10/16/2015IT 3271 All about binding n Variables are bound (dynamically) to values n values must be...

04/20/23 IT 327 1

All about binding

Variables are bound (dynamically) to values values must be stored somewhere in the memory.

Memory Locations for Variables (ch 12)

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Imperative Functional Imperative languages

a := 0– Store a zero in a’s memory location

Functional languages

val a = 0 – Bind a to the value zero

a := 0;

a := 1;

val a = 0;

val a = 1;

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Function Activations Activation of a function :

The lifetime of one execution of the function, from call to corresponding return.

most modern languages

If each activation has its own binding for variables, the variables are called activation-specific variable

(dynamic or automatic)

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Block Activations

A variable might be specific to a particular block (within a function):

fun fact n = if (n=0) then 1 else let val b = fact (n-1) in n*b end;

Do we have it in C++/JAVA?

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Other Lifetimes For Variables

We usually have a way to declare a variable that is bound to an independent memory (independent from any functions)

static allocation, the loader does the job of allocation

int count = 0;

int nextcount() {

int inc = 1; count = count + inc; return count;}

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Scope Activation

George Washington

America A.D. 1732-1799

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In most modern languages, variables with local scope have activation-specific lifetimes, by default

some exception

int nextcount() { static int count = 0; int inc = 1;

count = count + inc; return count;}

not activation-specific

(lifetime) Scope ActivationGeorge Washington

America A.D. 1732-1799

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there are other lifetimes for variables

In OOP, some variables’ lifetimes are associated with object lifetimes

Some variables may last across multiple executions of the program

In addition to activation-specific variables

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Activation Records

Each function being executed has an activation record

An activation record contains information about :

1. Activation-specific variables

2. Return address (or pointer to the current instructions)

3. Link to caller’s activation record

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Block Activation Records

When a block is entered, space (memory) must be found for the local variables of that block

Possibile implementations:– Preallocate in the containing function’s

activation record– Extend the function’s activation record when

the block is entered (and revert when exited)– Allocate separate block activation records

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Static Allocation The simplest approach: allocate one activation

record for every function, statically

Older dialects of Fortran and Cobol used this system

Simple and fast, but....(what’s the problem?)

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Fortran Example

FUNCTION AVG (ARR, N) DIMENSION ARR(N) SUM = 0.0 DO 100 I = 1, N SUM = SUM + ARR(I)100 CONTINUE AVG = SUM / FLOAT(N) RETURN END

return address

ARR address

N address

I

SUM

AVG

Static activation record

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Drawback Each function has one activation record, thus

There can be only one activation.

Modern languages (including modern dialects Cobol and Fortran) do not obey this restriction for:

1. Recursion2. Multithreading

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Stacks Of Activation Records

To support recursion, we need to allocate a new activation record for each activation

Dynamic allocation:

allocate deallocate

A stack of activation records: stack frames

push pop

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Current Activation Recordthe one for the function that is running

Static: location of activation record was determined before runtime

Dynamic: location of the current activation record is not known until runtime

A function must know how to find the address of its current activation record. A machine register is reserved to hold this

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C Example

int fact(int n) { int result; if (n<2) result = 1; else result = n * fact(n-1); return result;}

previous activation record

return address

n: 3

result: ?

current activation record

The evaluation of fact(3) before the 1st recursive call, fact(2)

....

cout << ... << fact(3) << ....

..... somewhere

calling fact(3)

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before calling fact(1)previous

activation record

return address

n: 2

result: ?



return address

n: 3

result: ?

After calling fact(2)

calling fact(3)calling fact(2)

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return address

n: 2

result: ?


return address

n: 3

result: ?


return address

n: 1

result: 1


Before fact(1)returns

calling fact(3)calling fact(2)calling fact(1)

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The second activation is about to return.


return address

n: 2

result: 2


return address

n: 3

result: ?


return address

n: 1

result: 1


12

2

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The first activation is about to return with the result fact(3) = 6.


return address

n: 2

result: 2


return address

n: 3

result: 6


return address

n: 1

result: 1


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ML Example

fun halve nil = (nil, nil)| halve [a] = ([a], nil)| halve (a::b::cs) = let val (x, y) = halve cs in (a::x, b::y) end;

halve [1,2,3,4]


return address

parameter: [1,2,3,4]

a: 1

b: 2

cs: [3,4]

x: ?

y: ?

value to return: ?


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return address

parameter: [3,4]

a: 3

b: 4

cs: []

x: ?

y: ?


return address


a: 1

b: 2

cs: [3,4]

x: ?

y: ?

value to return: ?

value to return: ?


This shows the contents of memory just before the third activation.

halve [3,4]

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return address

parameter: [3,4]

a: 3

b: 4

cs: []

x: ?

y: ?


return address


a: 1

b: 2

cs: [3,4]

x: ?

y: ?

value to return: ?

value to return: ?


return address

parameter: []

value to return: ([], [])


This shows the contents of memory just before the third activation returns.

halve []

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return address

parameter: [3,4]

a: 3

b: 4

cs: []

x: []

y: []


return address


a: 1

b: 2

cs: [3,4]

x: ?

y: ?

value to return: ([3], [4])

value to return: ?


return address

parameter: []



The second activation is about to return.

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return address

parameter: [3,4]

a: 3

b: 4

cs: []

x: []

y: []


return address


a: 1

b: 2

cs: [3,4]

x: [3]

y: [4]

value to return: ([3], [4])

value to return: ([1,3],[2,4])


return address

parameter: []



The first activation is about to return with the result halve [1,2,3,4] =([1,3],[2,4])

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Nesting Functions

– Function definitions can be nested inside other function definitions

– Inner functions can refer to local variables of the outer functions (under the usual block scoping rule)

ML, Ada, Pascal provide such feature; C, Java don’t. (for good reasons)

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Called by another inner function

An easy solution: further trace down the previous activation record.

