Compiler construction

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1 This lecture Compiler construction Lecture 5: Project etensions Magnus Mreen Spring 2018 Chalmers Universit o Technolog Gothenburg Universit Some project etensions: Arras Pointers and structures Object-oriented languages Module sstem (etension proposal) Warm-up: Boolean epressions and control low Warm-up: Boolean epressions and control low Possible code generation or one-armed i stmt cgstmt (Cond epr stmt) = do then_ <- newlabel k <- newlabel cgboolepr then_ k epr label then_ cgstmt stmt branch k label k Warm-up: Boolean epressions and control low Possible code generation or one-armed i stmt cgstmt (Cond epr stmt) = do then_ <- newlabel k <- newlabel cgboolepr then_ k epr label then_ cgstmt stmt branch k label k Arras Possible code or conjunction in condition epression cgboolepr then_ k (EAnd ) = do mid <- newlabel cgboolepr mid k label mid cgboolepr then_ k

2 Memor structure Memor organisation JavaLette restrictions Onl local variables and parameters no global variables or other non-local declarations Onl simple data tpes (int, double, boolean) with ied-size values Onl call-b-value parameter passing As a consequence, all data at runtime can be stored in the activation records on a stack. More general language eatures Global variables, nested procedures, linked structures and pointers, classes,, have other requirements: a stack is not suicient or runtime memor management. Tpical memor organisation Stack Heap Static data Code area Addr. 0 Comments Static area or global variables Stack and heap grow rom opposite ends to avoid predetermined size decisions Stack managed b LLVM Management o heap much more complicated than stack Arras in JavaLette Two etensions (each one credit) Java-like arras Arras in the JavaLette etensions are similar to Java arras: A variable o tpe e.g. int[] contains a reerence to a block o memor on the heap, where arra elements are stored. Arras must be eplicitl created as in int[] v = new int[20]; Lower bound o arra inde is alwas 0 Arra elements are initialized to 0/0.0/alse Arras have a length attribute, with dot-notation Arras can be both unction arguments and results First etension One-dimensional arras and oreach statement, as in int sum = 0; or (int : v) sum = sum + ; The ordinar or statement is not required. Two etensions (each one credit) Sample LLVM code Indeing in two-dimensional arra First etension One-dimensional arras and oreach statement, as in int sum = 0; or (int : v) sum = sum + ; The ordinar or statement is not required. Second etension Multidimensional arras: All indices must get upper bounds when an arra is created For n > 1, an n-dimensional arra is a one-dimensional arra, each o whose elements is an n 1-dimensional arra %arr1 = tpe %struct1* %arr2 = tpe %struct2* %struct1 = tpe {i32, [0 i32] %struct2 = tpe {i32, [0 %arr1] deine (%arr2 %m, i32 %i, i32 %j) { %p1 = getelementptr %struct2, %arr2 %m, i32 0, i32 1, i32 %i %p2 = load %arr1, %arr1* %p1 %p3 = getelementptr %struct1, %arr1 %p2, i32 0, i32 1, i32 %j %p4 = load i32, i32* %p3 ret i32 %p4 Your generated code ma well look dierent.

3 Hints or arra etension Hints or arra etension First etension LLVM tpe o arra hinted at in previous slide, use size 0 Use C unction calloc to allocate 0-initialized memor New orms o epression: arra indeing and new epression Indeed epressions also as L-values in assignments Not required to generate bounds-checking code Second etension First etension LLVM tpe o arra hinted at in previous slide, use size 0 Use C unction calloc to allocate 0-initialized memor New orms o epression: arra indeing and new epression Indeed epressions also as L-values in assignments Not required to generate bounds-checking code Second etension new epression with several indices involves generating code with loops and repeated calloc s Indeing requires several getelementptr instructions Structures and pointers etension Structures and pointers In addition to unction deinitions, a Javalette ile ma contain deinitions o structures and pointer tpes Structure objects are allocated on the heap, using new Pointer variable (on stack) ma reer to memor structure on the heap Static data Addr. 0 Code area Structures and pointers eamples Adding structures and pointers to JavaLette Code eamples in JavaLette. tpede struct Node *list; struct Node { int elem; list net; list cons(int, list s) { list res; res = new Node; res->elem = ; res->net = s; return res; int length(list s) { i (s == (list)null) return 0; else return 1 + length(s->net); list romto(int m, int n) { i (m > n) return (list)null; else return cons(m, romto(m + 1, n)); New toplevel deinitions Structure deinitions, eampliied b Node Pointer tpe deinitions, eampliied b list New epression orms New statement orms

