1. Overview of Compilation Prepared by Manuel E. Bermúdez, Ph.D. Associate Professor University of Florida Programming Language Principles Lecture 2

2. Overview of Translation Definition: A translator is an algorithm that converts source programs into equivalent target programs. Definition: A compiler is a translator whose target language is at a “lower” level than its source language. Translator Source Target

3. Overview of Translation (cont’d) When is one language’s level “lower” than another’s? Definition: An interpreter is an algorithm that simulates the execution of programs written in a given source language. Interpreter Source input output

4. Overview of Translation (cont’d) Definition: An implementation of a programming language consists of a translator (or compiler) for that language, and an interpreter for the corresponding target language. Interpreter Target input output Compiler Source

5. Translation A source program may be translated an arbitrary number of times before the target program is generated. Translator 1 Source Translator 2 Translator N Target......

6. Translation (cont’d) Each of these translations is called a phase, not to be confused with a pass, i.e., a disk dump. Q: How should a compiler be divided into phases? A: So that each phase can be easily described by some formal model of computation, and so the phase can be carried out efficiently.

7. Translation (cont’d) Q: How is a compiler usually divided? A: Two major phases, with many possibilities for subdivision. Phase 1: Analysis (determine correctness) Phase 2: Synthesis (produce target code) Another criterion: Phase 1: Syntax (form). Phase 2: Semantics (meaning).

8. Typical Compiler Breakdown Scanning (Lexical analysis). Goal: Group sequences of characters that occur on the source, into logical atomic units called tokens. Examples of tokens: Identifiers, keywords, integers, strings, punctuation marks, “white spaces”, end-of-line characters, comments, etc., … Scanner (Lexical analysis) Source Sequence of Tokens

9. Lexical Analysis Must deal with end-of-line and end-of- file characters. A preliminary classification of tokens is made. For example, both ‘ program ’ and ‘ Ex ’ are classified as Identifier. Someone must give unambiguous rules for forming tokens.

11. Screening Goals: Remove unwanted tokens. Classify keywords. Merge/simplify tokens. Screener Sequence of Tokens

12. Screening Keywords recognized. White spaces (and comments) discarded. The screener acts as an interface between the scanner and the next phase, the parser.

14. Parsing (Syntax Analysis) Goals To group together the tokens, into the correct syntactic structures, if possible. To determine whether the tokens appear in patterns that syntactically correct.

15. Parsing (Syntax Analysis) Syntactic structures: Expressions Statements Procedures Functions Modules Methodology: Use “re-write” rules (a.k.a. BNF).

16. String-To-Tree Transduction Goal: To build a “syntax tree” from the sequence of rewrite rules. The tree will be the functional representation of the source. Method: Build tree “bottom-up,” as the rewrite rules are emitted. Use a stack of trees.

18. Contextual Constraint Analysis Goal: To analyze static semantics, e.g., Are variables declared before they are used? Is there assignment compatibility? e.g., a:=3 Is there operator type compatibility? e.g., a+3 Do actual and formal parameter types match? e.g. int f(int n, char c) {…}... f('x', 3); Enforcement of scope rules.

19. Contextual Constraint Analysis Method: Traverse the tree recursively, deducing type information at the bottom, and passing it up. –Make use of a DECLARATION TABLE, to record information about names. –“Decorate” tree with reference information.

21. Example Chronologically, 1.Enter x into the DCLN table, with its type. 2.Check type compatibility for x=5. 3.X2 not declared! 4.Verify type of ’ > ’ is boolean. 5.Check type compatibility for ‘ + ’. 6.Check type compatibility between x and int, for assignment.

22. Code Generation Goal: Convert syntax tree to target code. Target code could be: Machine language. Assembly language. Quadruples for a fictional machine: label opcode operands (1 or 2) 

23. Code Generation Example: “pc” on UNIX generates assembly code “pi” on UNIX generates code for the “p” machine, which is interpreted by… an interpreter. pc: slow compilation, fast running code. pi: fast compilation, slow running code. Method: Traverse the tree again.

24. Code (for a stack machine) LOAD 5 STORE X LOAD X LOAD 10 BGT COND L1 L2 L1LOAD X LOAD 1 BADD STORE X GOTO L3 L2... L3

25. Code Optimization Goals: Reduce the size of the target program. Decrease the running time of the target. Note: “Optimization” is a misnomer. Code improvement would be better. Two types of optimization: Peephole optimization (local). Global optimization (improve loops, etc.).

26. Code Optimization (cont’d) Example (from previous slide): LOAD 5 can be LOAD 5 STORE X replaced STND X LOAD X with Store non-destructively, i.e., store in X, but do not destroy value on top of stack.

27. Summary Parser Source Constrainer Code Generator Code (for an abstract machine) Interpreter Screener Scanner InputOutput Table Routines Error Routines Tokens Tree

28. Overview of Compilation Prepared by Manuel E. Bermúdez, Ph.D. Associate Professor University of Florida Programming Language Principles Lecture 2