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108 lines
4.7 KiB
Markdown
108 lines
4.7 KiB
Markdown
# Interface Code Generator Design
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The Interface Code Generator (ICG) creates class/struct layout information, called
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attributes, used to convert string variable names to simulation memory
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addresses. The attributes are used with several of Trick's services
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including checkpointing, data recording, and the variable server.
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ICG execution is divided into 2 parts.
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1. Executing the Clang C++ frontend to parse all header files and extract
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class/struct information.
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1. Print class/struct information as attributes in input/output source code.
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## Clang C++ Frontend
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ICG uses the Clang frontend. Clang is a C/C++ frontend to the LLVM compiler.
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Clang and LLVM provide libraries that allow developers to create their own tools
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using the compiler. ICG calls the clang parser to process the header
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files and converts the source text into a tree form called an abstract syntax tree
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(AST). Clang completely handles all textual parsing. ICG provides classes to
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provide input parameters to the compiler and to extract the information out of the
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AST.
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### Parser Setup
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The frontend has some parameters that must be set in order for the parser to
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execute.
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#### Parsing Language
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The S_source.hh file where parsing begins is written in C++. The frontend is set to
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parse C++.
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#### Target Architecture
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The frontend requires a target architecture set. ICG does not pass the AST to the
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next stage of compiler tools after the frontend, so it doesn't matter what the
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target architecture is set to. Setting the target to the the default architecture
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works.
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### Preprocessor
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The frontend includes a preprocessor. The preprocessor handles include and define
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statements. ICG provides a pair of classes that interact with the clang preprocesor.
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The HeaderSearchDirs class to provide the preprocessor with include search
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directories and default defines. The CommentSaver class saves all comments parsed
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by the preprocessor. These classes are used throughout the rest of ICG execution.
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#### HeaderSearchDirs
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The HeaderSearchDirs manages the header search directories used by the preprocessor
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and provides an interface for querying the inclusion status of a directory. Since
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this class is closely related to the preprocessor, it also handles define macros.
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The HeaderSearchDirs class provides a list of include search directories to the
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clang preprocessor. There are two types of include directories, system include
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directories and user provided directories. Both types must be supplied to the
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preprocessor.
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The system include directories are retrieved by running the TRICK_CPPC compiler
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in verbose preprocessor mode with no input code. When the compiler is run
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in this mode, it prints the system include directories built into the compiler.
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This class parses that output for the system include directories and feeds the
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list into the preprocessor.
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The user include directories are retrieved from the command line when executing
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ICG. Typically ICG is called with the TRICK_CXXFLAGS environment variable
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included as an argument. TRICK_CXXFLAGS may include -I<include_dir> strings.
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These include directories are parsed from the command line and fed to the
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preprocessor.
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This class parses the TRICK_ICG_EXCLUDE environment variable and manages a list
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of directories ICG should not write any output source code.
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The HeaderSearchDirs class provides boolean responses to questions to whether a
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path is in a user directory or is in an TRICK_ICG_EXCLUDE directory.
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#### CommentSaver
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The CommentSaver class saves comments found in user code and provides an iterface
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for retrieving comments based on file name/line number. The class also provides
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an interface to retrieving several fields from a Trick header comment including
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the ICG and ICG_EXCLUDE_TYPES fields.
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The clang preprocessor includes a comment handler interface. ICG uses
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this interface to attach the CommentSaver class. Each comment handled by this
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class is stored in a map indexed by file name and line number.
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### Abstract Syntax Tree (AST) Traversal
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Traversing the resulting AST is bulk of ICG's work. The clang compiler provides
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an interface allowing ICG to attach handlers to process interesting events during
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code parsing. In this case, ICG only attaches a handler when clang is finished
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parsing the entire input file, called HandleTranslationUnit.
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When HandleTranslationUnit is start parsing the AST searching for top level
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declarations we want to process. From the top level recursively calls
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tree parsers that specialize in extracting information about typedefs,
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classes and structs, fields within each class/struct, and enumerations.
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The following diagram shows the recursive class instantiation sequence as well
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as the data returned to the caller.
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## Writing Class/Struct Attributes
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