Lecture 11 compiler ii

Lecture 11 compiler ii

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  Elements of ComputingSystems, Nisan & Schocken, MIT Press, www.nand2tetris.org , Chapter 1: Compiler II: Code Generation slide 1 www.nand2tetris.org Building a Modern Computer From First Principles Compiler II: Code Generation
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  Elements of ComputingSystems, Nisan & Schocken, MIT Press, www.nand2tetris.org , Chapter 1: Compiler II: Code Generation slide 2 Course map Assembler Chapter 6 H.L. Language & Operating Sys. abstract interface Compiler Chapters 10 - 11 VM Translator Chapters 7 - 8 Computer Architecture Chapters 4 - 5 Gate Logic Chapters 1 - 3 Electrical Engineering Physics Virtual Machine abstract interface Software hierarchy Assembly Language abstract interface Hardware hierarchy Machine Language abstract interface Hardware Platform abstract interface Chips & Logic Gates abstract interface Human Thought Abstract design Chapters 9, 12
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  Elements of ComputingSystems, Nisan & Schocken, MIT Press, www.nand2tetris.org , Chapter 1: Compiler II: Code Generation slide 3 The big picture (Chapter 11)Jack Program Toke- nizer Parser Code Gene -ration Syntax Analyzer Jack Compiler VM code XML code (Chapter 10) 1. Syntax analysis: extracting the semantics from the source code 2. Code generation: expressing the semantics using the target language this lecture previous lecture
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  Elements of ComputingSystems, Nisan & Schocken, MIT Press, www.nand2tetris.org , Chapter 1: Compiler II: Code Generation slide 4 Syntax analysis (review) Class Bar { method Fraction foo(int y) { var int temp; // a variable let temp = (xxx+12)*-63; ... ... Class Bar { method Fraction foo(int y) { var int temp; // a variable let temp = (xxx+12)*-63; ... ... <varDec> <keyword> var </keyword> <keyword> int </keyword> <identifier> temp </identifier> <symbol> ; </symbol> </varDec> <statements> <letStatement> <keyword> let </keyword> <identifier> temp </identifier> <symbol> = </symbol> <expression> <term> <symbol> ( </symbol> <expression> <term> <identifier> xxx </identifier> </term> <symbol> + </symbol> <term> <int.Const.> 12 </int.Const.> </term> </expression> ... <varDec> <keyword> var </keyword> <keyword> int </keyword> <identifier> temp </identifier> <symbol> ; </symbol> </varDec> <statements> <letStatement> <keyword> let </keyword> <identifier> temp </identifier> <symbol> = </symbol> <expression> <term> <symbol> ( </symbol> <expression> <term> <identifier> xxx </identifier> </term> <symbol> + </symbol> <term> <int.Const.> 12 </int.Const.> </term> </expression> ... Syntax analyzer The code generation challenge: Program = a series of operations that manipulate data Compiler: converts each “understood” (parsed) source operation and data item into corresponding operations and data items in the target language Thus, we have to generate code for o handling data o handling operations Our approach: morph the syntax analyzer (project 10) into a full-blown compiler: instead of generating XML, we’ll make it generate VM code.
