The second pass - code generation
6.4.3 The second pass - code generation
The second pass rescans the program by extracting partially assembled lines from the intermediate file, and then passing each of these to the code generator. The code generator has to keep track of further errors that might arise if any labels were not properly defined on the first pass. Because of the work already done in the first pass, handling the directives is now almost trivial in this pass. Once again, complete code for a simple implementation is to be found on the source diskette, and it should be necessary only to draw attention to the following points: For our simple machine, all the generated objected code can be contained in an array of length 256. A more realistic assembler might not be able to contain the entire object code in memory, because of lack of space. For a two-pass assembler few problems would arise, as the code could be written out to a file as soon as it was generated. Exactly how the object code is finally to be treated is a matter for debate. Here we have called on the listcode routine from the class defining the pseudo-machine, which dumps the 256 bytes in a form that is suitable for input to a simple loader. However, the driver program suggested earlier also allows this code to be interpreted immediately after assembly has been successful. An assembler program typically gives the user a listing of the source code, usually with assembled code alongside it. Occasionally extra frills are provided, like cross reference tables for identifiers and so on. Our one is quite simple, and an example of a source listing produced by this assembler was given earlier. Exercises 6.9 Make an extension to the ASSEMBLER language, to its grammar, and to the assembler program, to allow a character string as an operand in the DC directive. For example TYRANT DC TERRY should be treated as equivalent to TYRANT DC ’T’ DC ’E’ DC ’R’ DC ’R’ DC ’Y’ Is it desirable or necessary to delimit strings with different quotes from those used by single characters? 6.10 Change the table handler so that the symbol table is stored in a binary search tree, for efficiency. 6.11 The assembler will accept a line consisting only of a non-empty LABEL field. Is there any advantage in being able to do this? 6.12 What would happen if a label were to be defined more than once? 6.13 What would happen if a label were left undefined by the end of the first pass? 6.14 How would the symbol table handling alter if the source code were all held in memory throughout assembly see Exercise 6.1, or if a string table were used see Exercise 6.6?6.5 One-pass assembly
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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