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Chapter 8 - Control II: Procedures and Environments

Chapter 8 - Control II: Procedures and Environments. Three major parts of a runtime environment:. Static area allocated at load/startup time. Examples: global/static variables and load-time constants.

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Chapter 8 - Control II: Procedures and Environments

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  1. Chapter 8 - Control II: Procedures and Environments Louden, 2003

  2. Three major parts of a runtime environment: • Static area allocated at load/startup time. Examples: global/static variables and load-time constants. • Stack area for execution-time data that obeys a last-in first-out lifetime rule. Examples: method variables and temporaries. • Heap or dynamically allocated area for "fully dynamic" data, e.g. data allocated with new K. Louden, Programming Languages

  3. Procedure Overview • When functions are “first class” data items themselves, they can be dynamically created and used like values just like any other data structure. (e.g., Haskell curried functions, eval) • pass functions as arguments • A procedure is called or activated. • Activation record: collection of data needed to maintain a single execution of a procedure. • access to local and non-local references. • Static or dynamic environment (depending on scoping) must be accessible during runtime. • When a procedure depends only on parameters and fixed language features – closed form. • The code for a function together with its defining environment is called closure – as we can resolve all outstanding non-local environments. K. Louden, Programming Languages

  4. At seats: • Using what you understand about the call stack, what actions take place on call? • What actions take place on return? K. Louden, Programming Languages

  5. Implementing “Simple” Subprograms • Caller responsibilities 1. Save the execution status of the caller (calling environment) 2. Carry out the parameter-passing process by putting the parameters somewhere that the called function can access. 3. Pass the return address of the caller to the callee 4. Transfer control to the callee K. Louden, Programming Languages

  6. Implementing “Simple” Subprograms • Return Actions: 1. If it is a function, move the return value to a place the caller can get it 2. Restore the execution status of the caller 3. Transfer control back to the caller K. Louden, Programming Languages

  7. Implementing “Simple” Subprograms Called routine must • Create an activation record: • local variables • return address • points to other environments • parameters • See CSILM lesson “Call Stack” for an example csilm.usu.edu, Browse Resources, CSILM Activities, Programming Languages, Call Stack K. Louden, Programming Languages

  8. Parameter Passing • Aliases may be created • Type checking parameters – for legality and to pick between overloaded methods • a reference parameter is a nonlocal variable • the same data object passed for two parameters CALL S(X,X) causes aliasing • With aliasing, interesting problems in optimizations occur. x+2 y++ x+2 If x and y are aliases, x+2 isn’t a common subexpression K. Louden, Programming Languages

  9. Models of Parameter Passing K. Louden, Programming Languages

  10. 1. Pass-by-value (in mode) • Typically we copy the value in, but can do with a constant reference pointer. • Parameters are viewed as local variables of the procedure • Disadvantages of copy: • Requires more storage (duplicated space) • Cost of the moves (if the parameter is large) • Disadvantages of constant reference: • Must write-protect in the called subprogram or compiler check that there are no assignments. • Accesses cost more (indirect addressing) K. Louden, Programming Languages

  11. 2. Pass-by-result (out mode) • function return value(s) • Local’s value is passed back to the caller • Physical move is usually used (copy to call stack) • Disadvantages: • If value is passed, time and space costs to copy • order dependence may be a problem (if output values are aliased) procedure sub1(y: int, z: int);{y=0;z=5; } sub1(x, x); • Value of x in the caller depends on order of assignments at the return K. Louden, Programming Languages

  12. 3. Inout mode • Pass by value-result (aka copy-in copy-out or copy-restore) • Used to save cost of indirect access. Physical move, both ways • value-result (or pass by copy) • Disadvantages • ordering may be a problem with a call like doit(x,x) • time/space issues • Need to know whether address is computed again before copying back. doit(i,a[i]) K. Louden, Programming Languages

  13. 4.Pass by reference. Issues: • passing is faster (as no data copy) • formal parameter is local object of type pointerIf expression is passed as an in/out parameter: a temporary location may be passed(and then the copy is changed, not the original) • Disadvantages: • access slower as is indirect (always follow a pointer to access), but passing is fast (only copy a pointer, not a whole structure) • may make inadvertent changes to parameters K. Louden, Programming Languages

