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Building Interpreters

Building Interpreters. Mooly Sagiv html://www.cs.tau.ac.il/~msagiv/courses/wcc13.html. Chapter 4. Symbol Table. Structure of a simple compiler/interpreter. Runtime System Design. Lexical analysis. Code generation. Intermediate code (AST). Syntax analysis. Machine dependent.

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Building Interpreters

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  1. Building Interpreters Mooly Sagiv html://www.cs.tau.ac.il/~msagiv/courses/wcc13.html Chapter 4

  2. Symbol Table Structure of a simple compiler/interpreter Runtime System Design Lexical analysis Code generation Intermediate code (AST) Syntax analysis Machine dependent Context analysis Interpretation PL+pardigm dependent PL dependent

  3. Types of Interpreters • Recursive • Recursively traverse the tree • Uniform data representation • Conceptually clean • Excellent error detection • 1000x slower than compiler • Iterative • Closer to CPU • One flat loop • Explicit stack • Good error detection • 30x slower than compiler • Can invoke compiler on code fragments

  4. Input language (Overview) • Fully parameterized expressions • Arguments can be a single digit expression  digit | ‘(‘ expression operator expression ‘)’ operator  ‘+’ | ‘*’ digit  ‘0’ | ‘1’ | ‘2’ | ‘3’ | ‘4’ | ‘5’ | ‘6’ | ‘7’ | ‘8’ | ‘9’

  5. #include "parser.h" #include "backend.h" static int Interpret_expression(Expression *expr) { switch (expr->type) { case 'D': return expr->value; break; case 'P': { int e_left = Interpret_expression(expr->left); int e_right = Interpret_expression(expr->right); switch (expr->oper) { case '+': return e_left + e_right; case '*': return e_left * e_right; }} break; } } void Process(AST_node *icode) { printf("%d\n", Interpret_expression(icode)); }

  6. P P P * + * D D D D 4 9 3 2 AST for (2 * ((3*4)+9)) type right left oper

  7. Uniform self-identifying data representation • The types of the sizes of program data values are not known when the interpreter is written • Uniform representation of data types • Type • Size • The value is a pointer

  8. Example: Complex Number re: 3.0 4.0 im:

  9. Status Indicator • Direct control flow of the interpreter • Possible values • Normal mode • Errors • Jumps • Exceptions • Return

  10. Example: Interpreting C Return PROCEDURE Elaborate return with expression statement (RWE node): SET Result To Evaluate expression (RWE node . expression); IF Status . mode /= Normal mode: Return mode; SET Status . mode To Return mode; SET Status . value TO Result;

  11. Interpreting If-Statement

  12. Symbol table • Stores content of variables, named constants, … • For every variable V of type T • A pointer to the name of V • The file name and the line it is declared • Kind of declaration • A pointer to T • A pointer to newly allocated space • Initialization bit • Language dependent information (e.g. scope)

  13. Summary Recursive Interpreters • Can be implemented quickly • Debug the programming language • Not good for heavy-duty interpreter • Slow • Can employ general techniques to speed the recursive interpreter • Memoization • Tail call elimination • Partial evaluation

  14. Memoization int fib(int n) { if (n == 0) return 0 ; if (n==1) return 1; return fib(n-1) + fib(n-2) ; } int sfib[100] = {-1, -1, …, -1} int fib(int n) { if (sfib[n] > 0) return sfib[n]; if (n == 0) return 0 ; if (n==1) return 1; sfib[n] = fib(n-1) + fib(n-2) ; return sfib[n]; } 

  15. Tail Call Elimination void a(…) { … b(); } void b(){ code; } void a(…) { … code; } void b(){ code; } 

  16. Tail Call Elimination void a(int n) { code if (n > 0) a(n-1); } void a(int n) { loop: code if (n > 0) { n = n -1 ; goto loop } 

  17. Partial Evaluator Program’ Program Input 1 Input 2 Partial Evaluation • Partially interpret static parts in a program • Generates an equivalent program

  18. Example int pow4(int n) { return n * n * n *n; } int pow(int n, int e) { if (e==0) return 1; else return n * pow(n, e-1); } e=4

  19. Example2 Bool match(string, regexp) { switch(regexp) { …. } } regexp=a b*

  20. Partial Evaluation Generalizes Compilation Partial Evaluator Program Interpreter AST Program Input

  21. But ….

  22. Iterative Interpretation • Closed to CPU • One flat loop with one big case statement • Use explicit stack • Intermediate results • Local variables • Requires fully annotated threaded AST • Active-node-pointer (interpreted node)

  23. Demo Compiler

  24. Threaded AST • Annotated AST • Every node is connected to the immediate successor in the execution • Control flow graph • Nodes • Basic execution units • expressions • assignments • Edges • Transfer of control • sequential • while • …

  25. P P P * * + D D D D 2 9 3 4 Threaded AST for (2 * ((3*4)+9)) Start Dummy_node type right left oper

  26. Demo Compiler

  27. exit F T C Example while ((x > 0) && (x < 10)) { x = x + y ; y = y – 1 ; } while seq and ass ass > < id id y const + id const id x x x 0 10 id id x y + const id y 1

  28. Threading the AST(3.2.1) • One preorder AST pass • Every type of AST has its threading routine • MaintainsLast node pointer • Global variable • Set successor ofLast pointerwhen node isvisited

  29. Last node pointer main while seq and ass ass > < id id y const + id const id x x x 0 10 id id x y + const id y 1

  30. Last node pointer main while seq and ass ass > < id id y const + id const id x x x 0 10 id id x y + const id y 1

  31. Last node pointer main while seq and ass ass > < id id y const + id const id x x x 0 10 id id x y + const id y 1

  32. Last node pointer main while seq and ass ass > < id id y const + id const id x x x 0 10 id id x y + const id y 1

  33. Last node pointer main while seq and ass ass > < id id y const + id const id x x x 0 10 id id x y + const id y 1

  34. Last node pointer main while seq and ass ass > < id id y const + id const id x x x 0 10 id id x y + const id y 1

  35. Last node pointer main while seq and ass ass > < id id y const + id const id x x x 0 10 id id x y + const id y 1

  36. Last node pointer main while seq and ass ass > < id id y const + id const id x x x 0 10 id id x y + const id y 1

  37. T Last node pointer main while seq and ass ass > < id id y const + id const id x x x 0 10 id id x y + const id y 1

  38. Last node pointer main while seq and T ass ass > < id id y const + id const id x x x 0 10 id id x y + const id y 1

  39. Last node pointer main while seq and T ass ass > < id id y const + id const id x x x 0 10 id id x y + const id y 1

  40. Last node pointer main while seq and T ass ass > < id id y const + id const id x x x 0 10 id id x y + const id y 1

  41. main while seq and T ass ass > < id id y const + id const id x x x 0 10 id id x y + Last node pointer const id y 1

  42. Last node pointer main while seq and T ass ass > < id id y const + id const id x x x 0 10 id id x y + const id y 1

  43. First node pointer Last node pointer main while seq and T ass ass > < id id y const + id const id x x x 0 10 id id x y + const id y 1

  44. Demo Compiler

  45. Conditional Statement Last node pointer if cond then_part else_part

  46. Conditional Statement if F T cond then_part else_part End_If Last node pointer

  47. Iterative Interpretation • Closed to CPU • One flat loop with one big case statement • Use explicit stack • Intermediate results • Local variables • Requires fully annotated threaded AST • Active-node-pointer (interpreted node)

  48. Demo Compiler

  49. Conditional Statements

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