Called by a recursive call.

f(…) {int i = 3;

}

…… h(…) ……

g(…) {… i …

}

h(…) {

g(…)… …

}

f(…) {int i = 3;

… g(…) …}

g(…) {… g(…)…… i …

}

What is i in function g?

The tricky part is that, the previous activation record may not help

int i = 2;

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Static Scoping vs Dynamic Scoping

What is the vale of i in function g ? (suppose h f g)

f(…) {int i = 3;

}

h(…) {int i = 2;……f(…)……

}……g(…)……

g(…) {… i …

}dynamic scoping

static scoping

Static Scoping Dynamic Scoping

i = 3i = 3

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Static Scoping and Nesting Link

For Static Scoping: An inner function needs to find the address of the most recent activation of

the outer function, not the caller (caller is for dynamic) We can keep a nesting link in the activation record…

f(…) {int i = 3;

}

h(…) {int i = 2;……g(…)……

}……g(…)……

g(…) {… i …

}dynamic scoping

static scoping

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fun split nil = (nil,nil)| split [a] = ([a],nil)| split [a,b] =

if (a <= b) then ([a],[b])else ([b],[a])

| split (a::b::c) = let val (x,y) = split (a::c) in if (a < b) then (x, b::y) else (b::x, y) end; fun quicksort nil = nil| quicksort [a] = [a]| quicksort a = let val (x,y) = split a in quicksort(x)@quicksort(y) end;

Quick Sort in ML

pivot

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Better ML

fun quicksort nil = nil| quicksort (pivot::rest) = let fun split(nil) = (nil,nil) | split(x::xs) = let val (below, above) = split(xs) in if x < pivot then (x::below, above)

else (below, x::above) end; val (below, above) = split(rest) in quicksort below @ [pivot] @ quicksort above end;

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time

Current Activation Record

Register

Can’t find pivot here

QuickSort(…)

Return Address

Previous Activation Record

QuickSort local variable:pivote, rest,

…

Split(…)

Return Address


QuickSort local variable:

x, xs,…

Split(…)

Return Address



x, xs,…

Split(…)

Return Address



x, xs,…

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time


QuickSort(…)

Return Address


QuickSort local variable:pivote, rest,

…

Split(…)

Return Address



x, xs,…

Split(…)

Return Address



x, xs,…

Split(…)

Return Address



x, xs,…

Nesting link Nesting link Nesting linkNesting link

null

The rules to setup the nesting links: 1, 2, 31

233

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How to Set Nesting Links

For one level of nesting:

– 1. Calling from outer to outer: (non-nesting function, e.g. main calls quicksort)

set to null

– 2. Calling from outer to inner: (e.g. quicksort calls split, where split is defined inside quicksort)

set nesting link same to caller’s activation record

– 3. Calling from inner to inner: (e.g. split calls split) set nesting link same to caller’s nesting link

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Multiple Levels Of Nesting

References n nesting levels away chain back through n nesting links

f(…) {int fi;

}

g(…) { int gi;

}

h(…) {int hi… …hi, gi, fi

}

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Common methods for referring to non-local variables in outer functions

– Nesting links in activation records

– Displays: no nesting links in the activation records, but collected in a single static array

– Lambda lifting: passing all needed variables as parameters (i.e., as hidden parameters)

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Functions As Parameters

What really gets passed?

Source code, compiled code, pointer to code, or implementation in some other form

macro expanding OOP objects C’s way

functional languages

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Example

fun addXToAll (x,theList) = let fun addX y = y + x; in map addX theList end;

addXToAll (call) map, map (call) addX, addX (refers) x

In the previous example: quicksort (call) split

5, [2,3,4] [7,8,9]

where to?

Nesting link

Nesting link

(in addXToAll’s activation record)

map’s activation record?

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When map addX, what nesting link will addX be given?– Not map’s activation record: because addX is not

nested inside map – Not map’s nesting link: because map is not nested

inside any other funcion

Therefore, the parameter addX passed to map must include the nesting link to use when addX is called,

i.e., the nesting link of addx should be prepared when addToAll tries to pass it as an argument to map

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Examplefun addXToAll (x,theList) = let fun addX y = y + x; in map addX theList end;

Right before the call to map.

The variable addX is bound to a function-value including code and nesting link.


y=>y+xNesting link

Parameter

Return address

Nesting link

Previous activation

record

x

theList

addX

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Not Just For Parameters

Functional languages allow many more kinds of operations on function-values:– passed as parameters, – returned from functions, – constructed by expressions, etc.

Function-values include both code to call, and nesting link to use when calling it

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One More Complication

What happens if a function created is to be returned as a value?

fun funToAddX x = let fun addX y = y + x; in addX end;

fun test = let val f = funToAddX 3; in f 5 end;

return a function thatadd x

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Before funToAddX returns addx.

y=>y+xNesting link

Parameter :3

Return address

Nesting link:null

Previous activation record

x=3

addX


Parameter Return address

Nesting link…

f:?

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y=>y+xNesting link

Parameter :3

Return address

Nesting link:null

Previous activation record

x=3

addX


Parameter Return address

Nesting link…

f:

After funToAddX returns, f is the bound to the new function-value. Any Problem ?

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Problem -- This will fail if the language system deallocated that

activation record when the function returned

Solution: keep all activation records (ML’s way)

New problem:

Solution:

waste too much memory

Garbage collection

New problem: ??

10/16/2015IT 3271 All about binding n Variables are bound (dynamically) to values n values must be...

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