The environment of the computation

1. The environment of the computation • Declarations introduce names that denote entities. • At execution-time, entities are bound to values or to locations: name value (functional) name location value (imperative) • Value binding takes place during function invocation • Namesare boundtomemorylocationson scope entry.Locationsare bound tovaluesby assignment. • Compiler establishes symbolic mapping between names and locations: name offsetfromrun-time pointer • Run-time pointer is stack frame, TOC entry, static link, etc.

2. Run-time organization • Each subprogram invocation creates an activation record. • Recursion imposes stack allocation (all languages today) • Activation record hold actuals, linkage information, saved registers, local entities. • caller: place actuals on stack, return address, linkage information, then transfer control to callee. • Prologue: save registers, allocate space for locals • Epilogue: place return value in register or stack position, update actuals, restore registers, then transfer control to caller. • Binding of locations: actuals and locals are at fixed offsets from frame pointers • complications: variable no. of actuals, dynamic objects.

3. Activation record layout actual actual Handled by caller Frame pointer Return addr Save area local Handled by callee local Stack pointer

4. Save area • Linkage information: • Control link: previous value of frame pointer • Static link: address of stack frame of lexical parent • Return address: in caller • Other saved registers. • Compiler emits prologue code to save frame pointer, increment stack pointer, save registers. • Stack (sp) := fp; -- save frame pointer after actuals • fp := sp; -- new frame starts here

5. Functions with variable number of parameters printf (“this is %d a format %d string”, x, y); • within body of printf, need to locate as many actuals as place-holders in the format string. • Solution: place parameters on stack in reverse order. Actuals at positive offset from FP, locals at negative offset from FP. actual n actual n-1 … actual 1 (format string) return address

6. Objects of dynamic size declare x : string (1..N); -- N global, non-constant y : string (1..N); begin... • where is the start of y in the activation record? • Solution 1: use indirection: activation record hold pointers. Simpler implementation, costly dynamic allocation., deallocation. • solution 2:: local indirection: activation record holds offset into stack. • Faster allocation/deallocation, complex implementation.

7. Run-time access to globals procedure outer is -- recursive global : integer; procedure inner is -- recursive local : integer; begin ... if global = local then -- how do we locate global? • Need run-time structure to locate activation record of statically enclosing scopes. Environment includes current activation record AND activation records of parent scopes.

8. Global linkage • Static chain: pointer to activation record of statically enclosing scope • Display:: array of pointers to activation records • Neither works for function values (higher-order functions) • functional languages allocate activation records on heap • may not work for pointers to functions • simpler if there is no nesting (C, C++, Java) • can check static legality in many cases (Ada)

9. Static Links • Activation record hold pointer to activation record of enclosing scope. Setup as part of call prologue. To enclosing scope To retrieve entity 3 frames out: 3 dereference operations outer outer outer inner inner inner inner

10. Display • Global array of pointers to current activation records outermost display ... outer To retrieve entity 3 frames out: one indexing operation outer outer inner inner inner inner

11. Subprogram parameters type proc is access procedure (X : Integer); procedure Perform (Helper : proc) is begin … proc (42); end; procedure Action (X : Integer) is … procedure Proxy is begin Perform (Action’Access); end; • Proxy can see (and call) Action, therefore environment of Action ( e.g static link) Is known to Proxy. Proxy transmits to Perform both a pointer to the code for Action, and the proper static link for it. -- ‘Access creates pair (ptr to Action , environment of Action) • simplest implementation if Env is pointer (static link) but can also be display: more efficient to retrieve non-local entities, less efficient for subprogram parameters, because display is array of variable size.

12. Functions as first-class values forceheap allocation of activation record • The environment of definition of the function must be preserved until the point of call: activation record cannot be reclaimed if it creates functions. • Functional languages require more complex run-time management. • Higher-order functions: functions that return (build) functions, are powerful but complex mechanisms. Imperative languages restrict their use. • A function that returns a pointer to a function is a higher-order function.

13. Higher-order functions • Both arguments and result can be (pointers to) subprograms: type Func is access function (x : integer) return integer; function compose (first, second : Func) return Func is begin function result (x : integer) return integer is begin return (second (first (x)); -- implicit dereference on call end; -- in C++ as well. begin return result ’access; -- but first and second won’t exist at the point end; -- of call, soillegal in Ada.

14. Restricting higher-order functions • C: no nested definitions, so environment is always global. • C++: ditto, except for nested classes. • Ada: static checks to reject possible dangling references • Modula: pointers to function illegal if function not declared at top-level. • LISP: special syntax to indicate capture of environment • ML, Haskell: no restriction: compose is a primitive

15. Returning composite values • Intermediate problem: functions that return values of non-static sizes: function conc3 (x, y, z : string) return string is begin return x & “:” & y & “:” & z; end; example : string := conc3 (this, that, theother); • best not to use heap, but still need indirection. • Simplest : forbid it (Pascal, C) or use heap automatically (Java) • More efficient: local allocate/free on special stack. • Historical (algol68): two stacks.

16. Storage management • Data local to functions: stack • Global data: static areas • Class information, static members • Packages • Fortran common blocks • Linker establishes array of pointers to static areas (TOC). • Dynamic data: heap

17. Garbage collection • Run-time system manages heap, maintains free list • Allocation by first-fit or best-fit (variants with buddy system, etc). • Garbage collector must be able to distinguish data that is live from unreachable data. • Garbage collector attaches unreachable data (garbage) to free list. • Garbage collector may compact data in use, to improve program locality

18. Marking live data • Data is live if it can be referenced from any active function • Garbage collector must have run-time information on data layout: • Pointer data in activation records • Pointer data in data structures • Marking is a graph traversal starting from roots: pointers in the stack. • User cannot have control over pointer manipulation: • Languages with garbage collection do not have explicit pointers.

19. Sweeping and compacting garbage • Linear pass over the heap: all unmarked data is unreachable, and can be chained onto the free list • For compaction, rewrite live data contiguously, so that free space is also contiguous • Additional pass to adjust pointers: need to compute destination address of each block, and modify all pointers into that block. Needs inverse graph: each block has a chain of references to itself. • After pointers are adjusted, each block is copied to its destination.