6. Parameter Passing Parameter Passing CS 381 Spring 2016 Example - - PowerPoint PPT Presentation

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6. Parameter Passing Parameter Passing CS 381 Spring 2016 Example - - PowerPoint PPT Presentation

Jul 29, 2023 24 likes •248 views

CS 381 Spring 2016 6. Parameter Passing Parameter Passing CS 381 Spring 2016 Example (Formal) Parameter void f(int x, int y) { y := x+1 }; x := 3; What is the value of z? z := 1; f(2*x,z); ... it depends on the parameter

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CS 381 • Spring 2016 Parameter Passing

  • 6. Parameter Passing
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CS 381 • Spring 2016 Parameter Passing

What is the value of z? ... it depends on the 
 parameter passing mechanism.

Example

x := 3;
 z := 1; f(2*x,z); void f(int x, int y) { y := x+1 };

(Formal) Parameter Actual Parameter, or:
 Argument

2

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CS 381 • Spring 2016 Parameter Passing

Function Definition & Use

3

… parName * … … fName(parName, …)

Interface

Function/procedure/method name ➊ Parameter names and types ➋ Access to arguments ➌

Definition Use

fName(3,x+1,y,…)

Implementation Call

Defines: Pass arguments to parameters ➊ Pass control and execute code ➋ Return values as results ➌

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CS 381 • Spring 2016 Parameter Passing

Function Call

4

… 
 a := b+1
 c := a-b fName(a,b,c,…) fName(3,x+1,x,…)

Evaluate argument expressions ➊ Create activation record with parameter names on runtime stack ➋ Store values and locations in activation record ➌ Transfer control to function and run code ➍ ➎ Pass values back to call site ➏ Remove activation record from runtime stack

[〈a: ?, b: ?, c: ?〉, ... 〈x: 5〉 ...] [... 〈x: 5〉 ...] 6 [〈a: 3, b: 6, c: x〉, ... 〈x: 5〉 ...] [〈a: 7, b: 6, c: x〉, ... 〈x: 5〉 ...] [〈a: 7, b: 6, c: x〉, ... 〈x: 1〉 ...] N/A [... 〈x: 1〉 ...]

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CS 381 • Spring 2016 Parameter Passing

Call-By-Value

5

… 
 a := b+1
 c := a-b fName(a,b,c,…) fName(3,x+1,x,…)

Evaluate argument expressions ➊ Create activation record with parameter names on runtime stack ➋ Store values and locations in activation record ➌ Transfer control to function and run code ➍ ➎ Pass values back to call site ➏ Remove activation record from runtime stack

[〈a: ?, b: ?, c: ?〉, ... 〈x: 5〉 ...] [... 〈x: 5〉 ...] 6 [〈a: 3, b: 6, c: 5〉, ... 〈x: 5〉 ...] [〈a: 7, b: 6, c: 5〉, ... 〈x: 5〉 ...] [〈a: 7, b: 6, c: 1〉, ... 〈x: 5〉 ...] [... 〈x: 5〉 ...]

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CS 381 • Spring 2016 Parameter Passing

Call-By-Value

6

  • Evaluate argument
  • Bind resulting value to parameter
  • Look up value when needed
  • Local assignments ok, but are lost

after return from function call Synopsis

p := … … := p
 fName(p,…) p

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CS 381 • Spring 2016 Parameter Passing

Exercise: Compute under CBV

1 { int z; 2 int y; 3 y := 5; 4 { int f(int x){ 5 x := x+1; 6 y := x-4; 7 x := x+1; 8 return x; 9 }; 10 z := f(y)+y; 11 }; 12 }
 [] 1 [z:?] 2 [y:?, z:?] 3 [y:5, z:?] 4 [f:{}, y:5, z:?] 10 >> 
 [x:5, f:{}, y:5, z:?] 5 [x:6, f:{}, y:5, z:?] 6 [x:6, f:{}, y:2, z:?] 7 [x:7, f:{}, y:2, z:?] 8 [res:7, x:7, f:{}, y:2, z:?] << 10 [f:{}, y:2, z:9] ...


