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Tutorial part 4: Adding JIT-compilation to a toy interpreter

In this example we construct a “toy” interpreter, and add JIT-compilation to it.

Our toy interpreter

It’s a stack-based interpreter, and is intended as a (very simple) example of the kind of bytecode interpreter seen in dynamic languages such as Python, Ruby etc.

For the sake of simplicity, our toy virtual machine is very limited:

  • The only data type is int
  • It can only work on one function at a time (so that the only function call that can be made is to recurse).
  • Functions can only take one parameter.
  • Functions have a stack of int values.
  • We’ll implement function call within the interpreter by calling a function in our implementation, rather than implementing our own frame stack.
  • The parser is only good enough to get the examples to work.

Naturally, a real interpreter would be much more complicated that this.

The following operations are supported:

Operation Meaning Old Stack New Stack
DUP Duplicate top of stack. [..., x] [..., x, x]
ROT Swap top two elements of stack. [..., x, y] [..., y, x]
BINARY_ADD Add the top two elements on the stack. [..., x, y] [..., (x+y)]
BINARY_SUBTRACT Likewise, but subtract. [..., x, y] [..., (x-y)]
BINARY_MULT Likewise, but multiply. [..., x, y] [..., (x*y)]
BINARY_COMPARE_LT Compare the top two elements on the stack and push a nonzero/zero if (x<y). [..., x, y] [..., (x<y)]
RECURSE Recurse, passing the top of the stack, and popping the result. [..., x] [..., fn(x)]
RETURN Return the top of the stack. [x] []
PUSH_CONST arg Push an int const. [...] [..., arg]
JUMP_ABS_IF_TRUE arg Pop; if top of stack was nonzero, jump to arg. [..., x] [...]

Programs can be interpreted, disassembled, and compiled to machine code.

The interpreter reads .toy scripts. Here’s what a simple recursive factorial program looks like, the script factorial.toy. The parser ignores lines beginning with a #.

# Simple recursive factorial implementation, roughly equivalent to:
#
#  int factorial (int arg)
#  {
#     if (arg < 2)
#       return arg
#     return arg * factorial (arg - 1)
#  }

# Initial state:
# stack: [arg]

# 0:
DUP
# stack: [arg, arg]

# 1:
PUSH_CONST 2
# stack: [arg, arg, 2]

# 2:
BINARY_COMPARE_LT
# stack: [arg, (arg < 2)]

# 3:
JUMP_ABS_IF_TRUE 9
# stack: [arg]

# 4:
DUP
# stack: [arg, arg]

# 5:
PUSH_CONST 1
# stack: [arg, arg, 1]

# 6:
BINARY_SUBTRACT
# stack: [arg,  (arg - 1)

# 7:
RECURSE
# stack: [arg, factorial(arg - 1)]

# 8:
BINARY_MULT
# stack: [arg * factorial(arg - 1)]

# 9:
RETURN

The interpreter is a simple infinite loop with a big switch statement based on what the next opcode is:

int
toyvm_function::interpret (int arg, FILE *trace)
{
  toyvm_frame frame;
#define PUSH(ARG) (frame.push (ARG))
#define POP(ARG) (frame.pop ())

  frame.frm_function = this;
  frame.frm_pc = 0;
  frame.frm_cur_depth = 0;

  PUSH (arg);

  while (1)
    {
      toyvm_op *op;
      int x, y;
      assert (frame.frm_pc < fn_num_ops);
      op = &fn_ops[frame.frm_pc++];

      if (trace)
	{
	  frame.dump_stack (trace);
	  disassemble_op (op, frame.frm_pc, trace);
	}

      switch (op->op_opcode)
	{
	  /* Ops taking no operand.  */
	case DUP:
	  x = POP ();
	  PUSH (x);
	  PUSH (x);
	  break;

	case ROT:
	  y = POP ();
	  x = POP ();
	  PUSH (y);
	  PUSH (x);
	  break;

	case BINARY_ADD:
	  y = POP ();
	  x = POP ();
	  PUSH (x + y);
	  break;

	case BINARY_SUBTRACT:
	  y = POP ();
	  x = POP ();
	  PUSH (x - y);
	  break;

	case BINARY_MULT:
	  y = POP ();
	  x = POP ();
	  PUSH (x * y);
	  break;

	case BINARY_COMPARE_LT:
	  y = POP ();
	  x = POP ();
	  PUSH (x < y);
	  break;

	case RECURSE:
	  x = POP ();
	  x = interpret (x, trace);
	  PUSH (x);
	  break;

	case RETURN:
	  return POP ();

