1. Run-Time EnvironmentsRun Time Environments
TCS-502
Compiler Design
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2. Run-Time Environments
• How do we allocate the space for the generated target code and the data
object of our source programs?
• The places of the data objects that can be determined at compile time will beThe places of the data objects that can be determined at compile time will be
allocated statically.
• But the places for the some of data objects will be allocated at run‐time.
• The allocation of de‐allocation of the data objects is managed by the run‐time
support package.
– run‐time support package is loaded together with the generate target code.
– the structure of the run‐time support package depends on the semantics of the
programming language (especially the semantics of procedures in that language).
• Each execution of a procedure is called as activation of that procedure.p f p
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3. Procedure Activations
• An execution of a procedure starts at the beginning of the procedure body;
• When the procedure is completed, it returns the control to the point immediately
after the place where that procedure is called.
• Each execution of a procedure is called as its activation.
Lif ti f ti ti f d i th f th t b t th• Lifetime of an activation of a procedure is the sequence of the steps between the
first and the last steps in the execution of that procedure (including the other
procedures called by that procedure).
• If a and b are procedure activations, then their lifetimes are either non‐overlapping
or are nested.
• If a procedure is recursive a new activation can begin before an earlier activation of• If a procedure is recursive, a new activation can begin before an earlier activation of
the same procedure has ended.
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4. Activation Tree
• We can use a tree (called activation tree) to show the way control
enters and leaves activations.
• In an activation tree:
– Each node represents an activation of a procedure.
– The root represents the activation of the main program.
h d i f h d b iff h l fl f– The node a is a parent of the node b iff the control flows from a
to b.
– The node a is left to to the node b iff the lifetime of a occursThe node a is left to to the node b iff the lifetime of a occurs
before the lifetime of b.
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5. Activation Tree (cont.)
program main; enter main
procedure s; enter pp p
begin ... end; enter q
procedure p; exit q
dprocedure q; enter s
begin ... end; exit s
begin q; s; end; exit pbegin q; s; end; exit p
begin p; s; end; enter s
exit s
exit main
A Nested Structure
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A Nested Structure

6. Activation Tree (cont.)
mainmain
p sp s
q sq
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7. Procedure Activations
program sort(input, output)
var a : array [0..10] of integer;
procedure readarray;
var i : integer;
b i
Activations:
begin sortbegin
for i := 1 to 9 do read(a[i])
end;
function partition(y, z : integer) : integer
var i, j, x, v : integer;
begin sort
enter readarray
leave readarray
enter quicksort(1,9)
enter partition(1,9)
begin …
end
procedure quicksort(m, n : integer);
var i : integer;
begin
e te pa t t o ( ,9)
leave partition(1,9)
enter quicksort(1,3)
…
leave quicksort(1,3)begin
if (n > m) then begin
i := partition(m, n);
quicksort(m, i - 1);
quicksort(i + 1, n)
q ( , )
enter quicksort(5,9)
…
leave quicksort(5,9)
leave quicksort(1,9)
end
end;
begin
a[0] := -9999; a[10] := 9999;
readarray;
q ( , )
end sort.
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readarray;
quicksort(1, 9)
end.

8. Activation TreesActivation Trees
ss
q(1,9)r
q(1,3)
p(1,3) q(1,0) q(2,3)
p(1,9) q(5,9)
p(5,9) q(5,5) q(7,9)
q(2,1)p(2,3) q(3,3) q(7,7)p(7,9) q(9,9)
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9. Control Stack
• The flow of the control in a program corresponds to a depth‐• The flow of the control in a program corresponds to a depth‐
first traversal of the activation tree that:
– starts at the root, ,
– visits a node before its children, and
– recursively visits children at each node an a left‐to‐right order.
• A stack (called control stack) can be used to keep track of
live procedure activations.
– An activation record is pushed onto the control stack as the
activation starts.
That activation record is popped when that activation ends– That activation record is popped when that activation ends.
• When node n is at the top of the control stack, the stack
contains the nodes along the path from n to the root
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contains the nodes along the path from n to the root.

