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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 first
and last steps in the execution of the
procedure body
• A procedure is recursive if a new activation
can begin before an earlier activation of the
same 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 the
lifetime of a occurs before the lifetime of b
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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, z : integer) : integer
var i, j, x, v : integer;
begin …
end
procedure quicksort(m, n : integer);
var i : integer;
begin
if (n > m) then begin
i := partition(m, n);
quicksort(m, i - 1);
quicksort(i + 1, n)
end
end;
begin
a[0] := -9999; a[10] := 9999;
readarray;
quicksort(1, 9)
end.
Activations:
begin sort
enter readarray
leave readarray
enter quicksort(1,9)
enter partition(1,9)
leave partition(1,9)
enter quicksort(1,3)
…
leave quicksort(1,3)
enter quicksort(5,9)
…
leave quicksort(5,9)
leave quicksort(1,9)
end sort.
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Activation Trees: Example
s
r
q(1,9)
p(1,9)
q(1,3)
p(1,3)
q(1,0) q(2,3)
q(5,9)
p(5,9)
p(2,3) q(2,1) q(3,3)
q(5,5) q(7,9)
p(7,9) q(7,7) q(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
Activation tree:
s
r
p(1,9)
p(1,3)
q(1,9)
q(1,3)
q(1,0) q(2,3)
Control
stack:
s
q(1,9)
q(1,3)
q(2,3)
Activations:
begin sort
enter readarray
leave readarray
enter quicksort(1,9)
enter partition(1,9)
leave partition(1,9)
enter quicksort(1,3)
enter partition(1,3)
leave partition(1,3)
enter quicksort(1,0)
leave quicksort(1,0)
enter quicksort(2,3)
…
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Scope Rules
• Environment determines name-to-object
bindings: which objects are in scope?
Variable x locally declared in p
A function x
program prg;
var y : real;
function x(a : real) : real;
begin … end;
procedure p;
var x : integer;
begin
x := 1;
…
end;
begin
y := x(0.0);
…
end.
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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
environment
name
state
storage
var i;
…
i := 0;
…
i := i + 1;
value
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Mapping Names to Values
At compile time
At run time
environment
name
state
storage
value
var i; i := 0; i := i + 1;
Environment and states are different
An assignment changes the state but not the environment
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Static and Dynamic Notions of
Bindings
Static Notion
Dynamic Notion
Definition of a procedure
Activations 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
Code
Static
Data
Stack
Heap
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Stack Allocation
• Activation records (subroutine frames) on the runtime stack hold the state of a subroutine
• Calling sequences are code statements to create
activations records on the stack and enter data in
them
– 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)
fp
(frame pointer)
Returned value
Actual parameters
Optional control link
Optional access link
Caller’s
responsibility
to initialize
Save machine status
Local data
Temporaries
Callee’s
responsibility
to initialize
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Control Links
The control link is the old
value of the fp
Caller’s activation record
fp
sp
Control link
Stack
growth
Callee’s activation record
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Access Links (Static Links)
s
s
s
s
a x
a x
a x
a x
q(1,9)
access
k v
q(1,9)
access
k v
q(1,9)
access
k v
q(1,9)
access
k v
q(1,3)
access
k v
q(1,3)
access
k v
q(1,3)
access
k v
p(1,3)
access
i j
p(1,3)
access
i j
The access link points to the
activation record of the static
parent procedure:
s is parent of r, e, and q
q is parent of p
e(1,3)
access
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Accessing Nonlocal Data
• To implement access to nonlocal data a in
procedure p, the compiler generates code to
traverse np - na access links to reach the
activation record where a resides
– np is the nesting depth of procedure p
– na is the nesting depth of the procedure
containing a
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Parameter Passing Modes
• Call-by-value: evaluate actual parameters and
enter r-values in activation record
• Call-by-reference: enter pointer to the storage of
the actual parameter