	  /* Ops taking an operand.  */
	case PUSH_CONST:
	  PUSH (op->op_operand);
	  break;

	case JUMP_ABS_IF_TRUE:
	  x = POP ();
	  if (x)
	    frame.frm_pc = op->op_operand;
	  break;

	default:
	  assert (0); /* unknown opcode */

	} /* end of switch on opcode */
    } /* end of while loop */

#undef PUSH
#undef POP
}

Compiling to machine code

We want to generate machine code that can be cast to this type and then directly executed in-process:

typedef int (*toyvm_compiled_func) (int);

Our compiler isn’t very sophisticated; it takes the implementation of each opcode above, and maps it directly to the operations supported by the libgccjit API.

How should we handle the stack? In theory we could calculate what the stack depth will be at each opcode, and optimize away the stack manipulation “by hand”. We’ll see below that libgccjit is able to do this for us, so we’ll implement stack manipulation in a direct way, by creating a stack array and stack_depth variables, local within the generated function, equivalent to this C code:

int stack_depth;
int stack[MAX_STACK_DEPTH];

We’ll also have local variables x and y for use when implementing the opcodes, equivalent to this:

int x;
int y;

This means our compiler has the following state:

  toyvm_function &toyvmfn;

  gccjit::context ctxt;

  gccjit::type int_type;
  gccjit::type bool_type;
  gccjit::type stack_type; /* int[MAX_STACK_DEPTH] */

  gccjit::rvalue const_one;

  gccjit::function fn;
  gccjit::param param_arg;
  gccjit::lvalue stack;
  gccjit::lvalue stack_depth;
  gccjit::lvalue x;
  gccjit::lvalue y;

  gccjit::location op_locs[MAX_OPS];
  gccjit::block initial_block;
  gccjit::block op_blocks[MAX_OPS];

Setting things up

First we create our types:

void
compilation_state::create_types ()
{
  /* Create types.  */
  int_type = ctxt.get_type (GCC_JIT_TYPE_INT);
  bool_type = ctxt.get_type (GCC_JIT_TYPE_BOOL);
  stack_type = ctxt.new_array_type (int_type, MAX_STACK_DEPTH);

along with extracting a useful int constant:

  const_one = ctxt.one (int_type);

}

We’ll implement push and pop in terms of the stack array and stack_depth. Here are helper functions for adding statements to a block, implementing pushing and popping values:

void
compilation_state::add_push (gccjit::block block,
                             gccjit::rvalue rvalue,
                             gccjit::location loc)
{
  /* stack[stack_depth] = RVALUE */
  block.add_assignment (
    /* stack[stack_depth] */
    ctxt.new_array_access (
      stack,
      stack_depth,
      loc),
    rvalue,
    loc);

  /* "stack_depth++;".  */
  block.add_assignment_op (
    stack_depth,
    GCC_JIT_BINARY_OP_PLUS,
    const_one,
    loc);
}

void
compilation_state::add_pop (gccjit::block block,
                            gccjit::lvalue lvalue,
                            gccjit::location loc)
{
  /* "--stack_depth;".  */
  block.add_assignment_op (
    stack_depth,
    GCC_JIT_BINARY_OP_MINUS,
    const_one,
    loc);

  /* "LVALUE = stack[stack_depth];".  */
  block.add_assignment (
    lvalue,
    /* stack[stack_depth] */
    ctxt.new_array_access (stack,
                           stack_depth,
                           loc),
    loc);
}

We will support single-stepping through the generated code in the debugger, so we need to create gccjit::location instances, one per operation in the source code. These will reference the lines of e.g. factorial.toy.

void
compilation_state::create_locations ()
{
  for (int pc = 0; pc < toyvmfn.fn_num_ops; pc++)
    {
      toyvm_op *op = &toyvmfn.fn_ops[pc];

      op_locs[pc] = ctxt.new_location (toyvmfn.fn_filename,
                                       op->op_linenum,
                                       0); /* column */
    }
}

