CS Concepts of Compiler Design

Transcription

1 CS Concepts of Compiler Design Kenneth Louden Spring, 2006 Chapter 1: Terminology, and the Structure of a Compiler. Definition: A compiler is a translator from one notational system to another, usually from source code (high level code) to machine code (object code, target code, low level code). Spring, 2006 CS 153, Chapter 1 2

2 The operation of a compiler involves several different kinds of files A source code text file (.c,.cpp,.java, etc. file extensions). Intermediate code files: transformations of source code during compilation, usually kept in temporary files rarely seen by the user. An assembly code text file containing symbolic machine code, often produced as the output of a compiler (.asm,.s file extensions). Spring, 2006 CS 153, Chapter 1 3 Files, continued One or more binary object code files: machine instructions, not yet linked or executable (.obj,.o file extensions) A binary executable file: linked, independently executable (well, not always ) code (.exe,.out extensions, or no extension). Spring, 2006 CS 153, Chapter 1 4

3 A compiler consists of several phases (logical units) and helper utilities: Spring, 2006 CS 153, Chapter 1 5 Source Code Scanner Tokens Parser Syntax Tree Semantic Analyzer Annotated Tree Source Code Optimizer Intermediate Code Literal Table Symbol Table Error Handler Code Generator Target Code Target Code Optimizer Spring, 2006 CS 153, Chapter 1 6 Target Code

4 Extended Example Source code: a[index] = Tokens: ID Lbracket ID Rbracket AssignOp Num AddOp Num Parse tree (syntax tree with all steps of the parser in gory detail): Spring, 2006 CS 153, Chapter 1 7 Parse tree: expression assign-expression expression = expression subscript-expression additive-expression expression [ expression ] expression + expression identifier a identifier index number 4 number 2 Spring, 2006 CS 153, Chapter 1 8

5 Syntax tree: a "trimmed" version of the parse tree with only essential information: assign-expression subscript-expression additive-expression identifier a identifier index number 4 number 2 Spring, 2006 CS 153, Chapter 1 9 Annotated tree with attributes: assign-expression integer subscript-expression integer additive-expression integer identifier a array of integer identifier number number index 4 2 integer integer integer Spring, 2006 CS 153, Chapter 1 10

6 Target code (hypothetical, page 12): MOV R0, index MUL R0, 2 MOV R1, &a ADD R1, R0 MOV *R1, 6 movl _index, %eax movl $6, _a(,%eax,4) ;; value of index -> R0 ;; double value in R0 ;; address of a -> R1 ;; add R0 to R1 ;; const 6 -> addr in R1 Or (sample PC target code from gcc): (Note source level constant folding optimization.) Spring, 2006 CS 153, Chapter 1 11 Algorithmic Tools Tokens: defined using regular expressions. (Chapter 2) Scanner: an implementation of a finite state machine (deterministic automaton) that recognizes the token regular expressions (Chapter 2). Spring, 2006 CS 153, Chapter 1 12

7 Algorithmic Tools (continued) Parser: a push-down automaton (i.e. uses a stack), based on grammar rules in a standard format (BNF -- Backus Naur Form).(Chapters 3, 4, 5) Semantic Analyzer and Code Generator: recursive evaluators based on semantic rules for attributes (properties of language constructs). (Chapters 6, 7, 8) Spring, 2006 CS 153, Chapter 1 13 Other Phase Features Parser and scanner together typically operate as a unit (parser calls scanner repeatedly to generate tokens). Front end: Parser, scanner, semantic analyzer and source code optimizer depend primarily on source language. Back end: code generator and target code optimizer depend primarily on target language (machine architecture). Spring, 2006 CS 153, Chapter 1 14

8 Other Classifications Logical unit: phase Physical unit: separately compiled code file (see later) Temporal unit: pass Passes: trips through the source code (or intermediate code). These are not phases (but they could be). Spring, 2006 CS 153, Chapter 1 15 Data structure tools Syntax tree: see previous pictures. Literal table: "Hello, world!", , etc. If a literal is used more than once (as they often are in a program), we still want to store it only once. So we use a table (almost always a hash table or table of hash tables). Symbol table: all names (variables, functions, classes, typedefs, constants, namespaces). Again, a hash table or set of hash tables is the most likely data structure. Spring, 2006 CS 153, Chapter 1 16

9 Error handler One of the more difficult parts of a compiler to design. Must handle a wide range of errors Must handle multiple errors. Must not get stuck. Must not get into an infinite loop (typical simple-minded strategy:count errors, stop if count gets too high). Spring, 2006 CS 153, Chapter 1 17 Kinds of errors Syntax: iff (x == 0) y + = z + r; } Semantic: int x = "Hello, world!"; Runtime: int x = 2;... double y = / (x - 2); Spring, 2006 CS 153, Chapter 1 18

10 Errors, continued A compiler must handle syntax and semantic errors, but not runtime errors (whether a runtime error will occur is an undecidable question). Sometimes a compiler is required to generate code to catch runtime errors and handle them in some graceful way (either with or without exception handling). This, too, is often difficult. Spring, 2006 CS 153, Chapter 1 19 Sample compilers in this class ("Toys") TINY: a 4-pass compiler for the TINY language, based on Pascal (see text, pages 22-26) C-Minus: your project language (see text, pages and Appendix A). Based on C. Spring, 2006 CS 153, Chapter 1 20

11 TINY Example read x; if 0 < x then fact := 1; repeat fact := fact * x; x := x - 1 until x = 0; write fact end Spring, 2006 CS 153, Chapter 1 21 C-Minus Example int fact( int x ) { if (x > 1) return x * fact(x-1); else return 1; } void main( void ) { int x; x = read(); if (x > 0) write( fact(x) ); } Spring, 2006 CS 153, Chapter 1 22

12 Structure of the TINY Compiler globals.h util.h scan.h parse.h symtab.h analyze.h code.h cgen.h main.c util.c scan.c parse.c symtab.c analyze.c code.c cgen.c Spring, 2006 CS 153, Chapter 1 23 Conditional Compilation Options NO_PARSE: Builds a scanner-only compiler. NO_ANALYZE: Builds a compiler that parses and scans only. NO_CODE: Builds a compiler that performs semantic analysis, but generates no code. Spring, 2006 CS 153, Chapter 1 24

13 Listing Options (built in - not flags) EchoSource: Echoes the TINY source program to the listing, together with line numbers. TraceScan: Displays information on each token as the scanner recognizes it. TraceParse: Displays the syntax tree in a linearlized format. TraceAnalyze: Displays summary information on the symbol table and type checking. TraceCode: Prints code generation-tracing comments to the code file. Spring, 2006 CS 153, Chapter 1 25