  14. 5. Pass-by-name (Delayed evaluated parameters) • By textual substitution • parameter is evaluated everytime it is used, but it is evaluated in the caller’s environment • Purpose: flexibility of late binding • costly Thunks: a pass by name argument is implemented by a little procedure (called a thunk) which evaluates the argument. A thunk is a method to evaluate an expression that is yet to be evaluated. K. Louden, Programming Languages

  15. Pass-by-name How is it like other methods? • If actual is a scalar variable, it is pass-by-reference • If actual is a constant expression, it is pass-by-value • If actual is an array element, it is like nothing else e.g. procedure sub1(x: int; y: int); begin x := 1; Seems like nothing is happening y := 2;with first assignments but it is x := 2; y := 3; end; sub1(i, a[i]); K. Louden, Programming Languages

  16. Example: procedure R(vari,j: integer); begin m:boolean; m := true; i := i + 1; j := j + 1; end; m := 2; for(i=0;i<10;i++) c[i]=10*i; R(m,c[m]); • pass by reference: adds 1 to m and c[2]Pass by name: adds 1 to m and c[3] K. Louden, Programming Languages

  17. Parameter Passing Methods • Design Considerations for Parameter Passing 1. Efficiency 2. One-way or two-way • These two are in conflict with one another! • Good programming => limited access to variables, which means one-way whenever possible • Efficiency => pass by reference is fastest way to pass structures of significant size • Also, functions should not allow reference parameters K. Louden, Programming Languages

  18. Languages and Environments • Languages differ on where activation records must go in the environment: • Fortran is static: all data, including activation records, are statically allocated. (Each function has only one activation record—no recursion!) • Functional languages (Scheme,ML) and some OO languages (Smalltalk) are heap-oriented: all (or almost all) data, including activation records, are allocated dynamically. • Most languages are in between: data can go anywhere (depending on its properties); activation records go on the stack. K. Louden, Programming Languages

  19. Simple stack-based allocation • Described in Chapter 5. • Nested declarations are added to the stack as their code blocks are entered, and removed as their code blocks are exited. • Example: Stack at Point 1:{ int x; int y; { int z; } { int w; // Point 1 }} • Note ,z has been removed at point 1 as have exited scope K. Louden, Programming Languages

  20. Example (C): main →q →p int x; void p( int y) { int i = x; char c; ... } void q ( int a) { int x; p(1); } main() { q(2); return 0; } K. Louden, Programming Languages

  21. Local variable access using the ep • In a typical language with a stack-based runtime environment, the local declarations in a procedure are fixed at compile-time, both in size and in sequence. • This information can be used to speed up accesses to local variables, by precomputing these locations as offsets from the ep. • Then the local frame need not have a name-based lookup operation (unlike the symbol table). • In fact, names can be dispensed with altogether. K. Louden, Programming Languages

  22. Non-local variable access • Requires that the environment be able to identify frames representing enclosing scopes. • Using the control link results in dynamic scope (and also kills the fixed-offset property as you are not sure which method will contain the x. Thus, you can’t depend on a fixed location). • If procedures can't be nested (C, C++, Java), the enclosing scope is always locatable by other means: it is either global (accessed directly) or belongs to the current object. • If procedures can be nested, to maintain lexical scope a new link must be added to each frame: the access link, pointing to the activation of the defining environment of each procedure. K. Louden, Programming Languages

  23. Nested Subprograms • The process of locating a nonlocal reference: 1. Find the correct activation record instance 2. Determine the correct offset within that activation record instance May need to follow several links (access chaining) The number of links is known from compile time. If used stack of symbol tables, can count how many tables you had to search to find it. If used individual stacks for each value, you can record the nesting depth of each variable. K. Louden, Programming Languages

  24. Procedure values as pointer pairs • Each procedure becomes a pair of pointers: a code pointer (called the instruction pointer or ip in the text), and an environment pointer (ep in the text) pointing to the definition environment of the procedure (which will become the access link during a call). • Such an <ep,ip> pair is sometimes called a closure. K. Louden, Programming Languages

  25. The Process of Locating a Nonlocal Reference • Finding the offset is easy • Finding the correct activation record instance: • Static semantic rules guarantee that all nonlocal variables that can be referenced have been allocated in some activation record instance that is on the stack when the reference is made K. Louden, Programming Languages

