Case study - a simple adding machine
12.2 Case study - a simple adding machine
Preparation of completely attributed grammars suitable as input to CocoR requires an in-depth understanding of the Cocol specification language, including many features that we have not yet encountered. Sections 12.3 and 12.4 discuss these aspects in some detail, and owe much to the original description by Mössenböck 1990a. The discussion will be clarified by reference to a simple example, chosen to illustrate as many features as possible as a result, it may appear rather contrived. Suppose we wish to construct an adding machine that can add numbers arranged in various groups into subtotals, and then either add these subtotals to a running grand total, or reject them. Our numbers can have fractional parts; just to be perverse we shall allow a shorthand notation for handling ranges of numbers. Typical input is exemplified by clear start the machine 10 + 20 + 3 .. 7 accept one subtotal 10+20+3+4+5+6+7, accepted 3.4 + 6.875..50 cancel another one, but rejected 3 + 4 + 6 accept and a third, this time accepted total display grand total and then stop Correct input of this form can be described by a simple LL1 grammar that we might try initially to specify in Cocol on the lines of the following: COMPILER Calc CHARACTERS digit = 0123456789 . TOKENS number = digit { digit } [ . digit { digit } ] . PRODUCTIONS Calc = clear { Subtotal } total . Subtotal = Range { + Range } accept | cancel . Range = Amount [ .. Amount ] . Amount = number . END Calc. In general a grammar like this can itself be described in EBNF by Cocol = COMPILER GoalIdentifier ArbitraryText ScannerSpecification ParserSpecification END GoalIdentifier . . We note immediately that the identifier after the keyword COMPILER gives the grammar name, and must match the name after the keyword END . The grammar name must also match the name chosen for the non-terminal that defines the goal symbol of the phrase structure grammar. Each of the productions leads to the generation of a corresponding parsing routine. It should not take much imagination to see that the routines in our case study will also need to perform operations like converting the string that defines a number token into a corresponding numerical value. Thus we need mechanisms for extracting attributes of the various tokens from the scanner that recognizes them. adding such numbers into variables declared for the purpose of recording totals and subtotals, and passing these values between the routines. Thus we need mechanisms for declaring parameters and local variables in the generated routines, and for incorporating arithmetic statements. displaying the values of the variables on an output device. Thus we need mechanisms for interfacing our parsing routines to external library routines. reacting sensibly to input data that does not conform to the proper syntax. Thus we need mechanisms for specifying how error recovery should be accomplished. reacting sensibly to data that is syntactically correct, but still meaningless, as might happen if one was asked to process numbers in the range 6 .. 2. Thus we need mechanisms for reporting semantic and constraint violations. These mechanisms are all incorporated into the grammar by attributing it with extra information, as discussed in the next sections. As an immediate example of this, arbitrary text may follow the GoalIdentifier, preceding the ScannerSpecification. This is not checked by CocoR, but is simply incorporated directly in the generated parser. This offers the facility of providing code for IMPORT clauses in Modula-2, USES clauses in Turbo Pascal, or include directives in C ++ , and for the declaration of global objects constants, types, variables or functions that may be needed by later semantic actions.12.3 Scanner specification
Parts
» compilers and compiler generators 1996
» Objectives compilers and compiler generators 1996
» Systems programs and translators
» The relationship between high-level languages and translators
» T-diagrams Classes of translator
» Phases in translation compilers and compiler generators 1996
» Multi-stage translators compilers and compiler generators 1996
» Interpreters, interpretive compilers, and emulators
» Using a high-level host language Porting a high level translator
» Bootstrapping Self-compiling compilers compilers and compiler generators 1996
» The half bootstrap compilers and compiler generators 1996
» Bootstrapping from a portable interpretive compiler A P-code assembler
» Simple machine architecture compilers and compiler generators 1996
» Addressing modes compilers and compiler generators 1996
» Machine architecture Case study 1 - A single-accumulator machine
» Instruction set Case study 1 - A single-accumulator machine
» A specimen program Case study 1 - A single-accumulator machine
» An emulator for the single-accumulator machine
» A minimal assembler for the machine
» Machine architecture Case study 2 - a stack-oriented computer
