UT Dallas CS 4337 - #Sebesta ch10 subprogram imp (36 pages)

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#Sebesta ch10 subprogram imp



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#Sebesta ch10 subprogram imp

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Pages:
36
School:
University of Texas at Dallas
Course:
Cs 4337 - Organization of Programming Languages
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Chapter 10 Implementing Subprograms Chapter 10 Topics The General Semantics of Calls and Returns Implementing Simple Subprograms Implementing Subprograms with Stack Dynamic Local Variables Nested Subprograms Blocks Implementing Dynamic Scoping Copyright 2012 Addison Wesley All rights reserved 1 2 The General Semantics of Calls and Returns The subprogram call and return operations of a language are together called its subprogram linkage General semantics of calls to a subprogram Parameter passing methods Stack dynamic allocation of local variables Save the execution status of calling program Transfer of control and arrange for the return If subprogram nesting is supported access to nonlocal variables must be arranged Copyright 2012 Addison Wesley All rights reserved 1 3 The General Semantics of Calls and Returns General semantics of subprogram returns In mode and inout mode parameters must have their values returned Deallocation of stack dynamic locals Restore the execution status Return control to the caller Copyright 2012 Addison Wesley All rights reserved 1 4 Implementing Simple Subprograms Call Semantics Save the execution status of the caller Pass the parameters Pass the return address to the called Transfer control to the called Copyright 2012 Addison Wesley All rights reserved 1 5 Implementing Simple Subprograms continued Return Semantics If pass by value result or out mode parameters are used move the current values of those parameters to their corresponding actual parameters If it is a function move the functional value to a place the caller can get it Restore the execution status of the caller Transfer control back to the caller Required storage Status information parameters return address return value for functions temporaries Copyright 2012 Addison Wesley All rights reserved 1 6 Implementing Simple Subprograms continued Two separate parts the actual code and the non code part local variables and data that can change The format or layout of the non code part of an executing subprogram is called an activation record An activation record instance is a concrete example of an activation record the collection of data for a particular subprogram activation Copyright 2012 Addison Wesley All rights reserved 1 7 An Activation Record for Simple Subprograms Copyright 2012 Addison Wesley All rights reserved 1 8 Code and Activation Records of a Program with Simple Subprograms Copyright 2012 Addison Wesley All rights reserved 1 9 Implementing Subprograms with Stack Dynamic Local Variables More complex activation record The compiler must generate code to cause implicit allocation and deallocation of local variables Recursion must be supported adds the possibility of multiple simultaneous activations of a subprogram Copyright 2012 Addison Wesley All rights reserved 1 10 Typical Activation Record for a Language with Stack Dynamic Local Variables Copyright 2012 Addison Wesley All rights reserved 1 11 Implementing Subprograms with StackDynamic Local Variables Activation Record The activation record format is static but its size may be dynamic The dynamic link points to the top of an instance of the activation record of the caller An activation record instance is dynamically created when a subprogram is called Activation record instances reside on the runtime stack The Environment Pointer EP must be maintained by the run time system It always points at the base of the activation record instance of the currently executing program unit Copyright 2012 Addison Wesley All rights reserved 1 12 An Example C Function void sub float total int part int list 5 float sum Copyright 2012 Addison Wesley All rights reserved 1 13 Revised Semantic Call Return Actions Caller Actions Create an activation record instance Save the execution status of the current program unit Compute and pass the parameters Pass the return address to the called Transfer control to the called Prologue actions of the called Save the old EP in the stack as the dynamic link and create the new value Allocate local variables Copyright 2012 Addison Wesley All rights reserved 1 14 Revised Semantic Call Return Actions continued Epilogue actions of the called If there are pass by value result or out mode parameters the current values of those parameters are moved to the corresponding actual parameters If the subprogram is a function its value is moved to a place accessible to the caller Restore the stack pointer by setting it to the value of the current EP 1 and set the EP to the old dynamic link Restore the execution status of the caller Transfer control back to the caller Copyright 2012 Addison Wesley All rights reserved 1 15 An Example Without Recursion void fun1 float r int s t fun2 s void fun2 int x int y fun3 y void fun3 int q void main float p fun1 p r s t x y main fun1 fun2 calls fun1 calls fun2 calls fun3 q p Copyright 2012 Addison Wesley All rights reserved 1 16 An Example Without Recursion Copyright 2012 Addison Wesley All rights reserved 1 17 Dynamic Chain and Local Offset The collection of dynamic links in the stack at a given time is called the dynamic chain or call chain Local variables can be accessed by their offset from the beginning of the activation record whose address is in the EP This offset is called the local offset The local offset of a local variable can be determined by the compiler at compile time Copyright 2012 Addison Wesley All rights reserved 1 18 An Example With Recursion The activation record used in the previous example supports recursion int factorial int n 1 if n 1 return 1 else return n factorial n 1 2 void main int value value factorial 3 3 Copyright 2012 Addison Wesley All rights reserved 1 19 Activation Record for factorial Copyright 2012 Addison Wesley All rights reserved 1 20 1 Stacks for calls to factorial 1 21 2 Stacks for returns from factorial 1 22 Nested Subprograms Some non C based static scoped languages use stack dynamic local variables and allow subprograms to be nested Fortran 95 Ada Python JavaScript Ruby and Lua All variables that can be non locally accessed reside in some activation record instance in the stack The process of locating a non local reference 1 Find the correct activation record instance 2 Determine the correct offset within that activation record instance Copyright 2012 Addison Wesley All rights reserved 1 23 Locating a Non local Reference Finding the offset is easy Finding the correct activation record instance Static semantic rules guarantee that all nonlocal


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