CSC 8505Compiler Construction

Runtime Environments

2Runtime Environments

Outline

• Memory organization during program execution

• Static runtime environments• Stack-based runtime environments

– Without local procedure– With local procedure

• Parameter passing

3Runtime Environments

Memory organization during program execution

• Main memory– Store large amount of data– Slower access

• Registers– Very small amount of data– Faster access

Code area

Global/static area

stack

Free space

Heap

Main memory

registers

Data area

4Runtime Environments

Code Area• Addresses in code area

are static (i.e. no change during execution) for most programming language.

• Addresses are known at compile time.

proc 1

proc 2

proc n

entry point

entry point

entry point

5Runtime Environments

Data Area

• Addresses in data area are static for some data and dynamic for others.– Static data are located in static area.– Dynamic data are located in stack or heap.

• Stack (LIFO allocation) for procedure activation record, etc.

• Heap for user allocated memory, etc.

6Runtime Environments

Registers

• General-purpose registers– Used for calculation

• Special purpose registers– Program counter (pc)– Stack pointer (sp)– Frame pointer (fp)– Argument pointer (ap)

7Runtime Environments

Calling Sequence• Sequence of operations that must be done for

procedure calls– Call sequence

• Sequence of operations performed during procedure calls– Find the arguments and pass them to the callee. – Save the caller environment, i.e. local variables in activation

records, return address.– Create the callee environment, i.e. local variables in activation

records, callee’s entry point.– Return sequence

• Sequence of operations performed when return from procedure calls

– Find the arguments and pass them back to the caller. – Free the callee environment.– Restore the caller environment, including PC.

8Runtime Environments

Issues in Call Sequence

• Which part of the call sequence is included in the caller code? Which is in the callee code?– Save space in the code segment if the call sequence

is included in the callee code.– Normally, the caller finds the arguments and provides

them to the callee.• Which operation is supported in hardware?

– The more operations supported in hardware, the lower cost (i.e. execution time and space for code) is.

9Runtime Environments

Static runtime environments

• Static data– Both local and global variables are allocated once at

the beginning and deallocated at program termination– Fixed address

• No dynamic allocation• No recursive call

– Procedure calls are allowed, but no recursion.– One activation record for each procedure, allocated

statically

• Example:FORTRAN 77

10Runtime Environments

Memory Organization for Static Runtime Environment

PROGRAM TESTCOMMON MAXINTEGER MAXREAL TAB(10), TEMP…QMEAN(TAB, 3, TEMP)…END

SUBROUTINE QMEAN(A, SIZE, MEAN)COMMON MAXINTEGER MAX, SIZEREAL A(SIZE), MEAN, TEMPINTEGER K…END

MAX

TAB(1)TAB(2)…TAB(10)TEMP3

ASIZEMEANReturn addrTEMP K

Global area

Activation record for main

Activation record for QMEAN

11Runtime Environments

Stack-based runtime environments

• Handle recursive calls• Activation records are allocated in stack,

called rumtime stack or call stack• One procedure can have more than one

activation records in the stack at one time• Call sequence is more complex than the

sequence in static environment

12Runtime Environments

Stack-based Environments Without Local Procedures

• Maintain pointer to the current activation record– Store frame pointer in an fp register

• Record the link from an activation record to the previous record– In each activation record, a link from an

activation record to the activation record of the caller, called a dynamic link or control link is stored.

• Sometimes, the area for parameters and local variables need to be identified.

