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

incomingargs

locallocalvariablesvariables

return addressreturn address

tempstemps

saved saved registersregisters

arg narg n....

arg 1arg 1Static linkStatic link

arg narg n....

arg 1arg 1static linkstatic link

outgoingargs

currentframe

prevframe

nextframe

stackpointer

framepointer

low

er

mem

ory

ad

dre

sses

hig

her

ad

dre

sses

• 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 narg n....

arg 1arg 1stackpointer

framepointer

arg arg nn....

arg arg 11

stackpointer

framepointer

: frame sizeeither fixed or varies => Can be determined very late

Frame PointerFrame Pointer

• g calls f(a1, a2, ………, an)

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 karg k....

arg 1arg 1

arg narg n....

arg k+arg k+11

register

savearea

framepointer

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 Linksprettyprint 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 Callsprettyprint show ident

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.javapublic 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 1st 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(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()”

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(t481)

• 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 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

allocLocal()

allocLocal()

allocLocal()

v1

v2

v3

v1 might

use same space

framepointer

stackpointer

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 1st

formal parameter.