10. Control Stack
Activations:ControlActivation tree:
s
(1 9)r
Activations:
begin sort
enter readarray
leave readarray
Control
stack:
Activation tree:
s
q(1,9)
q(1,3)p(1,9)
r enter quicksort(1,9)
enter partition(1,9)
leave partition(1,9)
enter quicksort(1,3)
q(1,9)
q(1,3)
q(2,3)
p(1,3) q(1,0) q(2,3)
enter partition(1,3)
leave partition(1,3)
enter quicksort(1,0)
leave quicksort(1,0)
(2 3)enter quicksort(2,3)
…
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11. Variable Scopes
• The same variable name can be used in the different parts of the
program.
• The scope rules of the language determine which declaration of a• The scope rules of the language determine which declaration of a
name applies when the name appears in the program.
• An occurrence of a variable (a name) is:
– local: If that occurrence is in the same procedure in which that name is declared.
– non‐local: Otherwise (ie. it is declared outside of that procedure)
procedure p;
var b:real;
procedure p;procedure p;
var a: integer; a is local
begin a := 1; b := 2; end; b is non-local
begin end;
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begin ... end;

12. Run-Time Storage Organization
Code
Memory locations for code are
determined at compile time.
Static Data Locations of static data can also be
determined at compile time.
Stack
Data objects allocated at run‐time.
(Activation Records)(Activation Records)
Other dynamically allocated data
Heap
Other dynamically allocated data
objects at run‐time. (For example,
malloc area in C).
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13. Activation Records
• Information needed by a single execution of a procedure is
managed using a contiguous block of storage called
i i dactivation record.
• An activation record is allocated when a procedure is
entered, and it is de‐allocated when that procedure exited.
• Size of each field can be determined at compile timeSize of each field can be determined at compile time
(Although actual location of the activation record is
determined at run‐time).)
– Except that if the procedure has a local variable and its size depends
on a parameter, its size is determined at the run time.
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p

14. Activation Records (cont.)
return value
The returned value of the called procedure is returned
in this field to the calling procedure. In practice, we may
use a machine register for the return value.
actual parameters
optional control link
g
The field for actual parameters is used by the calling
procedure to supply parameters to the called procedure.
Th i l l li k i h i i doptional control link
optional access link
The optional control link points to the activation record
of the caller.
The optional access link is used to refer to nonlocal data
h ld i h i i d
saved machine status
local data
held in other activation records.
The field for saved machine status holds information about
the state of the machine before the procedure is called.
temporaries
The field of local data holds data that local to an execution
of a procedure..
Temporay variables is stored in the field of temporaries.
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Temporay variables is stored in the field of temporaries.

15. Activation Records (Ex1)
program main;
procedure p;
main
stack
procedure p;
var a:real;
procedure q;
var b:integer;
pbegin ... end;
begin q; end;
procedure s;
var c:integer;
p
a:
var c:integer;
begin ... end;
begin p; s; end; q
main
p
b:
p
q
s
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q

16. Activation Records for Recursive Procedures
program main;
procedure p;
function q(a:integer):integer;
main
function q(a:integer):integer;
begin
if (a=1) then q:=1;
else q:=a+q(a-1);
p
end;
begin q(3); end;
begin p; end;
q(3)
a: 3
q(2)
a:2
q(1)
a:1
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17. Creation of An Activation Record
• Who allocates an activation record of a procedure?
– Some part of the activation record of a procedure is created bySome part of the activation record of a procedure is created by
that procedure immediately after that procedure is entered.
– Some part is created by the caller of that procedure before that
procedure is entered.
• Who deallocates?• Who deallocates?
– Callee de‐allocates the part allocated by Callee.
– Caller de‐allocates the part allocated by Caller.Caller de allocates the part allocated by Caller.
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18. Creation of An Activation Record (cont.)
return value
actual parameters
ti l t l li k
Caller’s Activation Record
optional control link
optional access link
saved machine status
local data
temporaries
return value
Caller’s Responsibility
C ll ’ A i i R dreturn value
actual parameters
optional control link
Callee’s Activation Record
optional access link
saved machine status
local data
Callee’s Responsibility
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temporaries