Let’s create the function itself. As usual, we create its parameter first, then use the parameter to create the function:

void
compilation_state::create_function (const char *funcname)
{
  std::vector <gccjit::param> params;
  param_arg = ctxt.new_param (int_type, "arg", op_locs[0]);
  params.push_back (param_arg);
  fn = ctxt.new_function (GCC_JIT_FUNCTION_EXPORTED,
                          int_type,
                          funcname,
                          params, 0,
                          op_locs[0]);

We create the locals within the function.

  stack = fn.new_local (stack_type, "stack");
  stack_depth = fn.new_local (int_type, "stack_depth");
  x = fn.new_local (int_type, "x");
  y = fn.new_local (int_type, "y");

Populating the function

There’s some one-time initialization, and the API treats the first block you create as the entrypoint of the function, so we need to create that block first:

  initial_block = fn.new_block ("initial");

We can now create blocks for each of the operations. Most of these will be consolidated into larger blocks when the optimizer runs.

  for (int pc = 0; pc < toyvmfn.fn_num_ops; pc++)
    {
      char buf[16];
      sprintf (buf, "instr%i", pc);
      op_blocks[pc] = fn.new_block (buf);
    }

Now that we have a block it can jump to when it’s done, we can populate the initial block:

  /* "stack_depth = 0;".  */
  initial_block.add_assignment (stack_depth,
                                ctxt.zero (int_type),
                                op_locs[0]);

  /* "PUSH (arg);".  */
  add_push (initial_block,
	    param_arg,
            op_locs[0]);

  /* ...and jump to insn 0.  */
  initial_block.end_with_jump (op_blocks[0],
                               op_locs[0]);

We can now populate the blocks for the individual operations. We loop through them, adding instructions to their blocks:

  for (int pc = 0; pc < toyvmfn.fn_num_ops; pc++)
    {
      gccjit::location loc = op_locs[pc];

      gccjit::block block = op_blocks[pc];
      gccjit::block next_block = (pc < toyvmfn.fn_num_ops
                                  ? op_blocks[pc + 1]
                                  : NULL);

      toyvm_op *op;
      op = &toyvmfn.fn_ops[pc];

We’re going to have another big switch statement for implementing the opcodes, this time for compiling them, rather than interpreting them. It’s helpful to have macros for implementing push and pop, so that we can make the switch statement that’s coming up look as much as possible like the one above within the interpreter:

#define X_EQUALS_POP()\
      add_pop (block, x, loc)
#define Y_EQUALS_POP()\
      add_pop (block, y, loc)
#define PUSH_RVALUE(RVALUE)\
      add_push (block, (RVALUE), loc)
#define PUSH_X()\
      PUSH_RVALUE (x)
#define PUSH_Y() \
      PUSH_RVALUE (y)

Note

A particularly clever implementation would have an identical switch statement shared by the interpreter and the compiler, with some preprocessor “magic”. We’re not doing that here, for the sake of simplicity.

When I first implemented this compiler, I accidentally missed an edit when copying and pasting the Y_EQUALS_POP macro, so that popping the stack into y instead erroneously assigned it to x, leaving y uninitialized.

To track this kind of thing down, we can use gccjit::block::add_comment() to add descriptive comments to the internal representation. This is invaluable when looking through the generated IR for, say factorial:

      block.add_comment (opcode_names[op->op_opcode], loc);

We can now write the big switch statement that implements the individual opcodes, populating the relevant block with statements:

      switch (op->op_opcode)
	{
	case DUP:
	  X_EQUALS_POP ();
	  PUSH_X ();
	  PUSH_X ();
	  break;

	case ROT:
	  Y_EQUALS_POP ();
	  X_EQUALS_POP ();
	  PUSH_Y ();
	  PUSH_X ();
	  break;

	case BINARY_ADD:
	  Y_EQUALS_POP ();
	  X_EQUALS_POP ();
	  PUSH_RVALUE (
	   ctxt.new_binary_op (
	     GCC_JIT_BINARY_OP_PLUS,
	     int_type,
             x, y,
             loc));
	  break;

	case BINARY_SUBTRACT:
	  Y_EQUALS_POP ();
	  X_EQUALS_POP ();
	  PUSH_RVALUE (
           ctxt.new_binary_op (
	     GCC_JIT_BINARY_OP_MINUS,
	     int_type,
             x, y,
             loc));
	  break;

