Lecture#22.ppt

8/19/2019 Lecture#22.ppt

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Lecture # 22

Run-Time Environments

Chapter 7

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Runtime Environments

• Before code generation static source text of a

program needs to be related to the actions that

must occur at runtime to implement the program.• As execution proceeds same name in the source

text can denote different data objects in the target

machine.

• The allocation and de allocation of data objects is

managed by the runtime support package.

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7.1 Source Language Issues

• Each execution of a function is referred to

as an activation of the procedure/function

• If the function is recursive several of its

activations may be alive at the same time

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Procedure Activation and

Lifetime• A procedure is activated when called

• The lifetime of an activation of a procedure

is the sequence of steps between the firstand last steps in the execution of theprocedure body

• A procedure is recursive if a new activationcan begin before an earlier activation of thesame procedure has ended


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Activation Tree• We use an activation tree to depict the way control

enters and leaves activations. In an activation tree:

• Each node represents activation of a procedure

• The root represents activation of main program

• The node for a is the parent of the node for b if

the control flows from activation a to b

• The node for a is to the left of node for b if thelifetime of a occurs before the lifetime of b

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6

Procedure Activations: Example program sort(input, output) var a : array [0..10] of integer;

procedure readarray;

var i : integer;

begin

for i := 1 to 9 do read(a[i])

end;

function partition(y, : integer) : integer

var i, !, ", v : integer;

begin #

end

procedure $uic%sort(m, n : integer);

var i : integer;

begin

if (n & m) t'en begin

i := partition(m, n); $uic%sort(m, i 1);

$uic%sort(i 1, n)

end

end;

begin

a[0] := 9999; a[10] := 9999;

readarray;

$uic%sort(1, 9)

end.

Activations: begin sort

enter readarray

*eave readarray enter $uic%sort(1,9)

enter partition(1,9)

*eave partition(1,9)

enter $uic%sort(1,+)

#

*eave $uic%sort(1,+)

enter $uic%sort(,9)

#

*eave $uic%sort(,9)

*eave $uic%sort(1,9)

end sort.


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7

Activation Trees: Example

s

$(1,9)

$(1,+)

p(1,+) $(1,0) $(-,+)

$(-,1) p(-,+) $(+,+)

p(1,9)

r

$(,9)

p(,9) $(,) $(,9)

$(,) p(,9) $(9,9)

Activation tree for the sort program

Note: also referred to as the dynamic call graph


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Control Stack

• The flow of control of a program

corresponds to depth first traversal of the

activation tree

• Control stack is used to keep track of the

live procedure activations.

• The idea is to push the node when

activation begins and pop when it ends

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Factorial Example discussed

public static int Factorial(int num)

{

if (num == 1)

{

fact=1;

}

else

{

fact=fact+num*Factorial(--num);

} return fact;

}

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Addition of Elements of Array

recursively• public static int Add(int index)

• {

• if (index == 0)

• {• sum = arr[index];

• }

• else

• {

• sum = sum + Add(arr[index]);• }

• return sum;

• }

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Control Stack

s

$(1,9)

$(1,+)

p(1,+) $(1,0) $(-,+)

p(1,9)

r

Activations: begin sort

enter readarray

*eave readarray enter $uic%sort(1,9)

enter partition(1,9)

*eave partition(1,9)

enter $uic%sort(1,+)

enter partition(1,+)

*eave partition(1,+)

enter $uic%sort(1,0)

*eave $uic%sort(1,0)

enter $uic%sort(-,+)

#

Control

stack:

Activation tree:

s

$(1,9)

$(1,+)

$(-,+)


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Scope Rules

• Environment determines name-to-object

bindings: which objects are in scope? program prg; var y : rea*;

function "(a : rea*) : rea*;

begin # end;

procedure p;

var " : integer;

begin " := 1;

#

end;

begin

y := "(0.0);

#end.

Variable " locally declared in p

A function "


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Binding of Names

• If a name is declared once in a program it

can hold different values at runtime

• The term environment refers to a function

that maps name to a storage location

• The term state is referred to a function that

maps a storage location to the value held

there

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Mapping Names to Values

name storage value

environment state

var i;#

i := 0;

#

i := i 1;


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Mapping Names to Values

name storage value

environment state

var i; i := 0; i := i 1;

/nvironment and states are different

n assignment c'anges t'e state but not t'e

environment

At compile time At run time


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Static and Dynamic Notions of

Bindings

Static Notion Dynamic Notion

Definition of a procedureActivations of the

procedure

Declaration of a name Bindings of the name

Scope of a declaration Lifetime of a binding


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Storage Organization

• Runtime storage may be subdivided as:

• The generated target code

• Data objects

• Control stack to keep track of procedure

activations

• The size of target code is fixed at compile time

• Similarly size of data objects may be known

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Typical subdivision of runtime

memory• A separate area of runtime memory called

Heap holds all other runtime information

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Code

Static

DataStack

Heap


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Stack Allocation

• Activation records (subroutine frames) on the run-time stack hold the state of a subroutine

• Calling sequences are code statements to createactivations records on the stack and enter data inthem– Caller’s calling sequence enters actual arguments,

control link, access link, and saved machine state

– Callee’s calling sequence initializes local data

– Callee’s return sequence enters return value

– Caller’s return sequence removes activation record


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Activation Records

(Subroutine Frames)

Returned value

Actual parameters

Optional control link

Optional access link

Save machine status

Local data

Temporaries

Caller’s

responsibility

to initialize

Callee’s

responsibility

to initialize

fp

(frame pointer)


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Control Links

fp

spControl link

Stack

growth

Callee’s activation record

Caller’s activation record

The control link is the old

value of the fp


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Access Links (Static Links)s

a "

$(1,9)

access % v

s

a "

$(1,9)

access % v

$(1,+)

access

% v

s

a "

$(1,9)

access % v

$(1,+)

access

% v

p(1,+) access

i !

s

a "

$(1,9)

access % v

$(1,+)

access

% v

p(1,+) access

i !

e(1,+)

access

The access link points to the

activation record of the static

parent procedure:

s is parent of r, e, and $

$ is parent of p


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Accessing Nonlocal Data

• To implement access to nonlocal data a in

procedure p, the compiler generates code to

traverse n p - na access links to reach theactivation record where a resides

– n p is the nesting depth of procedure p

– na is the nesting depth of the procedurecontaining a


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Parameter Passing Modes

• Call-by-value: evaluate actual parameters andenter r-values in activation record

• Call-by-reference: enter pointer to the storage ofthe actual parameter

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