  26. Nested Subprograms • Technique 1 - Static Chains • A static chain is a chain of static links that connects certain activation record instances • The static link in an activation record instance for subprogram A points to one of the activation record instances of A's static parent • The static chain from an activation record instance connects it to all of its static ancestors K. Louden, Programming Languages

  27. Static Chains Show the static/dynamic chains when main →C →A →B →C main ----- static_depth = 0 A ----- static_depth = 1 B ----- static_depth = 2 C ----- static_depth = 1 K. Louden, Programming Languages

  28. Nested Subprograms • Static Chain Maintenance • At the call : • The activation record instance must be built • The dynamic link is just the old stack top pointer • The static link must point to the most recent ari of the static parent (in most situations) • Two Methods to set static chain: 1. Search the dynamic chain until the first ari for the static parent is found--easy, but slow K. Louden, Programming Languages

  29. Nested Subprograms 2. Treat procedure calls and definitions like variable references and definitions (have the compiler compute the nesting depth, or number of enclosing scopes between the caller and the procedure that declared the called procedure; store this nesting depth and send it with the call) • e.g. Look at MAIN_2 and the stack contents. At the call to SUB1 in SUB3, this nesting depth is 1, which is sent to SUB1 with the call. The static link in the new ari for SUB1 is set to point to the ari that is pointed to by the second static link in the static chain from the ari for SUB3 K. Louden, Programming Languages

  30. Nested Subprograms • Evaluation of the Static Chain Method • Problems: 1. A nonlocal reference is slow if the number of scopes between the reference and the declaration of the referenced variable is large 2. Time-critical code is difficult, because the costs of nonlocal references are not equal, and can change with code upgrades and fixes K. Louden, Programming Languages

  31. Nested Subprograms • Technique 2 (for locating non-local variables with static scope) - Displays • The idea: Put the static links in a separate stack called a display. The entries in the display are pointers to the ari's that have the variables in the referencing environment. • Represent references as (display_offset, local_offset) where display_offset is the same as chain_offset • Can access via computation. display offset of 10 is one lookup (not a chain of length 10) K. Louden, Programming Languages

  32. Main – level 0 p p level1 t level 1 q level 3 s level 2 r level 2 K. Louden, Programming Languages

  33. Stack is shown growing downwards. Display contains pointers to each activation record at each reachable level 100 main 200 t 300 s 400 main-> t -> s-> q q When s calls q, a single element is added to the table. K. Louden, Programming Languages

  34. At your seats… • Why do we do this? • This seems well and good – but how do we keep it current? K. Louden, Programming Languages

  35. 100 main 200 t 300 s 400 main-> t -> s-> q-> p q old level 1 is 200 When q calls p, a new level 1 entry is needed. Store the old one, so you can get it back. Level 2 and level 3 are unused (but unchanged) 500 p K. Louden, Programming Languages

  36. 100 main 200 t 300 s q 400 main-> t -> s-> q-> p->t old level 2 is 200 p When p calls t, a new level 2 entry is needed Level 3 and level 4 are unused (but unchanged) 500 old level 2 is 500 600 t K. Louden, Programming Languages

  37. 100 main 200 t 300 s q 400 main-> t -> s-> q-> p->t old level 2 is 200 p When p calls t, a new level 2 entry is needed Level 3 and level 4 are unused (but unchanged) 500 old level 2 is 500 600 t K. Louden, Programming Languages

  38. Blocks • Two Methods: 1. Treat blocks as parameterless subprograms • Use activation records 2. Allocate locals on top of the ari of the subprogram • Must use a different method to access locals • A little more work for the compiler writer K. Louden, Programming Languages

  39. Implementing Dynamic Scoping 1. Deep Access (search) - nonlocal references are found by searching the activation record instances on the dynamic chain • Length of chain cannot be statically determined • Every activation record instance must have variable names recorded K. Louden, Programming Languages

  40. Implementing Dynamic Scoping 2. Shallow Access - put locals in a central place • How implemented? a. One stack for each variable name b. Central referencing table with an entry for each variable name At subprogram entry, add location for each variable. At subprogram exit, remove location for each variable. K. Louden, Programming Languages

