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Compiler: High-level to Machine Code Translation

Learn how a compiler checks for syntax errors, translates high-level language programs to machine code, and executes them on any machine. Understand the importance of symbol tables and generating efficient assembly language code.

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Compiler: High-level to Machine Code Translation

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  1. Lecture 18 Compilers andLanguage Translation (S&G, ch. 9) CS 100 - Lecture 18

  2. Read S&G ch. 10(Models of Computation) CS 100 - Lecture 18

  3. Editor Not OK (error messages) Create high-level language program OK Compiler Check for syntax errors and translate to machine code Run-time system Execute the machine language program on any machine Program Development CS 100 - Lecture 18 slide < S. Levy

  4. .begin clear sum load one store count while: load count compare eleven jumpeq endwhile add sum store sum increment count jump while endwhile: out sum halt one: .data 1 eleven: .data 11 sum: .data 0 count: .data 0 .end Set sum to 0 Set count to 1 While count <= 10 do Set sum to sum + count Increment count Output sum Compiler: A Programming Language Translator Compile CS 100 - Lecture 18 slide < S. Levy

  5. Compiler: Requirements / Tasks • Recognize programming constructs (conditionals, loops) and translate them to assembly language (compare, jump) • Reject and report unrecognized programs • Keep track of symbols (sum, count) and declare each one using a .data directive • Generate efficient assembly language code CS 100 - Lecture 18 slide < S. Levy

  6. load x compare y jumpgt outx out y jump next outx: out x next: .... if x > y output x else output y Recognizing Constructs: Parsing • Could try to translate, e.g., from every possible if/else to its corresponding assembly code: Rule #18734 CS 100 - Lecture 18 slide < S. Levy

  7. load a compare b jumplt outa out b jump next outa: out a next: .... if a < b output a else output b Recognizing Constructs: Parsing (2) • Could try to translate, e.g., from every possible if/else to its corresponding assembly code: Rule #22102 • Impossible, because we’d need an infinite number of rules! CS 100 - Lecture 18 slide < S. Levy

  8. if/else output output < a b a b Recognizing Constructs: Parsing (3) • Instead, we recognize (abstract) structures, and work on their components one at a time: CS 100 - Lecture 18 slide < S. Levy

  9. Parse Generate < load a compare b a < b a b • When parsing fails, compiler reports a syntax error: Parse Unrecognized operator: << a << b Recognizing Constructs: Parsing (4) • Now we can work on smaller pieces: CS 100 - Lecture 18 slide < S. Levy

  10. BNF Grammar • A notation for expressing all the allowable statements in a language • A finite description of an infinite number of possibilities • Each distinct arrangement is shown in schematic form • Because languages are nested hierarchically, the definitions are recursive CS 100 - Lecture 18

  11. Example Grammar • assignment statement ::=variable=expression • expression ::= variable | expressionarith opvariable | (expression) • arith op ::= + | – | * | / • variable ::= x | y | z  Unrealistic! CS 100 - Lecture 18

  12. Additional Example • if statement ::=if(Boolean expression)assignment statement;else clause • Boolean expression ::= variable | expressionrelationexpression • relation ::= == | < | > • else clause ::= elseassignment statement |  CS 100 - Lecture 18

  13. Compile Set sum to 0 sum: .data 0 • Subsequent references to symbol don't need a new declaration: Compile load sum add count store sum Set sum to sum + count Keeping Track of Symbols: Symbol Tables • When a symbol is first set, compiler should declare it: CS 100 - Lecture 18 slide < S. Levy

  14. Set sum to 0 While count <= 10 do Keeping Track of Symbols: Symbol Tables (2) • Failure to set initial value should result in an error: “count” not declared • Compiler uses a symbol table (dictionary; phonebook) to keep track of values we’ve declared CS 100 - Lecture 18 slide < S. Levy

  15. a: int b: int s: String x: double • Then compiler can use symbol table to detect “type errors”: can't set int to String a = a + s; Symbol Tables • In a language like Java, symbol table would contain info about variable types: CS 100 - Lecture 18 slide < S. Levy

  16. Optimization: Generating Efficient Assembly Code • “Laundry metaphor”: Unoptimized • Load dirty clothes into washer • Run washer • Unload wet clothes from washer to basket • Load wet clothes into dryer from basket • Run dryer • Unoptimized version allows us to break down the task into simple parts • Requires less coordination between washer & dryer CS 100 - Lecture 18 slide < S. Levy

  17. Optimization: Generating Efficient Assembly Code (2) • “Laundry metaphor”: Optimized • Load dirty clothes into washer • Run washer • Load wet clothes from washer to dryer • Run dryer • Optimized version saves a step and doesn't require a basket • Requires more coordination between washer & dryer CS 100 - Lecture 18 slide < S. Levy

  18. unoptimized load a add b store a out a load a add c store a Set a to a + b Output a Set a to a + c optimized load a add b store a out a add c store a Optimization • Note coordination required between first and third steps of pseudocode CS 100 - Lecture 18 slide < S. Levy

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