4 Adding structures and pointers to JavaLette New toplevel deinitions Structure deinitions, eampliied b Node Pointer tpe deinitions, eampliied b list New epression orms Heap object creation, eampliied b new Node Pointer dereerencing, eampliied b s->net Null pointers, eampliied b (list)null New statement orms Adding structures and pointers to JavaLette New toplevel deinitions Structure deinitions, eampliied b Node Pointer tpe deinitions, eampliied b list New epression orms Heap object creation, eampliied b new Node Pointer dereerencing, eampliied b s->net Null pointers, eampliied b (list)null New statement orms Pointer dereerencing allowed in let hand sides o assignments, as in s->elem = 3; In absense o garbage collection, ou should have a ree statement Implementing structures and pointers in LLVM backend Implementing structures and pointers in LLVM backend Some hints Structure and pointer tpe deinitions translate to LLVM tpe deinitions Again, use calloc or allocating heap memor getelementptr and load will be used or pointer dereerencing Ino about struct laout ma be needed in the state o code generator Some hints Structure and pointer tpe deinitions translate to LLVM tpe deinitions Again, use calloc or allocating heap memor getelementptr and load will be used or pointer dereerencing Ino about struct laout ma be needed in the state o code generator From previous lecture: Computing the size o a tpe We use the getelementptr instruction: %p = getelementptr %T, %T* null, i32 1 %s = ptrtoint %T* %p to i32 Now, %s holds the size o %T. Other uses o pointers (not part o etension) Other uses o pointers (not part o etension) Code eample (in C) void swap (int *, int *) { int tmp = *; * = *; * = tmp; Code eample (in C) void swap (int *, int *) { int tmp = *; * = *; * = tmp; Parameter passing b reerence To make it possible to return results in parameters, one ma use pointer parameters Actual arguments are addresses Problem: makes code optimization much more diicult int main () { int a = 1; int b = 3; swap(&a, &b); print("a=%d\n", a); int main () { int a = 1; int b = 3; swap(&a, &b); print("a=%d\n", a); AR main AR swap a b

5 Call b reerence and aliasing Call b reerence and aliasing Code eamples void swap (int *, int *) { int tmp = *; * = *; * = tmp; swap (, ); Code eamples void swap (int *, int *) { int tmp = *; * = *; * = tmp; swap (, ); Comments With call b reerence and pointers, two dierent variables ma reer to the same location; aliasing Aliasing complicates code optimization: := 2 := 5 a := + 3 Here we might want to replace last instr b a := 5; but what i is an alias or? Deallocating heap memor Garbage collection The problem In contrast to stack memor, there is no simple wa to sa when heap allocated memor is not needed anmore. Two main approaches 1. Eplicit deallocation Programmer deallocates memor when no longer needed (using ree) Potentiall most eicient Ver eas to get wrong (memor leakage or premature returns) 2. Garbage collection Programmer does nothing; runtime sstem reclaims unneeded memor Secure but runtime penalt Acceptable in most situations Used in Java, Haskell, C#, General approach Runtime sstem keeps list(s) o ree heap memor, malloc returns a chunk rom suitable ree list. Man variations. Some approaches: 1. Reerence counting: each chunk keeps a reerence count o incoming pointers, when count becomes zero, chunk is returned to ree list; problem: cclic structures 2. When ree list is empt, collect in two phases: Mark Follow pointers rom global and local variables, marking reachable chunks Sweep Traverse heap and return unmarked chunks to ree list Object-oriented languages Class-based languages We consider onl languages where objects are created as instances o classes. A class describes: Object-orientation a collection o instance variables; each object gets its own cop o this collection a collection o methods to access and update the instance variables Each object contains, in addition to the instance variables, a pointer to a class descriptor. This descriptor contains addresses o the code o methods. Without inheritance, all this is straightorward; classes are just structures. We propose a little bit more: single inheritance without method override.