» Instruction set Case study 2 - a stack-oriented computer
» Specimen programs Case study 2 - a stack-oriented computer
» An emulator for the stack machine
» Syntax, semantics, and pragmatics
» Languages, symbols, alphabets and strings Regular expressions
» Grammars and productions compilers and compiler generators 1996
» Classic BNF notation for productions
» Simple examples compilers and compiler generators 1996
» Phrase structure and lexical structure -productions
» Semantic overtones The British Standard for EBNF Lexical and phrase structure emphasis
» One- and two-pass assemblers, and symbol tables
» Source handling Towards the construction of an assembler
» Lexical analysis Towards the construction of an assembler
» Syntax analysis Towards the construction of an assembler
» Symbol table structures The first pass - static semantic analysis
» The second pass - code generation
» Symbol table structures The first pass - analysis and code generation
» Error detection compilers and compiler generators 1996
» Simple expressions as addresses
» Improved symbol table handling - hash tables
» Macro processing facilities compilers and compiler generators 1996
» Conditional assembly compilers and compiler generators 1996
» Relocatable code compilers and compiler generators 1996
» Further projects compilers and compiler generators 1996
» Equivalent grammars Case study - equivalent grammars for describing expressions
» Useless productions and reduced grammars -free grammars
» Ambiguous grammars compilers and compiler generators 1996
» Type 1 Grammars Context-sensitive The Chomsky hierarchy
» The relationship between grammar type and language type
» EBNF description of Clang A sample program
» Context sensitivity Deterministic top-down parsing
» Terminal successors, the FOLLOW function, and LL1 conditions for non -free grammars
» Further observations Restrictions on grammars so as to allow LL1 parsing
» Alternative formulations of the LL1 conditions
» The effect of the LL1 conditions on language design
» Construction of simple recursive descent parsers
» Case studies compilers and compiler generators 1996
» Syntax error detection and recovery
» Construction of simple scanners
» LR parsing compilers and compiler generators 1996
» Automated construction of scanners and parsers
» Embedding semantic actions into syntax rules
» Attribute grammars compilers and compiler generators 1996
» Synthesized and inherited attributes
» Classes of attribute grammars
» Case study - a small student database
» Installation Input file preparation
» Execution Output from CocoR Assembling the generated system
» Case study - a simple adding machine
» Character sets Comments and ignorable characters
» Pragmas User names The scanner interface
» Syntax error recovery Parser specification
» Grammar checks Semantic errors
» Interfacing to support modules The parser interface
» Essentials of the driver program Customizing the driver frame file
» Case study - Understanding C declarations
» Case study - Generating one-address code from expressions
» Case study - Generating one-address code from an AST
» Case study - How do parser generators work?
» Project suggestions compilers and compiler generators 1996
» Overall compiler structure compilers and compiler generators 1996
» A hand-crafted source handler
» A hand-crafted scanner Lexical analysis
» A CocoR generated scanner Efficient keyword recognition
» A hand-crafted parser A CocoR generated parser
» Syntax error handling in hand-crafted parsers
» Error reporting The symbol table handler
» Other aspects of symbol table management - further types
» The code generation interface
» Code generation for a simple stack machine
» Machine tyranny Other aspects of code generation
» Abstract syntax trees as intermediate representations
» Simple optimizations - constant folding
» Simple optimizations - removal of redundant code
» Generation of assembler code
» Source handling, lexical analysis and error reporting Syntax
» The scope and extent of identifiers
» Symbol table support for the scope rules
» Parsing declarations and procedure calls
» Hypothetical machine support for the static link model
» Code generation for the static link model
» Hypothetical machine support for the display model
» Code generation for the display model
» Relative merits of the static link and display models
» Syntax and semantics compilers and compiler generators 1996
» Symbol table support for context-sensitive features
» Actual parameters and stack frames
» Hypothetical stack machine support for parameter passing
» Semantic analysis and code generation in a hand-crafted compiler
» Semantic analysis and code generation in a CocoR generated compiler
» Scope rules Language design issues
» Function return mechanisms Language design issues
» Context sensitivity and LL1 conflict resolution Fundamental concepts
» Parallel processes, exclusion and synchronization
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