	case BINARY_MULT:
	  Y_EQUALS_POP ();
	  X_EQUALS_POP ();
	  PUSH_RVALUE (
           ctxt.new_binary_op (
	     GCC_JIT_BINARY_OP_MULT,
	     int_type,
             x, y,
             loc));
	  break;

	case BINARY_COMPARE_LT:
	  Y_EQUALS_POP ();
	  X_EQUALS_POP ();
	  PUSH_RVALUE (
	     /* cast of bool to int */
	     ctxt.new_cast (
	       /* (x < y) as a bool */
	       ctxt.new_comparison (
		 GCC_JIT_COMPARISON_LT,
                 x, y,
                 loc),
	       int_type,
               loc));
	  break;

	case RECURSE:
	  {
	    X_EQUALS_POP ();
	    PUSH_RVALUE (
	      ctxt.new_call (
		fn,
		x,
                loc));
	    break;
	  }

	case RETURN:
	  X_EQUALS_POP ();
	  block.end_with_return (x, loc);
	  break;

	  /* Ops taking an operand.  */
	case PUSH_CONST:
	  PUSH_RVALUE (
	    ctxt.new_rvalue (int_type, op->op_operand));
	  break;

	case JUMP_ABS_IF_TRUE:
	  X_EQUALS_POP ();
	  block.end_with_conditional (
	    /* "(bool)x".  */
            ctxt.new_cast (x, bool_type, loc),
	    op_blocks[op->op_operand], /* on_true */
	    next_block, /* on_false */
            loc); 
	  break;

	default:
	  assert(0);
	} /* end of switch on opcode */

Every block must be terminated, via a call to one of the gccjit::block::end_with_ entrypoints. This has been done for two of the opcodes, but we need to do it for the other ones, by jumping to the next block.

      if (op->op_opcode != JUMP_ABS_IF_TRUE
	  && op->op_opcode != RETURN)
	block.end_with_jump (next_block, loc);

This is analogous to simply incrementing the program counter.

Verifying the control flow graph

Having finished looping over the blocks, the context is complete.

As before, we can verify that the control flow and statements are sane by using gccjit::function::dump_to_dot():

fn.dump_to_dot ("/tmp/factorial.dot");

and viewing the result. Note how the label names, comments, and variable names show up in the dump, to make it easier to spot errors in our compiler.

image of a control flow graph

Compiling the context

Having finished looping over the blocks and populating them with statements, the context is complete.

We can now compile it, extract machine code from the result, and run it:

class compilation_result
{
public:
  compilation_result (gcc_jit_result *result) :
    m_result (result)
  {
  }
  ~compilation_result ()
  {
    gcc_jit_result_release (m_result);
  }

  void *get_code (const char *funcname)
  {
    return gcc_jit_result_get_code (m_result, funcname);
  }

private:
  gcc_jit_result *m_result;
};
  compilation_result compiler_result = fn->compile ();

  const char *funcname = fn->get_function_name ();
  toyvm_compiled_func code
    = (toyvm_compiled_func)compiler_result.get_code (funcname);

  printf ("compiler result: %d\n",
	  code (atoi (argv[2])));

Single-stepping through the generated code

It’s possible to debug the generated code. To do this we need to both:

Having done this, we can put a breakpoint on the generated function:

$ gdb --args ./toyvm factorial.toy 10
(gdb) break factorial
Function "factorial" not defined.
Make breakpoint pending on future shared library load? (y or [n]) y
Breakpoint 1 (factorial) pending.
(gdb) run
Breakpoint 1, factorial (arg=10) at factorial.toy:14
14    DUP

We’ve set up location information, which references factorial.toy. This allows us to use e.g. list to see where we are in the script:

(gdb) list
9
10    # Initial state:
11    # stack: [arg]
12
13    # 0:
14    DUP
15    # stack: [arg, arg]
16
17    # 1:
18    PUSH_CONST 2

and to step through the function, examining the data:

(gdb) n
18    PUSH_CONST 2
(gdb) n
22    BINARY_COMPARE_LT
(gdb) print stack
$5 = {10, 10, 2, 0, -7152, 32767, 0, 0}
(gdb) print stack_depth
$6 = 3

You’ll see that the parts of the stack array that haven’t been touched yet are uninitialized.