  41. Using Shallow Access to Implement Dynamic Scoping K. Louden, Programming Languages

  42. Parameter Passing Conventions • Actual/Formal Parameter Correspondence: 1. Positional (this is what we are used to) 2. Keyword • e.g. SORT(LIST => A, LENGTH => N); • Advantage: order is irrelevant • Disadvantage: user must know the formal parameter’s names 3. Default Values: e.g. procedure SORT(LIST : LIST_TYPE; LENGTH : INTEGER := 100); ... SORT(LIST => A); K. Louden, Programming Languages

  43. Overloaded Subprograms • Def: An overloaded subprogram is one that has the same name as another subprogram in the same referencing environment • C++ and Ada have overloaded subprograms built-in, and users can write their own overloaded subprograms • Overloaded subprograms provide ad hoc (non-generalizable) polymorphism K. Louden, Programming Languages

  44. Generic Subprograms • Analogy: Would you rather have: • Vacuum that can adjust to any carpet height versus • A different vacuum for each type of carpet. • A generic or polymorphic subprogram is one (not many) subprogram that takes parameters of different types on different activations • A subprogram that takes a generic parameter needs to figure out what type was passed (Haskell) K. Louden, Programming Languages

  45. Separate & Independent Compilation • Def: Independent compilation is compilation of some of the units of a program separately from the rest of the program, without the benefit of interface information • Def: Separate compilation is compilation of some of the units of a program separately from the rest of the program, using interface information to check the correctness of the interface between the two parts. K. Louden, Programming Languages

  46. Memory Management Benefit (of system controlled storage management): • ability to delay the binding of a storage segment's size and/or location • reuse of a storage segment for different jobs (from system supervisor point of view) • reuse of storage for different data structures • increased generality, not have to specify maximum data structure size • dynamic data structures • recursive procedures - garbage collection is automatic Adv/Disadvof programmer controlled storage management • Disadvantage: burden on programmer & may interfere with necessary system controlMay lead to subtle errorsMay interfere with system-controlled storage management • Advantage: difficult for system to determine when storage may be most effectively allocated and freed K. Louden, Programming Languages

  47. Heap management • Single-size cells vs. variable-size cells • Reference counters (eager approach) vs. garbage collection (lazy approach) 1. Reference counters: maintain a counter in every cell that store the number of pointers currently pointing at the cell • Disadvantages: space required, complications for cells connected circularly Expensive - when making a pointer assignment p=q • decrement count for old value of p • if 0, return to free storage. Check if contains references to other blocks. Could be recursive • do pointer assignment • Increment reference count for q K. Louden, Programming Languages

  48. One-bit reference counting • Another variation on reference counting, called one-bit reference counting, uses a single bit flag to indicate whether each object has either "one" or "many" references. • If a reference to an object with "one" reference is removed, then the object can be recycled. • If an object has "many" references, then removing references does not change this, and that object will never be recycled. It is possible to store the flag as part of the pointer to the object, so no additional space is required in each object to store the count. • One-bit reference counting is effective in practice because most actual objects have a reference count of one. K. Louden, Programming Languages

  49. 2. Garbage collection: allocate until all available cells allocated; then begin gathering all garbage • Every heap cell has an extra bit used by collection algorithm • All cells initially set to garbage • All pointers traced into heap, and reachable cells marked as not garbage • All garbage cells returned to list of available cells Disadvantages: when you need it most, it works worst (takes most time when program needs most of cells in heap) K. Louden, Programming Languages

  50. Mark-Sweep - Java uses • In a mark-sweep collection, the collector first examines the program variables; any blocks of memory pointed to are added to a list of blocks to be examined. • For each block on that list, it sets a flag (the mark) on the block to show that it is still required, and also that it has been processed. It also adds to the list any blocks pointed to by that block that have not yet been marked. In this way, all blocks that can be reached by the program are marked. • In the second phase, the collector sweeps all allocated memory, searching for blocks that have not been marked. If it finds any, it returns them to the allocator for reuse • Can find circular references. • Easy if regular use of pointers (Like in LISP) • All elements must be reachable by a chain of pointers which begins outside the heap • Have to be able to know where all pointers are - both inside the heap and outside. How can a chain be followed from a pointer if there is no predefined location for that pointer in the pointed-to cell? K. Louden, Programming Languages

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