Activation Records.ppt

8/10/2019 Activation Records.ppt

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Activation RecordsModern Compiler Implementation in JavaAppel

Chapter 6

CMSC 432

Shon Vick


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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) fcalls -> Many xs and ys

Runtime Stack


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arg n..

arg 1stackpointer

framepointer

arg n..

arg 1stackpointer

framepointer

: frame sizeeither fixed

or varies =>Can bedeterminedvery late

Frame Pointer

g calls f(a1, a2, , an)

5


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Registers

register : local vars, temporary results Can save load/store instructions

general purpose vs. special purposeregisters 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 dofine register save scheduling.


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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 callinganotherfunction

Register windows


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Parameter Passing (Contd)

argument passing in reg+ stack

Sometimes formalparameters are atconsecutive addresses :register save area bycallee

call-by-reference Code for dereferencing

formal parameter access

arg k..

arg 1

arg n..

arg k+1

registersavearea

framepointer


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Return Address

g calls f : f returns Need gs 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)


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Frame-resident Variables

Variables are written to memory only whennecessary Variable will be passed by reference or &

(address of) operator is applied Variable is accessed by a procedure nestedinside the current one

Value is too big to fit into a single register

Variable is an array Register holding variable is needed for specialpurpose (parameter passing)

Too many local variables (spilled into frame)


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Escaped Variable

A variable escapes if it is passed byreference, its address is taken, or it

is accessed from a nested function.

Variables are bound to register or

memory in later phase in compiling.


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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


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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, AccessListt) {

head=h; tail=t;}

}


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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);}


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Making new Frames

Hold information fo parameters & localvariables

Frame for function f with k formals

newFrame(f, l) where f: Label l:BoolList

Ex: a three-argument function named g with 1stargumentescaped (No parameters will be escapes in MiniJava.)

frame,newFrame(g,

new BoolList(true,

new BoolList(false,

newBoolList(false,null))))


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Class Access

Describe formal & local vars in the frameor in registers

Abstract data type whose implementaion isvisible 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; }


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Access and Allocate the Vars

InFrame(X) : a memory location at offsetX 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()


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Representation of FrameDescriptions

Implementation of frame is an objectholding:

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 functionsmachine code is to begin

See Table 6.4 on page 129


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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 FPex) 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

f ti f()


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Allocating LocalStorage in frame

with the SameName

function f() =

let var v1:= 6in

print(v1);

let var v2:= 7in print(v2);

end

print(v1);

let var v3:= 8

in print(v3);endprint(v1);

end

allocLocal()

allocLocal()

allocLocal()

v1v2v3

v1mightuse samespace

frame

pointer

stackpointer


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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 avariable

integers and booleans are passed by value

object, including integer arrays, can be

represented as pointers that are passed byvalue


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Calculating Escapes

FindEscape(): looks for escaping variablesand records this information in the escapefields 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


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Temporaries and Labels

Temps 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 inpackage Temp Packages Frame and Temp provide machine

independent views of variables


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Managing Static Links

Static Link management is somewhattedious?

MiniJava does not have nested functiondeclararions: thus Frameshould not knowanything about static links.

It will be handled in the Translation phase.

Static links may be passed to the callee bythe 1stformal parameter.


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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
http://www.cs.rutgers.edu/~ryder/415/lectures/runtimeSyst3.pdfhttp://www.cs.rutgers.edu/~ryder/415/lectures/runtimeSyst3.pdfhttp://pds.twi.tudelft.nl/~koen/compilerbouw/slides/week11.ppthttp://pds.twi.tudelft.nl/~koen/compilerbouw/slides/week11.ppthttp://pds.twi.tudelft.nl/~koen/compilerbouw/slides/week11.ppthttp://pds.twi.tudelft.nl/~koen/compilerbouw/slides/week11.ppthttp://www.cs.rutgers.edu/~ryder/415/lectures/runtimeSyst3.pdfhttp://www.cs.rutgers.edu/~ryder/415/lectures/runtimeSyst3.pdf

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