Activation Records Chapter 6

2 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

3 Higher-order Function 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 nested function + higher-order function -> not in stack-mode Pascal -> no higher function C -> no nested function  runtime stack

Stack Frames incoming args localvariables return address tempssavedregisters arg n.. arg 1 Static link arg n.. arg 1 static link outgoing args current frame prev frame next frame stack pointer frame pointer lower memory addresses higher addresses 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 4

5 arg n.. arg 1 stack pointer frame pointer arg n.. arg 1 stack pointer frame pointer : frame size either fixed or varies => Can be determined very late Frame Pointer g calls f(a 1, a 2, ………, a n ) 5

6 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.

7 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

8 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 arg k.. arg 1 arg n.. arg k+ 1 register save area frame pointer

9 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)

10 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)

11 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.

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

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

14 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()

15 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;} }

16 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); }

17 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

18 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 1 st argument escaped (No parameters will be escapes in MiniJava.) frame,newFrame(g, new BoolList(true, new BoolList(false, new BoolList(false,null))))

19 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; }

20 Access and Allocate the Vars InFrame(X) : a memory location at offset X from the FP(frame pointer) InReg(t 84 ) : in register t 84 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() ”

21 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

22 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(t 481 ) allocLocal(bool) –Called when frame is create –Called when nested block is entered

Allocating Local Storage in frame with the Same Name function f() = let var v 1 := 6 in print(v 1 ); let var v 2 := 7 in print(v 2 ); end print(v 1 ); let var v 3 := 8 in print(v 3 ); end print(v 1 ); end allocLocal() v1v1 v2v2 v3v3 v 1 might use same space frame pointer stack pointer

24 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

25 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

26 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

27 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 1 st formal parameter.