Note

Turning on optimizations may lead to unpredictable results when stepping through the generated code: the execution may appear to “jump around” the source code. This is analogous to turning up the optimization level in a regular compiler.

Examining the generated code

How good is the optimized code?

We can turn up optimizations, by calling gccjit::context::set_int_option() with GCC_JIT_INT_OPTION_OPTIMIZATION_LEVEL:

ctxt.set_int_option (GCC_JIT_INT_OPTION_OPTIMIZATION_LEVEL, 3);

One of GCC’s internal representations is called “gimple”. A dump of the initial gimple representation of the code can be seen by setting:

ctxt.set_bool_option (GCC_JIT_BOOL_OPTION_DUMP_INITIAL_GIMPLE, 1);

With optimization on and source locations displayed, this gives:

factorial (signed int arg)
{
  <unnamed type> D.80;
  signed int D.81;
  signed int D.82;
  signed int D.83;
  signed int D.84;
  signed int D.85;
  signed int y;
  signed int x;
  signed int stack_depth;
  signed int stack[8];

  try
    {
      initial:
      stack_depth = 0;
      stack[stack_depth] = arg;
      stack_depth = stack_depth + 1;
      goto instr0;
      instr0:
      /* DUP */:
      stack_depth = stack_depth + -1;
      x = stack[stack_depth];
      stack[stack_depth] = x;
      stack_depth = stack_depth + 1;
      stack[stack_depth] = x;
      stack_depth = stack_depth + 1;
      goto instr1;
      instr1:
      /* PUSH_CONST */:
      stack[stack_depth] = 2;
      stack_depth = stack_depth + 1;
      goto instr2;

      /* etc */

You can see the generated machine code in assembly form via:

ctxt.set_bool_option (GCC_JIT_BOOL_OPTION_DUMP_GENERATED_CODE, 1);
result = ctxt.compile ();

which shows that (on this x86_64 box) the compiler has unrolled the loop and is using MMX instructions to perform several multiplications simultaneously:

        .file   "fake.c"
        .text
.Ltext0:
        .p2align 4,,15
        .globl  factorial
        .type   factorial, @function
factorial:
.LFB0:
        .file 1 "factorial.toy"
        .loc 1 14 0
        .cfi_startproc
.LVL0:
.L2:
        .loc 1 26 0
        cmpl    $1, %edi
        jle     .L13
        leal    -1(%rdi), %edx
        movl    %edx, %ecx
        shrl    $2, %ecx
        leal    0(,%rcx,4), %esi
        testl   %esi, %esi
        je      .L14
        cmpl    $9, %edx
        jbe     .L14
        leal    -2(%rdi), %eax
        movl    %eax, -16(%rsp)
        leal    -3(%rdi), %eax
        movd    -16(%rsp), %xmm0
        movl    %edi, -16(%rsp)
        movl    %eax, -12(%rsp)
        movd    -16(%rsp), %xmm1
        xorl    %eax, %eax
        movl    %edx, -16(%rsp)
        movd    -12(%rsp), %xmm4
        movd    -16(%rsp), %xmm6
        punpckldq       %xmm4, %xmm0
        movdqa  .LC1(%rip), %xmm4
        punpckldq       %xmm6, %xmm1
        punpcklqdq      %xmm0, %xmm1
        movdqa  .LC0(%rip), %xmm0
        jmp     .L5
        # etc - edited for brevity

This is clearly overkill for a function that will likely overflow the int type before the vectorization is worthwhile - but then again, this is a toy example.

Turning down the optimization level to 2:

ctxt.set_int_option (GCC_JIT_INT_OPTION_OPTIMIZATION_LEVEL, 2);

yields this code, which is simple enough to quote in its entirety:

        .file   "fake.c"
        .text
        .p2align 4,,15
        .globl  factorial
        .type   factorial, @function
factorial:
.LFB0:
        .cfi_startproc
.L2:
        cmpl    $1, %edi
        jle     .L8
        movl    $1, %edx
        jmp     .L4
        .p2align 4,,10
        .p2align 3
.L6:
        movl    %eax, %edi
.L4:
.L5:
        leal    -1(%rdi), %eax
        imull   %edi, %edx
        cmpl    $1, %eax
        jne     .L6
.L3:
.L7:
        imull   %edx, %eax
        ret
.L8:
        movl    %edi, %eax
        movl    $1, %edx
        jmp     .L7
        .cfi_endproc
.LFE0:
        .size   factorial, .-factorial
        .ident  "GCC: (GNU) 4.9.0 20131023 (Red Hat 0.2-%{gcc_release})"
        .section        .note.GNU-stack,"",@progbits

Note that the stack pushing and popping have been eliminated, as has the recursive call (in favor of an iteration).

