -> x=3 int (*j)() = f(4); -> x=4 int z = h(5) ; <- no x int w = j(7) ; <- no x Pascal -> no higher function C -> no nested function  runtime stack

prev frame next frame Stack Frames arg n . . arg 1 Static link incoming args • Push/pop frames • Access variables in deeper frames -> nonlocal variables • Stack frame • Local varialbes • Parameters • Return address • Temporaries • Register save area • Usually has “standard” frame layout for several languages • Depends on architecture frame pointer local variables return address temps saved registers lower memory addresses higher addresses current frame arg n . . arg 1 static link outgoing args stack pointer 4

Frame Pointer frame pointer • g calls f(a1, a2, ………, an) arg n . . arg 1 stack pointer : frame size either fixed or varies => Can be determined very late frame pointer arg n . . arg 1 stack pointer 5

Registers • register : local vars, temporary results… • Can save load/store instructions • general purpose vs. special purpose registers • caller save vs. callee save register • Ex: MIPS r16-r23 are preserved across procedure calls(callee-save) r0-r15 not preserved (caller-save) • If we do interprocedure analysis, we can do fine register save scheduling.

Parameter Passing • passing with stack • passing some in registers and others in stack • k=6 or 4 • Need to save register when call another function • need not to save “argument registers”, when • Leaf procedure • Interprocedural register allocation • Arguments become dead variables when calling another function • Register windows

arg n . . arg k+1 arg k . . arg 1 Parameter Passing (Cont’d) • argument passing in reg + stack • Sometimes formal parameters are at consecutive addresses : register save area by callee • call-by-reference • Code for dereferencing formal parameter access frame pointer register save area

Return Address • g calls f : f returns • Need g’s address (resume point) -> return address • Can be saved • On stack • In special register • In special memory location • Hardware “call” instruction dependent • Usually in designated registers • Need to save (no-leaf proc) • No need to save (leaf proc)

Frame-resident Variables • Variables are written to memory only when necessary • Variable will be passed by reference or & (address of) operator is applied • Variable is accessed by a procedure nested inside the current one • Value is too big to fit into a single register • Variable is an array • Register holding variable is needed for special purpose (parameter passing) • Too many local variables (“spilled” into frame)

Escaped Variable • A variable “escape”s if it is passed by reference, its address is taken, or it is accessed from a nested function. • Variables are bound to register or memory in later phase in compiling.

prettyprint output write --output show n ident i,s --output --n -- i,s prettyprint show ident show , , Static Links • Variable References • Static Links • Display • Lambda lifting (passing all nonlocals as parameters) • Procedure Calls

Lambda lifting • remove static links, only global routines • out-of-scope variables • referencing: pass additional pointers • creating: heap allocation int aap(int *i) {return *i+7;} fptr mies(int i) { int *_i = malloc(sizeof(i)); *_i = i; return closure(aap,_i); } typedef int (*fptr)(); fptr mies(int i) { int aap() {return i+7;} return aap; } nested lifted

Frames in MiniJava • Package Frame • Access.java AccessList.java Frame.java • PackageTemp • Temp.java, TempList.java, Label.java, LabelList.java • Package Util • BoolList.java • Package T (Mips, Sparcs) • T(Mips/Sparcs)Frame.java • Inframe(), InReg(), newFrame(), allocLocal()

Package Frame • Abstraction of Actual Frames package Frame; import Temp.Templ import Temp.Label; Public abstract class Access{ … } public class AccessList { public Access head; public AccessList tail; public AccessList(Access h, AccessList t) { head=h; tail=t;} }

Frame.java public abstract class Frame { public abstract Frame newFrame(Temp.Label name, Util.BoolList formals); public Temp.Label name; public AccessList formals; public abstract Access allocLocal(boolean escape); public abstract Temp.Temp FP(); public abstract Temp.Temp RV(); /* ..other stuff, eventually … */ // public abstract int wordSize(); // public abstract Tree.Exp externalCall(String func, Tree.ExpList args); }

TFrame : specific to Target Machine • For T machine… package T; class Frame extends Frame.Frame { /* real definitions of Frame */ …. } • In machine independent part of compiler // in class Main.Main: Frame.Frame frame = new T.Frame(…); • To hide the identity of the target machine

Making new Frames • Hold information fo parameters & local variables • Frame for function f with k formals newFrame(f, l) where f : Label l:BoolList Ex: a three-argument function named g with 1st argument escaped (No parameters will be escapes in MiniJava.) frame,newFrame(g, new BoolList(true, new BoolList(false, new BoolList(false,null))))

Class Access • Describe formal & local vars in the frame or in registers • Abstract data type whose implementaion is visible only inside the Frame module: package T class InFrame extends Frame.Access { int offset; InFrame (int o) {offset = o; } } class InReg extends Frame.Access { Temp.Temp temp; InReg(Temp.Temp t) {temp = t; }

Access and Allocate the Vars • InFrame(X) : a memory location at offset X from the FP(frame pointer) • InReg(t84) : in register t84 • formals in Frame.java • A list of k “accesses” denoting locations where the formal parameters will be kept at runtime , as seen from inside the callee • May be seen differently by the caller and calle : “shift of view” • View shift must be handled by “newFrame()”

Representation of Frame Descriptions • Implementation of frame is an object holding: • the location of all the formals • instructions required to implement the “view shift” • the number of locals allocated so far • the “label” at which the function’s machine code is to begin • See Table 6.4 on page 129

Local Variables • To allocation a new local var in a frame f • f.allocLocal(true) //allocate in Memory • will return InFrame() access with an offset from FP ex) two local vars in Sparcs => InFrame(-4), InFrame(-8) • f.allocLocal(false) // allocate in register • will return InReg() ex) on register-allocated vars => InReg(t481) • allocLocal(bool) • Called when frame is create • Called when nested block is entered

allocLocal() allocLocal() allocLocal() function f() = let var v1 := 6 in print(v1); let var v2 := 7 in print(v2); end print(v1); let var v3 := 8 in print(v3); end print(v1); end Allocating Local Storage in frame with the Same Name frame pointer v1 might use same space v1 v2 v3 stack pointer

Escape Variables • No variables escape in MiniJava, because • there is no nesting of classes and methods • it is not possible to take the address of a variable • integers and booleans are passed by value • object, including integer arrays, can be represented as pointers that are passed by value

Calculating Escapes • FindEscape(): looks for escaping variables and records this information in the escape fields of AST • Traverse the entire AST before semantic analysis • When the variables are encounted, • Set 0 when first encountered • Set 1 when referenced at inner block! when address is taken by & when call-by-reference

Temporaries and Labels • Temp’s are virtual registers • May not be enough registers available to store all temporaries in a register • Delay decision until later • Label’s are like labels in assembler, a location of a machine language instruction • Classes Temp and Label in package Temp • Packages Frame and Temp provide machine independent views of variables

Managing Static Links • Static Link management is somewhat tedious? • MiniJava does not have nested function declararions: thus Frame should not know anything about static links. • It will be handled in the Translation phase. • Static links may be passed to the callee by the 1st formal parameter.

References • http://www.cs.rutgers.edu/~ryder/415/lectures/runtimeSyst3.pdf • http://pds.twi.tudelft.nl/~koen/compilerbouw/slides/week11.ppt • Modern Compiler Implementation in Java, Appel , Cambridge University Press, Chapter 6

Activation Records