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  Elements of ComputingSystems, Nisan & Schocken, MIT Press, www.nand2tetris.org , Chapter 1: Compiler II: Code Generation slide 5 Memory segments (review) Where i is a non-negative integer and segment is one of the following: static: holds values of global variables, shared by all functions in the same class argument: holds values of the argument variables of the current function local: holds values of the local variables of the current function this: holds values of the private (“object”) variables of the current object that: holds array values (silly name, sorry) constant: holds all the constants in the range 0 … 32767 (pseudo memory segment) pointer: used to anchor this and that to various areas in the heap temp: fixed 8-entry segment that holds temporary variables for general use; Shared by all VM functions in the program. VM memory Commands: pop segment i push segment i VM memory Commands: pop segment i push segment i
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  Elements of ComputingSystems, Nisan & Schocken, MIT Press, www.nand2tetris.org , Chapter 1: Compiler II: Code Generation slide 6 Code generation example method int foo() { var int x; let x = x + 1; ... method int foo() { var int x; let x = x + 1; ... <letStatement> <keyword> let </keyword> <identifier> x </identifier> <symbol> = </symbol> <expression> <term> <identifier> x </identifier> </term> <symbol> + </symbol> <term> <constant> 1 </constant> </term> </expression> </letStatement> <letStatement> <keyword> let </keyword> <identifier> x </identifier> <symbol> = </symbol> <expression> <term> <identifier> x </identifier> </term> <symbol> + </symbol> <term> <constant> 1 </constant> </term> </expression> </letStatement> Syntax analysis (note that x is the first local variable declared in the method) push local 0 push constant 1 add pop local 0 push local 0 push constant 1 add pop local 0 Code generation
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  Elements of ComputingSystems, Nisan & Schocken, MIT Press, www.nand2tetris.org , Chapter 1: Compiler II: Code Generation slide 7 Handling variables When the compiler encounters a variable, say x, in the source code, it has to know: What is x’s data type? Primitive, or ADT (class name) ? (Need to know in order to properly allocate RAM resources for its representation) What kind of variable is x? local, static, field, argument ? ( We need to know in order to properly allocate it to the right memory segment; this also implies the variable’s life cycle ).
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  Elements of ComputingSystems, Nisan & Schocken, MIT Press, www.nand2tetris.org , Chapter 1: Compiler II: Code Generation slide 8 Handling variables: mapping them on memory segments (example) When compiling this class, we have to create the following mappings: The class variables nAccounts , bankCommission are mapped on static 0,1 The object fields id, owner, balance are mapped on this 0,1,2 The argument variables sum, bankAccount, when are mapped on arg 0,1,2 The local variables i, j, due are mapped on local 0,1,2. The target language uses 8 memory segments Each memory segment, e.g. static, is an indexed sequence of 16-bit values that can be referred to as static 0, static 1, static 2, etc.
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  Elements of ComputingSystems, Nisan & Schocken, MIT Press, www.nand2tetris.org , Chapter 1: Compiler II: Code Generation slide 9 Handling variables: symbol tables How the compiler uses symbol tables: The compiler builds and maintains a linked list of hash tables, each reflecting a single scope nested within the next one in the list Identifier lookup works from the current symbol table back to the list’s head (a classical implementation).
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  Elements of ComputingSystems, Nisan & Schocken, MIT Press, www.nand2tetris.org , Chapter 1: Compiler II: Code Generation slide 10 Handling variables: managing their life cycle Variables life cycle static variables: single copy must be kept alive throughout the program duration field variables: different copies must be kept for each object local variables: created on subroutine entry, killed on exit argument variables: similar to local variables. Good news: the VM implementation already handles all these details !
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  Elements of ComputingSystems, Nisan & Schocken, MIT Press, www.nand2tetris.org , Chapter 1: Compiler II: Code Generation slide 11 class Complex { // Fields (properties): int re; // Real part int im; // Imaginary part ... /** Constructs a new Complex number */ public Complex (int re, int im) { this.re = re; this.im = im; } ... } class Foo { public void bla() { Complex a, b, c; ... a = new Complex(5,17); b = new Complex(12,192); ... c = a; // Only the reference is copied ... } class Complex { // Fields (properties): int re; // Real part int im; // Imaginary part ... /** Constructs a new Complex number */ public Complex (int re, int im) { this.re = re; this.im = im; } ... } class Foo { public void bla() { Complex a, b, c; ... a = new Complex(5,17); b = new Complex(12,192); ... c = a; // Only the reference is copied ... } Java code Handling objects: construction / memory allocation How to compile: foo = new ClassName(…) ? The compiler generates code affecting: foo = Memory.alloc(n) Where n is the number of words necessary to represent the object in question, and Memory.alloc is an OS method that returns the base address of a free memory block of size n words. Following compilation:
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  Elements of ComputingSystems, Nisan & Schocken, MIT Press, www.nand2tetris.org , Chapter 1: Compiler II: Code Generation slide 12 Handling objects: accessing fields class Complex { // Properties (fields): int re; // Real part int im; // Imaginary part ... /** Constructs a new Complex number */ public Complex(int re, int im) { this.re = re; this.im = im; } ... /** Multiplies this Complex number by the given scalar */ public void mult (int c) { re = re * c; im = im * c; } ... } class Complex { // Properties (fields): int re; // Real part int im; // Imaginary part ... /** Constructs a new Complex number */ public Complex(int re, int im) { this.re = re; this.im = im; } ... /** Multiplies this Complex number by the given scalar */ public void mult (int c) { re = re * c; im = im * c; } ... } Java code *(this + 1) = *(this + 1) times (argument 0) *(this + 1) = *(this + 1) times (argument 0) How to compile: im = im * c ? 1. look up the two variables in the symbol table 2. Generate the code: This pseudo-code should be expressed in the target language.