7

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CS 381 • Spring 2016 Parameter Passing

Call-By-Reference

8

… 
 c := c-2 fName(ref c,…) fName(3,x+1,x,…)

Evaluate argument expressions ➊ Create activation record with parameter names on runtime stack ➋ Store values and locations in activation record ➌ Transfer control to function and run code ➍ ➎ Pass values back to call site ➏ Remove activation record from runtime stack

[〈c: ?〉, ... 〈x: 5〉 ...] [... 〈x: 5〉 ...] [... 〈x: 3〉 ...] [〈c: 〉, ... 〈x: 5〉 ...] [〈c: 〉, ... 〈x: 3〉 ...]

Alias, or:
 Synonym Indicating parameter passing schema Only variables can be arguments

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CS 381 • Spring 2016 Parameter Passing

p := … … := p


Call-By-Reference

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  • Parameter p points to variable x passed as argument
  • Parameter p is just another name for x
  • Every read/write access to p acts on x
  • Only variables can be passed as arguments

Synopsis

fName(x,…) fName(ref p,…) x

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CS 381 • Spring 2016 Parameter Passing

Exercise: Compute under CBR

1 {int z; 2 int y; 3 y := 5; 4 {int f(ref int x){ 5 x := x+1; 6 y := x-4; 7 x := x+1; 8 return x; 9 }; 10 z := f(y)+y; 11 }; 12 }
 [] 1 [z:?] 2 [y:?, z:?] 3 [y:5, z:?] 4 [f:{}, y:5, z:?] 10 >> [x->y, f:{}, y:5, z:?] 5 [x->y, f:{}, y:6, z:?] 6 [x->y, f:{}, y:2, z:?] 7 [x->y, f:{}, y:3, z:?] 8 [res:3, x->y, f:{}, y:3, z:?] 
 << 10 [f:{}, y:3, z:6] ...


10

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CS 381 • Spring 2016 Parameter Passing

Call-By-Value-Result

11

… 
 c := c-2 fName(valres c,…) fName(3,x+1,x,…)

Evaluate argument expressions ➊ Create activation record with parameter names on runtime stack ➋ Store values and locations in activation record ➌ Transfer control to function and run code ➍ ➎ Pass values back to call site ➏ Remove activation record from runtime stack

[〈c: ?〉, ... 〈x: 5〉 ...] [... 〈x: 5〉 ...] [... 〈x: 3〉 ...] [〈c: 5〉, ... 〈x: 5〉 ...] [〈c: 3〉, ... 〈x: 5〉 ...]

Indicating parameter passing schema

[〈c: 3〉, ... 〈x: 3〉 ...]

Only variables can be arguments

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CS 381 • Spring 2016 Parameter Passing

p := … … := p


Call-By-Value-Result

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  • Copy value of argument variable x to parameter p
  • Every read/write access to p acts on p
  • Copy value of parameter p back to argument variable x
  • Only variables can be passed as arguments

Synopsis

fName(x,…) fName(valres p,…) x

copy in copy out

p

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CS 381 • Spring 2016 Parameter Passing

When exactly does the copy-out step happen when returning from a function call? Before or after the function result is set?

x := f(x); int f(valres int y) { 
 y := 7;
 return 5 }

Call-By-Value-Result & Function Results

Does it matter?

13

[〈y: 0〉, 〈x: 0〉 ...] [〈x: 0〉 ...] [〈y: 7〉, 〈x: 0〉 ...] [〈res: 5, y: 7〉, 〈x: 0〉 ...] [〈x: 5〉 ...]

“before”

[〈x: 7〉 ...]

“after” Yes! defined behavior

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CS 381 • Spring 2016 Parameter Passing

\

Exercise: Compute under CBVR

1 {int z; 2 int y; 3 y := 5; 4 {int f(valres x){ 5 x := x+1; 6 y := x-4; 7 x := x+1; 8 return x; 9 }; 10 z := f(y)+y; 11 }; 12 }
 [] 1 [z:?] 2 [y:?, z:?] 3 [y:5, z:?] 4 [f:{}, y:5, z:?] 10 >> [x:5, f:{}, y:5, z:?] 5 [x:6, f:{}, y:5, z:?] 6 [x:6, f:{}, y:2, z:?] 7 [x:7, f:{}, y:2, z:?] 8 [res:7, x:7, f:{}, y:2, z:?] 9 [res:7, x:7, f:{}, y:7, z:?] 
 << 10 [f:{}, y:7, z:14] ...


