cc

A simple C compiler

The C language grammar (c.y and c.l files) have been taken from here

Using a C11 working draft, available here

Running

This project uses the LLVM C++ API, so it needs to be present on the system for compilation. Version 11.0.0 has been used for development, but it should work on lower versions too. Other libraries which need to be present are Flex and Bison (used for lexing and parsing C files); and popt (used for parsing command line args, should be present on most systems already)

We use CMake for building this project. In the repository's root directory, run the following commands to compile:

cmake .
make

The cc binary is generated in the same directory. To compile a file prog.c to LLVM IR file prog.ll, run the following:

./cc prog.c -o=prog

There are more flags available, run ./cc --help for more information.

Running tests

There are some test .c files in the tests/ directory which can be used to test the compiler. The method used for testing is that these files are compiled through both cc and a compiler on the system, and their output is compared. These tests are run through CTest which comes with CMake. To build and run all tests, run:

make run_tests

NOTE: It is recommended to supply make with -j8 (or any other appropriate number) so it can utilise multiple threads which makes compilation and running tests much faster

Select supported constructs

• All integer types such as int, long, long long, etc. along with signed and unsigned specifiers
• Implicit type casting between integer types
• double support, with no implicit casting
• if and switch, while and do-while
• Use of break and continue wherever applicable
• goto and return statements
• Global and local variable declaration (with initializer support)

Supported optimizations

For now, the compiler supports some local optimizations which are listed:

• Constant folding
• Dead code removal: Some optimizations supported under this would include not generating the while body if the condition is constant and evaluates to zero. Similarly for the if statement.

Limitations

One major limitation of dead code removal is the presence of labeled statements, since a goto can the jump into the body of a while loop for which we didn't generate code which causes problems. Currently, if the while loop body has any labeled statement, the entire body is always generated. This can be improved in with better bookkeeping by seeing which labels are jumped into, etc. Note that a similar issue also occurs with having a constant condition in the if statement.

Code formatting

All code has been formatted through clang-format using the google style. Settings for it are present in the .vscode folder included, which formats the file on save when using Visual Studio Code.

All supported constructs

Different C constructs' implementation has been put in different files, listed below:

• decl_common.hpp
  • Storage-class specifiers
  • Type specifiers (currently not supporting atomic, struct, union, enum, typedef)
  • Declaration specifiers
  • Pointer
• declarator.hpp
  • Direct Declarator
    • Function Declarator
  • Declarator
  • Parameter list (Using same class for Parameter Type List)
  • Parameter Declaration
• expression.hpp
  • Expression
  • Assignment Expression
  • Binary Expression (Multiplicative Expression through Logical Or Expression)
  • Conditional Expression
  • Unary Expression
    • Unary operators (does not support * and & operators)
    • Increment and Decrement
  • Postfix Expression
    • Increment and Decrement
    • Function Call
  • Primary Expression
    • Identifier
    • Constant (only Integer Constant)
    • Parenthesized Expression
    • String
• func_def.hpp
  • External Declaration
  • Function Definition
  • Translation Unit
• block_item.hpp
  • Block Item
• statement.hpp
  • Statement
  • Labeled Statement
    • goto corresponding labeled statements
    • case and default
  • Compound Statement
  • Expression Statement
  • Selection Statements
    • if
    • switch
  • Iteration Statements
    • while
    • do while
  • Jump Statements
    • goto
    • continue
    • break
    • return

Other files' use has been given below:

• printer.hpp

  Extends std::cout to allow indented printing. Keeps track of the indentation level and prints each new line according to that. A caveat is that it only considers a new line when std::endl is provided to it.

• ast.hpp

  Contains the ast_node class which is a base class for all other classes which are a part of an AST. Statically holds a printer object and a pointer to the base node of the AST generated.

• symbol_table.hpp

  Contains implementation of the symbol table. Takes care of function scopes as well as block scopes