6 JavaLette classes, code eample 1 JavaLette classes, code eample 2 class Counter { int val; void incr () { val++; return; int value () { return val; int main () { Counter c; c = new Counter; c.incr(); c.incr(); c.incr(); int = c.value(); printint(); return 0; class Point2 { int ; int ; void move(int d, int d) { = + d; = + d; int getx() { return ; int gety() { return ; class Point3 etends Point2 { int z; void movez(int dz) { z = z + dz; int getz() { return z; int main() { Point2 p; Point3 q = new Point3; q.move(2, 4); q.movez(7); p = q; Adding classes to basic JavaLette (etension 1) New toplevel deinitions Class deinitions, consisting o a number o instance variable declarations and a number o method deinitions Instance variables are onl visible within methods o the class All methods are public All classes have one deault constructor, which initializes instance variables to deault values (0, alse, null) A class ma etend another one, adding more instance variables and methods, but without overriding Classes are tpes; variables can be declared to be reerences to objects o a class We have subtping; i S etends C, then S is a subtpe o C; whenever an object o tpe C is epected, we ma suppl an object o tpe S. Hints or object etension 1 New orms o epressions Object creation, eampliied b new Point2, which allocates a new object on the heap with deault values or instance variables Method calls, eampliied b p.move(3,5) Null reerences, eampliied b (Point)null Sel reerence. Within a class, sel reers to the current object; all calls to sibling methods within a class must use method calls to sel Implementation hints Much o ideas (and code) rom the pointers/structures etension can be reused. Method calls will be translated to unction call with receiving object as etra, irst parameter. Overriding / dnamic dispatch (etension 2) Objects at runtime Eample with overriding We consider the ollowing classes: A obja = new A(); B objb = new B(); C objc = new C(); class A { int ; void (int z) { = z; class B etends A { int ; int g() {return 0; g g class C etends B { void (int z) { = z; = 0; code entr A_ code entr B_g code entr C_ Each object includes instance variables and pointer to class descriptor.

7 Objects at runtime, variant Dnamic dispatch A obja = new A(); B objb = new B(); C objc = new C(); null g Code eample A obj = obja; obj.(5); obj = objc; obj.(3); code entr A_ code entr B_g code entr C_ Class descriptors linked into list. List searched at runtime. Dnamic dispatch What code is run? Code eample A obj = obja; obj.(5); obj = objc; obj.(3); The method to eecute is determined at runtime b ollowing the link to the class descriptor Static tpe checking guarantees that there is a method with proper signature in the descriptor There is an eicienc penalt in dnamic dispatch (so optimization tries to remove it) Modules Module sstems Module sstems Programmer s perspective: Modularit Reusabilit Inormation hiding Name space control Programmer s perspective: Modularit Reusabilit Inormation hiding Name space control Compiler s perspective: Separate compilation Smaller compilation units Recompilation onl o changed units Librar modules released as binaries

8 Some approaches A possible module sstem or basic JavaLette Increasing levels o sophistication Inclusion mechanism: concatenate all iles beore compilation Include with header iles: headers with tpe inormation included or compilation and separate linking Import mechanism: Compilation requires interace ino rom imported iles Compilation generates interace and object iles Oten in OO languages, module = class Etension proposal One module per ile All modules in same director (urther etension: deine search path mechanism) Observations Mainl sstem or name space control and libraries I ou want to implement it, ou ma get credits Diicult: not much support in LLVM Import in JavaLette Import and dependenc New snta I M is a module name, then import M is a new orm o declaration M.(e 1,, e n ) is a new orm o epression Unqualiied use A unction in an imported module ma be used without the module qualiication i the name is unique. Name clashes are resolved as ollows: I two imported modules deine a unction, we must use the qualiied orm I the current module and an imported module both deine, the unqualiied name reers to the local unction Import To use unctions deined in M, another module must eplicitl import M. Hence, import is not transitive, i.e, i M imports L and L imports K, it does not ollow that M imports K. Dependenc I M imports L, then M depends on L I M imports L and L depends on K, then M depends on K; dependenc is transitive We assume that dependenc is non-cclic: i M depends on N, then N ma not depend on M. Compiling a module, 1 Compiling a module, 2 Compiler s tasks When called b jlc M.jl, the compiler must 1. Read the import statements o M to get list o imported modules 2. Recursivel, read the import statements o these modules (and report an error i some module not ound) 3. Build dependenc graph o involved modules 4. Sort modules topologicall (and report error i cclic import ound) 5. Go through modules in topological order (M last) and check timestamps to see i recompilation is necessar Hint: It is OK to require that import statements are in the beginning o the ile and with one import per line to avoid need o complete parsing. Smbol table You need a smbol table with tpes o unctions rom all imported modules. This ino is readil available in LLVM iles, but needs to be collected (and parsed). Build the smbol table so that unqualiied names will ind the correct tpe signature (i.e., ou must check or name clashes). Note 1 It is a good idea to replace unqualiied names b qualiied (or code generation) Note 2 Tpe declaration or all imported unctions must be added to LLVM ile

Compiler construction 2009

Compiler construction 2009 Lecture 6 Some project extensions. Pointers and heap allocation. Object-oriented languages. Module systems. Memory structure Javalette restrictions Only local variables and parameters