shecc is built from scratch, targeted at 32-bit Arm architecture, as
a self-compiling compiler for a subset of the C language.
- 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, bootstrapping 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.
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:
+=,-=,*= - local variable initializations for supported data types
- e.g.
int i = [expr]
- e.g.
The backend targets armv7hf with Linux ABI, verified on Raspberry Pi 3.
The steps to validate shecc bootstrapping:
sheccsource code is initially compiled using an ordinary compilerg which generates an x86/x86_64 binary.- The built binary reads its own source code as input and generates an ARMv7-A binary.
- 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.
- If outputs generated in steps 2. and 3. are identical, the bootstrapping
would be successful. That is,
shecccan compile its own source code and produces new versions of that same program.
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-userIt is still possible to build shecc on macOS or Microsoft Windows. However,
the second stage bootstrapping would fail due to qemu-arm absence.
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 fibshecc comes with unit tests. To run the tests, give "check" as an argument:
$ make checkReference 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
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.cLine-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- 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"). - Global variable initialization is not supported.
Therefore, you can not initialize a global such as
int i = [expr]. - Dereference is incomplete. Consider
int x = 5; int *ptr = &x;and it is forbidden to use*ptr. However, it is valid to useptr[0], which behaves the same of*ptr. - The support of varying number of function arguments is incomplete. No
<stdarg.h>can be used. Alternatively, check the implementationprintfin sourcelib/c.cforvar_arg. - The memory region allocated by
malloccan not be released. In fact, there is nofreefunction. Memory allocator should be introduced in embedded libc implementation. - If you attempt to return values in
mainfunction, the value will not be reserved after the program exited. You have to modify thereturnstatement toexitcall. Check the test items in filetests/driver.shfor details. - The C front-end is a bit dirty because there is no effective AST.
- No function pointer is supported.
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.