Lecture 15: Control Abstraction

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1 Lecture 15: Control Abstraction COMP 524 Programming Language Concepts Aaron Block March 8, 2007 Based on notes by N. Fisher, F. Hernandez-Campos, and D. Stotts

2 Closures Deep binding is implemented using closures A closure is the combination of a reference to a subroutine and an explicit representation of its referencing environment Typically implemented with A pointer to the subroutines code If the scoping is dynamic, we need a way to temporarily unroll al the changes since the reference was created. 2

3 FunArgs: Problem The failure of traditional stack-based implementations of procedure calls in the presence of first-class functions (functions that can be passed as procedure parameters and returned as procedure results). Upwards funarg problem: The problem of returning a function as a procedure result; requires (i) allocating stack frame on the heap and (ii) returning a closure containing a pointer to code and a pointer o the enclosed stack frame. Downwards funarg problem: the problem of passing a function as a procedure parameter; requires a tree structure for stack frames. 3

4 Abstraction Programming languages support the binding of names with potentially complex program fragments that can be used through an interface Programmers only need to know about the purpose of the fragment rather than its implementation => Abstraction A control abstraction performs a well-defined operation Subroutines A data abstraction represents information Data structures most data structures include some number of control abstractions 4

5 Subroutines Execute an operation on behalf of a calling program unit Subroutines can be parameterized The parameters in the definition of the function are known as formal parameters The parameters passed in the subroutine call are known as actual parameters or arguments At the time of the call, actual parameters are mapped to formal parameters Functions are subroutines that return a value, while 5

6 Subroutine Frames 1001: A(3) Actual Parameters : int A(int n){ Formal Parameters int m = n *n; return m + A(n-1); } Each subroutine requires a subroutine frame (a.k.a. activation record) to keep track of Arguments and return values Local variables and temporaries Bookkeeping in formation When a subroutine returns, its frame is removed 6

7 1001: A(3) Actual Parameters : int A(int n){ Formal Parameters int m = n *n; return m + A(n-1); } Stack growth A A A Temps Local Variables Misc. Bookkeeping Return Address A Arguments & A returns n 7 m

8 , Space to build argument lists Local variables and temporaries Saved Register Arguments & Returns 8 Current frame Previous frame Stack Pointer Top of frame stack Frame Pointer Access to arguments and locals via offset of The differ if temporary ace is allocated in stack

9 Statically determined offset Space to build argument lists Dynamic Space Local variables and temporaries Saved Register Stack Pointer Top of frame stack Frame Pointer Access to arguments and locals via offset of The differ if temporary ace is allocated in stack Arguments & 9 Returns

10 Calling Sequence On procedure call and return compilers generate code that execute to manage the runtime stack. Setup at call to procedure foo(a,b). Prologue before foo code executes. Epilogue at the end of foo code. Teardown right after calling the code. 10

11 Setup foo(a,b) Move to allocate a new stack frame Copy args a,b into frame Copy return address into frame Set to point to new frame Maintain static chain or dilay Move PC to procedure address 11

12 Setup foo(a,b) Move to allocate a new stack frame Copy args a,b into frame Copy return address into frame Set to point to new frame Maintain static chain or dilay Move PC to procedure address Subroutine B Subroutine A (called from main) 12

13 Setup foo(a,b) Move to allocate a new stack frame Copy args a,b into frame Copy return address into frame Set to point to new frame Maintain static chain or dilay Move PC to procedure address Subroutine foo Subroutine B Subroutine A (called from main) 13

14 Setup foo(a,b) Move to allocate a new stack frame Copy args a,b into frame Copy return address into frame Set to point to new frame Maintain static chain or dilay Move PC to procedure address Subroutine foo A & B Subroutine B Subroutine A (called from main) 14

15 Setup foo(a,b) Move to allocate a new stack frame Copy args a,b into frame Copy return address into frame Set to point to new frame Maintain static chain or dilay Move PC to procedure address Subroutine foo Return Ad A & B Subroutine B Subroutine A (called from main) 15

16 Setup foo(a,b) Move to allocate a new stack frame Copy args a,b into frame Copy return address into frame Set to point to new frame Maintain static chain or dilay Move PC to procedure address Subroutine foo Return Ad A & B Subroutine B Subroutine A (called from main) 16

17 Setup foo(a,b) If the callee is nested inside Move to allocate a new stack frame the caller, then the callee s static link should refer to the Copy args a,b into frame Copy return address into frame Set to point to new frame Maintain static chain or dilay Move PC to procedure address caller s frame. Subroutine foo Return Ad A & B Subroutine B Subroutine A (called from main) 17

18 Setup foo(a,b) If the callee is k>= 0 levels closer Move to allocate a new stack frame to the outer-level of lexical nesting. Copy args a,b into frame In this case the caller derefences Subroutine foo its own static pointer by k and Copy return address into frame Set to point to new frame Maintain static chain or dilay Move PC to procedure address Return Ad passes that to the callee. A & B Subroutine B Subroutine A (called from main) 18

19 Setup foo(a,b) Move to allocate a new stack frame Copy args a,b into frame Copy return address into frame Set to point to new frame Maintain static chain or dilay Move PC to procedure address Subroutine foo Return Ad This changes A & B where the code is executed. Subroutine B Subroutine A (called from main) 19

20 Prologue Copy registers into local slots Object initialization. 20

21 Prologue Copy registers into local slots Object initialization. Return Ad A & B 21

22 Prologue Copy registers into local slots Object initialization. All old reg. Old Return Ad A & B 22

23 Prologue Copy registers into local slots Object initialization. Objects that are All old used reg. are Old initialized. Return Ad A & B 23

24 Epilogue Place return value into slot in frame. Restore registers. Restore PC to return address. Subroutine foo Return Ad A & B Subroutine B Subroutine A (called from main) 24

25 Epilogue Place return value into slot in frame. Restore registers. Restore PC to return address. Subroutine foo Return Ad A & B Return Value Subroutine B Subroutine A (called from main) 25

26 Epilogue Place return value into slot in frame. Restore registers. Restore PC to return address. Registers stored from foo s subroutine are registered. Subroutine foo Return Ad A & B Return Value Subroutine B Subroutine A (called from main) 26

27 Epilogue Place return value into slot in frame. Restore registers. Restore PC to return address. The program resumes from where it began. Subroutine foo Return Ad A & B Return Value Subroutine B Subroutine A (called from main) 27

28 Teardown Move & (deallocate frame) Move return values (if in registers) Subroutine foo Return Ad A & B Return Value Subroutine B Subroutine A (called from main) 28

29 Teardown Move & (deallocate frame) Move return values (if in registers) Return Value Subroutine B Subroutine A (called from main) 29

30 Teardown Move & (deallocate frame) Move return values (if in registers) If the return value was placed in a register, put it in the stack. Return Value Subroutine B Subroutine A (called from main) 30

Organization of Programming Languages CS320/520N. Lecture 05. Razvan C. Bunescu School of Electrical Engineering and Computer Science bunescu@ohio.

Organization of Programming Languages CS320/520N Razvan C. Bunescu School of Electrical Engineering and Computer Science bunescu@ohio.edu Names, Bindings, and Scopes A name is a symbolic identifier used