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  Elements of ComputingSystems, Nisan & Schocken, MIT Press, www.nand2tetris.org , Chapter 1: Compiler II: Code Generation slide 13 Assume that b and r were passed to the function as its first two arguments. How to compile (in Java): b.radius = r ? // Get b's base address: push argument 0 // Point the this segment to b: pop pointer 0 // Get r's value push argument 1 // Set b's third field to r: pop this 2 Assume that b and r were passed to the function as its first two arguments. How to compile (in Java): b.radius = r ? // Get b's base address: push argument 0 // Point the this segment to b: pop pointer 0 // Get r's value push argument 1 // Set b's third field to r: pop this 2 120 80 50radius: x: y: 3color: 120 80 50 3012 3013 3014 33015 412 3012 ... ... High level program view RAM view 0 ... b following compilation b object b object(Actual RAM locations of program variables are run-time dependent, and thus the addresses shown here are arbitrary examples.) 00 1 Virtual memory segments just before the operation b.radius=17: 3012 17 0 1 ... ... 120 80 17 0 1 2 30120 1 3 3012 17 0 1 argument pointer this ... 3 (this 0 is now alligned with RAM[3012]) ... Virtual memory segments just after the operation b.radius=17: argument pointer this Handling objects: establishing access to the object’s fields Background: Suppose we have an object named b of type Ball. A Ball has x,y coordinates, a radius, and a color.
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  Elements of ComputingSystems, Nisan & Schocken, MIT Press, www.nand2tetris.org , Chapter 1: Compiler II: Code Generation slide 14 Handling objects: method calls General rule: each method call foo.bar(v1,v2,...) is translated into: push foo push v1 push v2 ... call bar class Complex { // Properties (fields): int re; // Real part int im; // Imaginary part ... /** Constructs a new Complex object. */ public Complex(int re, int im) { this.re = re; this.im = im; } ... } class Foo { ... public void bla() { Complex x; ... x = new Complex(1,2); x.mult(5); ... } } class Complex { // Properties (fields): int re; // Real part int im; // Imaginary part ... /** Constructs a new Complex object. */ public Complex(int re, int im) { this.re = re; this.im = im; } ... } class Foo { ... public void bla() { Complex x; ... x = new Complex(1,2); x.mult(5); ... } } Java code push x push 5 call mult push x push 5 call mult How to compile: x.mult(5) ? This method call can also be viewed as: mult(x,5) Generate the following code:
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  Elements of ComputingSystems, Nisan & Schocken, MIT Press, www.nand2tetris.org , Chapter 1: Compiler II: Code Generation slide 15 class Bla { ... void foo(int k) { int x, y; int[] bar; // declare an array ... // Construct the array: bar = new int[10]; ... bar[k]=19; } ... Main.foo(2); // Call the foo method ... class Bla { ... void foo(int k) { int x, y; int[] bar; // declare an array ... // Construct the array: bar = new int[10]; ... bar[k]=19; } ... Main.foo(2); // Call the foo method ... Java code How to compile: bar = new int(n) ? Generate code affecting: bar = Memory.alloc(n) Handling arrays: declaration / construction 19 4315 4316 4317 4324 (bar array) ... 4318 ... ... 4315 ... 0 bar x y 2 k (local 0) (local 1) (local 2) (argument 0) 275 276 277 504 RAM state ... Following compilation:
