Babyc

An educational foray into compiler writing. Written in C, compiling C to x86 assembly (handy x86 reference site, assembly directives reference, System V ABI reference).

Technically targetting C11 (standard PDF), but we will implement such a small subset of C that it's academic.

Current feature set:

• positive integers (no other types yet)
• integer constants
• logical negation (!FOO)
• bitwise negation (~FOO)
• addition (foo + bar)
• subtraction (foo - bar binary only)
• multiplication (foo * bar)
• less than comparison (foo < bar, foo <= bar)
• comments (// foo and /* foo */)
• sequences of statements (foo; bar)
• return statements
• if statements (if (foo) { bar }, no else yet)
• local variables (int only, function scope only, must be initialised)
• variable assignment (int only)
• while loops (while (foo) { bar })
• function calls (only int foo() i.e. no arguments, returning int)
• preprocessor usage (we shell out to gcc)

License

GPL v2 license.

Usage

Compiling babyc:

# Compile the compiler.
$ make

Usage:

# Run it, producing an assembly file.
$ build/babyc test_programs/immediate__return_1.c
# Use the GNU toolchain to assemble and link.
$ ./link

Viewing the code after preprocessing:

$ build/babyc --dump-expansion test_programs/if_false__return_2.c

Viewing the AST:

$ build/babyc --dump-ast test_programs/if_false__return_2.c

Running tests:

$ make test

Debugging

If you're debugging a compiled program that segfaults, you may want to simply read the out.s file.

To use gdb (given we have no signal table, function prologues or other conveniences), do the following:

$ gdb out
(gdb) run
... it segfaults
(gdb) layout asm
... shows which line the segfault occurred on
(gdb) info registers
... shows the current state of the registers (`layout reg' also
... provides this data)

If you want to debug a program that doesn't segfault, you can set a breakpoint to the entrypoint:

$ gdb out
(gdb) info files
    ...
    Entry point: 0x80000000
    ...
(gdb) break *0x80000000
(gdb) run

Improving code quality

The make command will generate warnings, fix them. You can also run with clang-analyzer to catch further issues:

$ scan-build make

For code formatting, run:

$ make format