CS 242 Revised class schedule Scope, Function Calls and Storage Management � Friday Oct 17 • No lecture; discussion section as usual � Friday Oct 24 • No section John Mitchell � Monday Oct 27 • Review section during class meeting time, Gates B01 � Wednesday Oct 29 • No lecture • Evening exam: 7PM, Gates B01 and B03 Topics Block-Structured Languages � Block-structured languages and stack storage � Nested blocks, local variables � In-line Blocks • Example new variables declared in nested blocks { int x = 2; • activation records { int y = 3; • storage for local, global variables outer inner local variable block x = y+ 2; � First-order functions block global variable } • parameter passing } • tail recursion and iteration � Higher-order functions • Storage management • deviations from stack discipline – Enter block: allocate space for variables • language expressiveness = > implementation complexity – Exits block: some or all space may be deallocated Examples Simplified Machine Model � Blocks in common languages Registers Code Data • C { … } • Algol begin … end Stack • ML let … in … end � Two forms of blocks • I n-line blocks Program • Blocks associated with functions or procedures Counter � Topic: block-based memory management, Heap Environment Pointer access to local variables, parameters,global vars 1
Interested in Memory Mgmt Only Some basic concepts � Registers, Code segment, Program counter � Scope • Ignore registers • Region of program text where declaration is visible � Lifetime • Details of instruction set will not matter � Data Segment • Period of time when location is allocated to program • Stack contains data related to block entry/exit { int x = … ; • Inner declaration of x hides outer one. • Heap contains data of varying lifetime { int y = … ; • Called “hole in scope” • Environment pointer points to current stack position { int x = … ; • Lifetime of outer x includes time when – Block entry: add new activation record to stack …. inner block is executed – Block exit: remove most recent activation record } ; Lifetime ≠ scope • } ; • Lines indicate “contour model” of scope. } ; In-line Blocks Activation record for in-line block � Activation record � Control link Control link • Data structure stored on run -time stack • pointer to previous record Local variables • Contains space for local variables on stack Intermediate results � Example � Push record on stack: • Set new control link to Push record with space for x, y Control link { int x= 0; point to old env ptr Set values of x, y int y= x+ 1; Local variables Push record for inner block • Set env ptr to new record { int z= (x+ y)* (x-y); Intermediate results Set value of z � Pop record off stack }; Pop record for inner block • Follow control link of } ; Pop record for outer block Environment current record to reset Pointer environment pointer May need space for variables and intermediate results like (x+ y), (x-y ) Example Scoping rules � Global and local variables { int x= 0; Control link int y= x+ 1; • x, y are local to outer block { int x= 0; x 0 { int z= (x+ y)* (x-y); • z is local to inner bock int y= x+ 1; y 1 • x, y are global to inner block }; { int z= (x+ y)* (x-y); } ; }; Control link } ; � Static scope z -1 Push record with space for x, y Set values of x, y x+ y 1 • global refers to declaration in closest enclosing block Push record for inner block x-y -1 � Dynamic scope Set value of z • global refers to most recent activation record Pop record for inner block Environment Pop record for outer block Pointer These are same until we consider function calls. 2
Functions and procedures Activation record for function � Syntax of procedures (Algol) and functions (C) � Return address Control link procedure P (< pars> ) < type> function f(< pars> ) • Location of code to Return address begin { execute on function return Return-result addr < local vars> < local vars> � Return-result address Parameters < proc body> < function body> • Address in activation end; } ; record of calling block to Local variables � Activation record must include space for receive return address Intermediate results � Parameters • parameters • location to put return • Locations to contain data value on function exit • return address Environment from calling block • return value Pointer (an intermediate result) Example Function call fact(k) � Function fact(3) Control link Control link Control link Return-result addr fact(n) = if n< = 1 then 1 Return-result addr Return address n 3 n k else n * fact(n-1) fact(n-1) fact(n-1) Return result addr � Return result address fact(2) Control link Parameters Environment • location to put fact(n) Return-result addr Pointer � Parameter Local variables n 2 fact(n-1) • set to value of n by calling Intermediate results fact(1) sequence Control link fact(n) = if n< = 1 then 1 � Intermediate result Return-result addr else n * fact(n-1) Environment n 1 • locations to contain value Pointer fact(n-1) of fact(n-1) Return address omitted; would Function return next slide → be ptr into code segment Function return Topics for first-order functions � Parameter passing fact(3) fact(3) Control link Control link Return result addr Return result addr • use ML reference cells to describe pass-by -value, n 3 n 3 pass-by -reference fact(n-1) fact(n-1) 2 � Access to global variables fact(2) fact(2) Control link Control link • global variables are contained in an activation record Return result addr Return result addr higher “up” the stack n 2 n 2 � Tail recursion fact(n-1) 1 fact(n-1) 1 • an optimization for certain recursive functions fact(1) Control link Return result addr fact(n) = if n< = 1 then 1 n 1 See this yourself: write factorial and run under debugger else n * fact(n-1) fact(n-1) 3
ML imperative features (review) Parameter passing � General terminology: L-values and R-values � Pass-by-reference • Assignment y := x+ 3 • Caller places L-value (address) – Identifier on left refers to location, called its L-value of actual parameter in activation record – Identifier on right refers to contents, called R-value • Function can assign to variable that is passed � ML reference cells and assignment � Pass-by-value • Different types for location and contents • Caller places R-value (contents) x : int non-assignable integer value y : int ref location whose contents must be integer of actual parameter in activation record !y the contents • Function cannot change value of caller’s variable ref x expression creating new cell initialized to x • Reduces aliasing (alias: two names refer to same loc) • ML form of assignment y := x+ 3 place value of x+ 3 in location (cell) y y := !y + 3 add 3 to contents of y and store in location y Example Access to global variables pseudo-code Standard ML � Two possible scoping conventions fun f (x : int ref) = e f r - y • Static scope: refer to closest enclosing block b s - ( x := !x+ 1; !x ); s a p y = ref 0 : int ref; • Dynamic scope: most recent activation record on stack f(y) + !y; � Example function f (x) = { x := x+ 1; return x }; var y : int = 0; outer block x 1 int x= 1; print f(y)+ y; function g(z) = x+ z; y 3 fun f (z : int ) = f(3) pass-by-value function f(y) = let x = ref z in x 4 x := !x+ 1; !x { int x = y+1; end; return g(y* x) } ; g(12) z 12 y = ref 0 : int ref; f(3); f(!y) + !y; Which x is used for expression x+ z ? Activation record for static scope Complex nesting structure � Control link function m(…) { Control link int x= 1; • Link to activation record of … int x= 1; Access link previous (calling) block function n( … ){ function g(z) = x+ z; � Access link Return address function g(z) = x+ z; function f(y) = Simplify to … • Link to activation record of { int x = y+ 1; Return result addr { … closest enclosing block in return g(y* x) } ; Parameters function f(y) { f(3); program text Local variables int x = y+1; � Difference return g(y* x) } ; Intermediate results • Control link depends on … dynamic behavior of prog Simplified code has same block nesting, f(3); … } Environment if we follow convention that each • Access link depends on Pointer … n( … ) … } static form of program text declaration begins a new block. … m(…) 4
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