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  Elements of ComputingSystems, Nisan & Schocken, MIT Press, www.nand2tetris.org , Chapter 1: Compiler II: Code Generation slide 16 class Bla { ... void foo(int k) { int x, y; int[] bar; // declare an array ... // Construct the array: bar = new int[10]; ... bar[k]=19; } ... Main.foo(2); // Call the foo method ... class Bla { ... void foo(int k) { int x, y; int[] bar; // declare an array ... // Construct the array: bar = new int[10]; ... bar[k]=19; } ... Main.foo(2); // Call the foo method ... Java code How to compile: bar[k] = 19 ? // bar[k]=19, or *(bar+k)=19 push bar push k add // Use a pointer to access x[k] pop addr // addr points to bar[k] push 19 pop *addr // Set bar[k] to 19 // bar[k]=19, or *(bar+k)=19 push bar push k add // Use a pointer to access x[k] pop addr // addr points to bar[k] push 19 pop *addr // Set bar[k] to 19 VM Code (pseudo) // bar[k]=19, or *(bar+k)=19 push local 2 push argument 0 add // Use the that segment to access x[k] pop pointer 1 push constant 19 pop that 0 // bar[k]=19, or *(bar+k)=19 push local 2 push argument 0 add // Use the that segment to access x[k] pop pointer 1 push constant 19 pop that 0 VM Code (actual) 19 4315 4316 4317 4324 (bar array) ... 4318 ... ... 4315 ... 0 bar x y 2 k (local 0) (local 1) (local 2) (argument 0) 275 276 277 504 RAM state, just after executing bar[k] = 19 ... Following compilation: Handling arrays: accessing an array entry by its index
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  Elements of ComputingSystems, Nisan & Schocken, MIT Press, www.nand2tetris.org , Chapter 1: Compiler II: Code Generation slide 17 syntax analysis parse tree Handling expressions ((5+z)/-8)*(4^2)((5+z)/-8)*(4^2) High-level code push 5 push z add push 8 neg call div push 4 push 2 call power call mult push 5 push z add push 8 neg call div push 4 push 2 call power call mult code generation To generate VM code from a parse tree exp, use the following logic: The codeWrite(exp) algorithm: if exp is a constant n then output "push n" if exp is a variable v then output "push v" if exp is op(exp1) then codeWrite(exp1); output "op"; if exp is (exp1 op exp2) then codeWrite(exp1); codeWrite(exp2); output "op"; if exp is f (exp1, ..., expn) then codeWrite(exp1); ... codeWrite(exp1); output "call f"; VM code
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  Elements of ComputingSystems, Nisan & Schocken, MIT Press, www.nand2tetris.org , Chapter 1: Compiler II: Code Generation slide 18 Handling program flow if (cond) s1 else s2 ... if (cond) s1 else s2 ... High-level code VM code to compute and push !(cond) if-goto L1 VM code for executing s1 goto L2 label L1 VM code for executing s2 label L2 ... VM code to compute and push !(cond) if-goto L1 VM code for executing s1 goto L2 label L1 VM code for executing s2 label L2 ... VM code code generation while (cond) s ... while (cond) s ... High-level code label L1 VM code to compute and push !(cond) if-goto L2 VM code for executing s goto L1 label L2 ... label L1 VM code to compute and push !(cond) if-goto L2 VM code for executing s goto L1 label L2 ... VM code code generation
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  Elements of ComputingSystems, Nisan & Schocken, MIT Press, www.nand2tetris.org , Chapter 1: Compiler II: Code Generation slide 19 Final example
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  Elements of ComputingSystems, Nisan & Schocken, MIT Press, www.nand2tetris.org , Chapter 1: Compiler II: Code Generation slide 20 Perspective Jack simplifications that are challenging to extend: Limited primitive type system No inheritance No public class fields, e.g. must use r = c.getRadius() rather than r = c.radius Jack simplifications that are easy to extend: : Limited control structures, e.g. no for, switch, … Cumbersome handling of char types, e.g. cannot use let x=‘c’ Optimization For example, c=c+1 is translated inefficiently into push c, push 1, add, pop c. Parallel processing Many other examples of possible improvements …