• A stack pointer is maintained in an sp register.

local var.sreturn addr

control link

parameters

SP

FP

13Runtime Environments

Call Sequence in Stack-based Environments Without Local Procedures

• Calling sequence– Push arguments– Push fp as control link– Copy sp to fp– Store return address– Jump to callee– Reserve space for local

variables• Return sequence

– Copy fp to sp– Load control link into fp– Jump to return address– Change sp to pop arguments

arguments

activation record of

main

control link

return addr

sp

fp

fp

sp

splocal var.s

sp

sp

argumentscontrol link

return addr

sp

splocal var.s

sp

sp

fp

Compute arguments and push into stackStore fp as control linkMove fpPush return addressReserve area for local variables

Calling sequenceReturn sequence

Load fp into sp (to pop local var.s and return addr)Load control link into fp

fp

Jump to return addrPop arguments

Global area

Dire

ctio

n of

sta

ck g

row

th

14Runtime Environments

Activation Treemain(){ …;

g(x); return(0); }

void g(int m){ …

f(y); … g(y);…

}

void f(int n){ g(n);

…}

Global area

activation record for main

activation record for g(2)

activation record for f(1)

activation record for g(1)

activation record for g(1)

15Runtime Environments

Calculating Address in Stack-Based Environments

• Address of local variables and parameters are calculated dynamically during execution time

• Offset of variables and parameters from fp is:– static– known during compile time– Stored in symbol table

Address of x =fp+xOffset

Address of a =fp+aOffset

parameters

control link

return address

local variables

fp

sp

x

a

aOffset

xOffset

16Runtime Environments

Considerations in Address Calculation

• Sizes of variables depend on data types• Addressing elements in array

– Number of dimensions– Size of array– Ordering in storage

• Row-major order• Column-major order

17Runtime Environments

Local Temporaries

X = (a+b)/((c-d)*f(i))

• Values of (a+b) and (c-d) must be stored before f can be called.

• Thus, area for temporary variables must be reserved in the stack.

• Usually, temporaries are pushed into stacks after local variables.

18Runtime Environments

Nested Declarations

• Local variables of blocks can be defined.

void f{ int x; int i;

…{ char x; int j;

…}…

}

• A block could be treated as a procedure, but it is inefficient.• Another simpler method is to:

– push local variables in stack when the block is entered– pop the local variables when the block is exited

control link of f

xj

return addr of f

xi

rest of stack

activationrecord

of f

local var. of block

19Runtime Environments

Call Sequence• Calling sequence

– Push arguments– Push access link– Push fp as control link– Copy sp to fp– Store return address– Jump to callee– Reserve space for local variables

• How to find access link

• Return sequence– Copy fp to sp– Load control link

into fp– Pop access link– Jump to return

address– Change sp to pop

arguments

20Runtime Environments

Parameter Passing

• Pass by value• Pass by reference• Pass by value-result• Pass by name

21Runtime Environments

Pass by Value• Value parameters are

not changed during the execution

• Only the value is sent into the procedure, and are used locally

• When the control is returned from the callee, the value of the parameter is not passed back to the caller.

void change(int x){ x++;return;

}

void main(){ int y=0;change(y);printf(“%d\n”,y);return;

}

Output:0

22Runtime Environments

Pass by Reference• The reference to a variable is

passed to the callee.• The callee used the reference

(address) to refer to the variable.– Indirect addressing is needed to

refer to parameters• The variable in the caller and the

referenced memory in the callee share the same memory location.

• The value of the variable in the caller is also changed when the referenced in the callee is changed.

void change (int &x)

{ x++;return;

}

void main(){ int y=0;

change(y);printf(“%d\n”,y);return;

}Output:

1

23Runtime Environments

Pass by Value-Result• The value of the parameter is

copied into the callee when the callee is entered.

• New memory location is provided for the parameter in the callee’s activation record.– No indirect address is needed

• When the control is returned to the caller, the value is copied back.

void change(int x)

{ x++;return;

}

void main(){ int y=0;change(y);printf(“%d\n”,y);return;

}

Output:1

24Runtime Environments

Pass by Name

• The argument is replaced by the textual representation of the parameter passed from the caller.

• The evaluation of the actual parameters must be delayed until the execution.

• A procedure (thunk) is called to find the value of the parameter in the callee.

void change(int x){ i=4;x++;return;

}

void main(){ int i=0;inta[5]={0,0,0,0,0};change(a[i]);return;

}

After the call:a={0,0,0,0,1}