19. Variable Length Data
return value
actual parameters
ti l t l li k
Variable length data is allocated after
optional control link
optional access link
saved machine status
temporaries, and there is a link to from
local data to that array.
local data
pointer a
pointer b
temporaries
array a
array b
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20. Dangling Reference
• Whenever a storage is de-allocated, the problem of dangling references
may accur.y
main () {
i *int *p;
p = dangle();
}
int *dangle*() {
int i=2;
return &i;return &i;
}
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21. Access to Nonlocal Names
• Scope rules of a language determine the treatment of references to
nonlocal names.
• Scope Rules:
– Lexical Scope (Static Scope)
• Determines the declaration that applies to a name by examining
the program text alone at compile‐time.
• Most closely nested rule is used• Most‐closely nested rule is used.
• Pascal, C, ..
– Dynamic ScopeDynamic Scope
• Determines the declaration that applies to a name at run‐time.
• Lisp, APL, ...
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22. Lexical Scope
• The scope of a declaration in a block‐structured language is given
by the mostly closed rule.y y
• Each procedure (block) will have its own activation record.
– procedure
begin end blocks– begin‐end blocks
• (treated same as procedure without creating most part of its activation record)
• A procedure may access to a nonlocal name using:
– access links in activation records, or
– displays (an efficient way to access to nonlocal names)
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23. Access Links
program main;
var a:int;
procedure p;
main
access link
p p;
var d:int;
begin a:=1; end;
procedure q(i:int);
a:
q(1)
access link
var b:int;
procedure s;
var c:int;
i,b:
q(0)
access link
Access
Links
begin p; end;
begin
if (i<>0) then q(i-1)
else s;
i,b:
s
access linkelse s;
end;
begin q(1); end;
c:
p
access link
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access link
d:

24. Access Links
s s s s
a x
q(1,9)
a x
q(1,9)
a x
q(1,9)
a x
q(1,9)q(1,9)
access
k v
q(1,9)
access
k v
q(1 3)
q(1,9)
access
k v
q(1 3)
q(1,9)
access
k v
q(1 3)q(1,3)
access
k v
q(1,3)
access
k v
p(1 3)
q(1,3)
access
k v
p(1 3)p(1,3)
access
i j
p(1,3)
access
i j
e(1 3)
24
e(1,3)
access

25. Procedure Parameters
program main;
procedure p(procedure a);
main
access link
procedure p(procedure a);
begin a; end;
procedure q;
procedure s;
q
access linkprocedure s;
begin ... end;
begin p(s) end;
begin q; end;
access link
p
li k
g q
access link
sAccess links must be passed with
access link
p
procedure parameters.
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26. Displays
• An array of pointers to activation records can be used to access
activation recordsactivation records.
• This array is called as displays.
• For each level, there will be an array entry.
1:
2
Current activation record at level 1
C t ti ti d t l l 22:
3:
Current activation record at level 2
Current activation record at level 3
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27. Accessing Nonlocal Variables
program main;
var a:int;
main
access link
addrC := offsetC(currAR)
t := *currARvar a:int;
procedure p;
var b:int;
begin q; end;
a:
p
addrB := offsetB(t)
t := *t
addrA := offsetA(t)begin q; end;
procedure q();
var c:int;
begin
p
access link
b:
ADD addrA,addrB,addrC
g
c:=a+b;
end;
begin p; end;
q
access link
c:
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28. Accessing Nonlocal Variables using Display
program main;
var a:int;
main
access link
addrC := offsetC(D[3])
addrB := offsetB(D[2])
D[1]
var a:int;
procedure p;
var b:int;
begin q; end;
a:
p
addrB : offsetB(D[2])
addrA := offsetA(D[1])
ADD addrA,addrB,addrC
begin q; end;
procedure q();
var c:int;
begin
p
access link
b:
D[2]
g
c:=a+b;
end;
begin p; end;
q
access link
D[3]
c:
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29. Parameter Passing Methods
• Call-by-value
• Call-by-referenceCall by reference
• Call-by-name (used by Algol)
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30. Parameter Passing Methods
• 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
• Copy-restore (value-result): evaluate actual parameters and
enter r-values, after the call copy r-values of formal
parameters into actualsparameters into actuals
• Call-by-name: use a form of in-line code expansion (thunk) to
evaluate parameterseva uate pa a ete s
30

31. SOME OTHER ISSUES
• Symbol Tables hold scope information.
• Dynamic Scope:
– uses control link (similar to access links)
• Dynamic Storage Allocation (Heap Management)
different allocation techniques– different allocation techniques
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