shecc : self-hosting and educational C compiler

Introduction

shecc is built from scratch, targeted at 32-bit Arm architecture, as a self-compiling compiler for a subset of the C language.

Features

• Generate executable Linux ELF binaries for ARMv7-A;
• Provide a minimal C standard library for basic I/O on GNU/Linux;
• The cross-compiler is written in ANSI C, arguably running on most platforms;
• Self-contained C language front-end and machine code generator;
• Two-pass compilation: on the first pass it checks the syntax of statements and constructs a table of symbols, while on the second pass it actually translates program statements into Arm machine code.

Compatibility

shecc is capable of compiling C source files written in the following syntax:

• data types: char, int, struct, and pointer
• condition statements: if, while, for, switch, case, break, return, and general expressions
• compound assignments: +=, -=, *=
• global/local variable initializations for supported data types
  • e.g. int i = [expr]

The backend targets armv7hf with Linux ABI, verified on Raspberry Pi 3.

Bootstrapping

The steps to validate shecc bootstrapping:

1. stage0: shecc source code is initially compiled using an ordinary compiler which generates a native executable. The generated compiler can be used as a cross-compiler.
2. stage1: The built binary reads its own source code as input and generates an ARMv7-A binary.
3. stage2: The generated ARMv7-A binary is invoked (via QEMU or running on Arm devices) with its own source code as input and generates another ARMv7-A binary.
4. bootstrap: Build the stage1 and stage2 compilers, and verify that they are byte-wise identical. If so, shecc can compile its own source code and produce new versions of that same program.

Prerequisites

Code generator in shecc does not rely on external utilities. You only need ordinary C compilers such as gcc and clang. However, shecc would bootstrap itself, and Arm ISA emulation is required. Install QEMU for Arm user emulation on GNU/Linux:

$ sudo apt-get install qemu-user

It is still possible to build shecc on macOS or Microsoft Windows. However, the second stage bootstrapping would fail due to qemu-arm absence.

Build and Verify

Run make and you should see this:

  CC+LD	out/inliner
  GEN	out/libc.inc
  CC	out/src/main.o
  LD	out/shecc
  SHECC	out/shecc-stage1.elf
  SHECC	out/shecc-stage2.elf

File out/shecc is the first stage compiler. Its usage:

shecc [-o output] [-no-libc] [--dump-ir] <infile.c>

Compiler options:

• -o : output file name (default: out.elf)
• --no-libc : Exclude embedded C library (default: embedded)
• --dump-ir : Dump intermediate representation (IR)

Example:

$ out/shecc -o fib tests/fib.c
$ chmod +x fib
$ qemu-arm fib

shecc comes with unit tests. To run the tests, give "check" as an argument:

$ make check

Reference output:

...
int main(int argc, int argv) { exit(sizeof(char)); } => 1
int main(int argc, int argv) { int a; a = 0; switch (3) { case 0: return 2; case 3: a = 10; break; case 1: return 0; } exit(a); } => 10
int main(int argc, int argv) { int a; a = 0; switch (3) { case 0: return 2; default: a = 10; break; } exit(a); } => 10
OK

Intermediate Representation

Once the option --dump-ir is passed to shecc, the intermediate representation (IR) will be generated. Take the file tests/fib.c for example. It consists of a recursive Fibonacci sequence function.

int fib(int n)
{
    if (n == 0)
        return 0;
    else if (n == 1)
        return 1;
    return fib(n - 1) + fib(n - 2);
}

Execute the following to generate IR:

$ out/shecc --dump-ir -o fib tests/fib.c

Line-by-line explanation between C source and IR:

 C Source            IR                         Explanation
-------------------+--------------------------+----------------------------------------------------

int fib(int n)      fib:                        Reserve stack frame for function fib
{                     {
    if (n == 0)         x0 = &n                 Get address of variable n
                        x0 = *x0 (4)            Read value from address into x0, length = 4 (int)
                        x1 := 0                 Set x1 to zero
                        x0 == x1 ?              Compare x0 with x1
                        if false then goto 1641 If x0 != x1, then jump to label 1641
        return 0;       x0 := 0                 Set x0 to zero. x0 is the return value.
                        return (from fib)       Jump to function exit
                    1641:
    else if (n == 1)    x0 = &n                 Get address of variable n
                        x0 = *x0 (4)            Read value from address into x0, length = 4 (int)
                        x1 := 1                 Set x1 to 1
                        x0 == x1 ?              Compare x0 with x1
                        if true then goto 1649  If x0 != x1, then jump to label 1649
        return 1;       x0 := 1                 Set x0 to 1. x0 is the return value.
                        return (from fib)       Jump to function exit
                    1649:
    return              x0 = &n                 Get address of variable n
       fib(n - 1)       x0 = *x0 (4)            Read value from address into x0, length = 4 (int)
                        x1 := 1                 Set x1 to 1
                        x0 -= x1                Subtract x1 from x0 i.e. (n - 1)
       +                x0 := fib() @ 1631      Call function fib() into x0
                        push x0                 Store the result on stack
       fib(n - 2);      x0 = &n                 Get address of variable n
                        x0 = *x0 (4)            Read value from address into x0, length = 4 (int)
                        x1 := 2                 Set x1 to 2
                        x0 -= x1                Subtract x1 from x0 i.e. (n - 2)
                        x1 := fib() @ 1631      Call function fib() into x1
                        pop x0                  Retrieve the result off stack into x0
                        x0 += x1                Add x1 to x0 i.e. the result of fib(n-1) + fib(n-2)
                        return (from fib)       Jump to function exit
                      }                         Restore the previous stack frame
                      exit fib

Known Issues

1. Any non-zero value is NOT treated as logical truth by all ops. That is, the expression 0 == strcmp(ptr, "hello") is not equivalent to !strcmp(ptr, "hello").
2. The generated ELF lacks of .bss and .rodata section
3. Dereference is incomplete. Consider int x = 5; int *ptr = &x; and it is forbidden to use *ptr. However, it is valid to use ptr[0], which behaves the same of *ptr.
4. The support of varying number of function arguments is incomplete. No <stdarg.h> can be used. Alternatively, check the implementation printf in source lib/c.c for var_arg.
5. If you attempt to return values in main function, the value will not be reserved after the program exited. You have to modify the return statement to exit call. Check the test items in file tests/driver.sh for details.
6. The C front-end is a bit dirty because there is no effective AST.
7. No function pointer is supported.

License

shecc is freely redistributable under the BSD 2 clause license. Use of this source code is governed by a BSD-style license that can be found in the LICENSE file.