14

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CS 381 • Spring 2016 Parameter Passing

\

Comparison: CBV, CBR, CBVR

1 {int z; 2 int y; 3 y := 5; 4 {int f(? x){ 5 x := x+1; 6 y := x-4; 7 x := x+1; 8 return x; 9 }; 10 z := f(y)+y; 11 }; 12 }


[x:5, f:{}, y:5, z:?] 5 [x:6, f:{}, y:5, z:?] 6 [x:6, f:{}, y:2, z:?] 7 [x:7, f:{}, y:2, z:?] 8 [res:7, x:7, f:{}, y:2, z:?] 9 [res:7, x:7, f:{}, y:7, z:?] <<

10 [f:{}, y:7, z:14] 


15 [x->y, f:{}, y:5, z:?] 5 [x->y, f:{}, y:6, z:?] 6 [x->y, f:{}, y:2, z:?] 7 [x->y, f:{}, y:3, z:?] 8 [res:3, x->y, f:{}, y:3, z:?] <<

10 [f:{}, y:3, z:6]


[x:5, f:{}, y:5, z:?] 5 [x:6, f:{}, y:5, z:?] 6 [x:6, f:{}, y:2, z:?] 7 [x:7, f:{}, y:2, z:?] 8 [res:7, x:7, f:{}, y:2, z:?] <<

10 [f:{}, y:2, z:9]


CBV CBR CBVR

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CS 381 • Spring 2016 Parameter Passing

Call-By-Name

16

z := a-2
 x := 2
 z := 2*a fName(name a,…) fName(x+1,…)

Evaluate argument expressions ➊ Create activation record with parameter names on runtime stack ➋ Store values and locations in activation record ➌ Transfer control to function and run code ➍ ➎ Pass values back to call site ➏ Remove activation record from runtime stack

[〈a: ?〉, ... 〈x: 5〉 ...] [... 〈x: 5〉 ...] [... 〈x: 2〉 ...] [〈a: x+1〉, ... 〈x: 5〉 ...] [〈z: 4, a: x+1〉, ... 〈x: 2〉 ...]

Indicating parameter passing schema

expressions

[〈z: 4, a: x+1〉, ... 〈x: 5〉 ...] [〈z: 6, a: x+1〉, ... 〈x: 2〉 ...]

local variable

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CS 381 • Spring 2016 Parameter Passing

… := … p


Call-By-Name

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  • Bind argument expression e to parameter p
  • Every read access to p evaluates e anew
  • Imagine substituting (e) for p in the body of fName
  • Assignments to parameter are not allowed

Synopsis

fName(e,…) fName(name p,…) … := … (e)

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CS 381 • Spring 2016 Parameter Passing

Call-By-Need

18

z := a-2
 x := 2
 z := 2*a fName(need a,…) fName(x+1,…)

Evaluate argument expressions ➊ Create activation record with parameter names on runtime stack ➋ Store values and locations in activation record ➌ Transfer control to function and run code ➍ ➎ Pass values back to call site ➏ Remove activation record from runtime stack

[〈a: ?〉, ... 〈x: 5〉 ...] [... 〈x: 5〉 ...] [... 〈x: 2〉 ...] [〈a: x+1〉, ... 〈x: 5〉 ...] [〈z: 4, a: 6〉, ... 〈x: 2〉 ...]

Indicating parameter passing schema

expressions, replaced by values

[〈z: 4, a: 6〉, ... 〈x: 5〉 ...] [〈z: 12, a: 6〉, ... 〈x: 2〉 ...]

local variable

[〈z: ?, a: x+1〉, ... 〈x: 5〉 ...]

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CS 381 • Spring 2016 Parameter Passing

Call-By-Need

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  • Similar to call-by-name, except:
  • First read access to p evaluates e to v and stores v in p
  • Subsequent accesses to p retrieve v

Synopsis

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CS 381 • Spring 2016 Parameter Passing

Call-By-... Value Reference Value-Result Name Need restriction on argument var only var only restriction on parameter no assignment no assignment what is stored in activation record value pointer value expr expr/value how to access parameter value lookup deref lookup eval eval, then lookup

  • n return from call

copy value to actual param

Comparison

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CS 381 • Spring 2016 Parameter Passing

Classification

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Value Flow in only in & out CBV CBR CBVR CBN CBNeed Activation
 Record
 Content Value Pointer Expression

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CS 381 • Spring 2016 Parameter Passing

Example Languages

22

Call-by-Need

Haskell

Call-by-Value Call-by-Reference Call-by-Value-Result Call-by-Name