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shecc : self-hosting and educational C optimizing compiler

logo image

Introduction

shecc is built from scratch, targeting both 32-bit Arm and RISC-V architectures, as a self-compiling compiler for a subset of the C language. Despite its simplistic nature, it is capable of performing basic optimization strategies as a standalone optimizing compiler.

Features

  • Generate executable Linux ELF binaries for ARMv7-A and RV32IM.
  • Provide a minimal C standard library for basic I/O on GNU/Linux.
  • The cross-compiler is written in ANSI C, making it compatible with most platforms.
  • Include a self-contained C front-end with an integrated machine code generator; no external assembler or linker needed.
  • Utilize a two-pass compilation process: the first pass checks syntax and breaks down complex statements into basic operations, while the second pass translates these operations into Arm/RISC-V machine code.
  • Develop a register allocation system that is compatible with RISC-style architectures.
  • Implement an architecture-independent, static single assignment (SSA)-based middle-end for enhanced optimizations.

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]
  • limited support for preprocessor directives: #define, #ifdef, #elif, #endif, #undef, and #error
  • non-nested variadic macros with __VA_ARGS__ identifier

The backend targets armv7hf with Linux ABI, verified on Raspberry Pi 3, and also supports RISC-V 32-bit architecture, verified with QEMU.

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/RV32IM binary.
  3. stage2: The generated ARMv7-A/RV32IM binary is invoked (via QEMU or running on Arm and RISC-V devices) with its own source code as input and generates another ARMv7-A/RV32IM 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/RISC-V ISA emulation is required. Install QEMU for Arm/RISC-V 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.

To execute the snapshot test, install the packages below:

$ sudo apt-get install graphviz jq

Build and Verify

Configure which backend you want, shecc supports ARMv7-A and RV32IM backend:

$ make config ARCH=arm
# Target machine code switch to Arm

$ make config ARCH=riscv
# Target machine code switch to RISC-V

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] [+m] [--no-libc] [--dump-ir] <infile.c>

Compiler options:

  • -o : Specify output file name (default: out.elf)
  • +m : Use hardware multiplication/division instructions (default: disabled)
  • --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

IR Regression Tests

To ensure the consistency of frontend (lexer, parser) behavior when working on it, the snapshot test is introduced. The snapshot test dumps IRs from the executable and compares the structural identity with the provided snapshots.

Verify the emitted IRs by specifying check-snapshots target when invoking make:

$ make check-snapshots

If the compiler frontend is updated, the emitted IRs might be changed. Thus, you can update snapshots by specifying update-snapshots target when invoking make:

$ make update-snapshots

Notice that the above 2 targets will update all backend snapshots at once, to update/check current backend's snapshot, use update-snapshot / check-snapshot instead.

Unit Tests

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

$ make check

Reference output:

  TEST STAGE 0
...
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
  TEST STAGE 2
...
int main(int argc, int argv) { exit(sizeof(char*)); }
exit code => 4
output => 
int main(int argc, int argv) { exit(sizeof(int*)); }
exit code => 4
output => 
OK

To clean up the generated compiler files, execute the command make clean. For resetting architecture configurations, use the command make distclean.

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)      def int @fib(int %n)                    Indicate a function definition
{                   {
  if (n == 0)         const %.t1001, $0                     Load constant 0 into a temporary variable ".t1001"
                      %.t1002 = eq %n, %.t1001              Test "n" equals ".t1001" or not, and write the result in temporary variable ".t1002"
                      br %.t1002, .label.1177, .label.1178  If ".t1002" equals zero, goto false label ".label.1178", otherwise,
                                                            goto true label ".label.1177"
                    .label.1177
    return 0;         const %.t1003, $0                     Load constant 0 into a temporary variable ".t1003"
                      ret %.t1003                           Return ".t1003"
                      j .label.1184                         Jump to endif label ".label.1184"
                    .label.1178
  else if (n == 1)    const %.t1004, $1                     Load constant 1 into a temporary variable ".t1004"
                      %.t1005 = eq %n, %.t1004              Test "n" equals ".t1004" or not, and write the result in temporary variable ".t1005"
                      br %.t1005, .label.1183, .label.1184  If ".t1005" equals zero, goto false label ".label.1184". Otherwise,
                                                            goto true label ".label.1183"
                    .label.1183
    return 1;         const %.t1006, $1                     Load constant 1 into a temporary variable ".t1006"
                      ret %.t1006                           Return ".t1006"
                    .label.1184
  return
    fib(n - 1)        const %.t1007, $1                     Load constant 1 into a temporary variable ".t1007"
                      %.t1008 = sub %n, %.t1007             Subtract ".t1007" from "n", and store the result in temporary variable ".t1008"
                      push %.t1008                          Prepare parameter for function call
                      call @fib, 1                          Call function "fib" with one parameter
    +                 retval %.t1009                        Store return value in temporary variable ".t1009"
    fib(n - 2);       const %.t1010, $2                     Load constant 2 into a temporary variable ".t1010"
                      %.t1011 = sub %n, %.t1010             Subtract ".t1010" from "n", and store the result in temporary variable ".t1011"
                      push %.t1011                          Prepare parameter for function call
                      call @fib, 1                          Call function "fib" with one parameter
                      retval %.t1012                        Store return value in temporary variable ".t1012"
                      %.t1013 = add %.t1009, %.t1012        Add ".t1009" and ".t1012", and store the result in temporary variable ".t1013"
                      ret %.t1013                           Return ".t1013"
}                   }

Known Issues

  1. The generated ELF lacks of .bss and .rodata section
  2. 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.
  3. The C front-end is a bit dirty because there is no effective AST.

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.