Putting it all together

The complete example can be seen in the source tree at gcc/jit/docs/examples/tut04-toyvm/toyvm.cc

along with a Makefile and a couple of sample .toy scripts:

$ ls -al
drwxrwxr-x. 2 david david   4096 Sep 19 17:46 .
drwxrwxr-x. 3 david david   4096 Sep 19 15:26 ..
-rw-rw-r--. 1 david david    615 Sep 19 12:43 factorial.toy
-rw-rw-r--. 1 david david    834 Sep 19 13:08 fibonacci.toy
-rw-rw-r--. 1 david david    238 Sep 19 14:22 Makefile
-rw-rw-r--. 1 david david  16457 Sep 19 17:07 toyvm.cc

$ make toyvm
g++ -Wall -g -o toyvm toyvm.cc -lgccjit

$ ./toyvm factorial.toy 10
interpreter result: 3628800
compiler result: 3628800

$ ./toyvm fibonacci.toy 10
interpreter result: 55
compiler result: 55

Behind the curtain: How does our code get optimized?

Our example is done, but you may be wondering about exactly how the compiler turned what we gave it into the machine code seen above.

We can examine what the compiler is doing in detail by setting:

state.ctxt.set_bool_option (GCC_JIT_BOOL_OPTION_DUMP_EVERYTHING, 1);
state.ctxt.set_bool_option (GCC_JIT_BOOL_OPTION_KEEP_INTERMEDIATES, 1);

This will dump detailed information about the compiler’s state to a directory under /tmp, and keep it from being cleaned up.

The precise names and their formats of these files is subject to change. Higher optimization levels lead to more files. Here’s what I saw (edited for brevity; there were almost 200 files):

intermediate files written to /tmp/libgccjit-KPQbGw
$ ls /tmp/libgccjit-KPQbGw/
fake.c.000i.cgraph
fake.c.000i.type-inheritance
fake.c.004t.gimple
fake.c.007t.omplower
fake.c.008t.lower
fake.c.011t.eh
fake.c.012t.cfg
fake.c.014i.visibility
fake.c.015i.early_local_cleanups
fake.c.016t.ssa
# etc

The gimple code is converted into Static Single Assignment form, with annotations for use when generating the debuginfo:

$ less /tmp/libgccjit-KPQbGw/fake.c.016t.ssa
;; Function factorial (factorial, funcdef_no=0, decl_uid=53, symbol_order=0)

factorial (signed int arg)
{
  signed int stack[8];
  signed int stack_depth;
  signed int x;
  signed int y;
  <unnamed type> _20;
  signed int _21;
  signed int _38;
  signed int _44;
  signed int _51;
  signed int _56;

initial:
  stack_depth_3 = 0;
  # DEBUG stack_depth => stack_depth_3
  stack[stack_depth_3] = arg_5(D);
  stack_depth_7 = stack_depth_3 + 1;
  # DEBUG stack_depth => stack_depth_7
  # DEBUG instr0 => NULL
  # DEBUG /* DUP */ => NULL
  stack_depth_8 = stack_depth_7 + -1;
  # DEBUG stack_depth => stack_depth_8
  x_9 = stack[stack_depth_8];
  # DEBUG x => x_9
  stack[stack_depth_8] = x_9;
  stack_depth_11 = stack_depth_8 + 1;
  # DEBUG stack_depth => stack_depth_11
  stack[stack_depth_11] = x_9;
  stack_depth_13 = stack_depth_11 + 1;
  # DEBUG stack_depth => stack_depth_13
  # DEBUG instr1 => NULL
  # DEBUG /* PUSH_CONST */ => NULL
  stack[stack_depth_13] = 2;

  /* etc; edited for brevity */

We can perhaps better see the code by turning off GCC_JIT_BOOL_OPTION_DEBUGINFO to suppress all those DEBUG statements, giving:

$ less /tmp/libgccjit-1Hywc0/fake.c.016t.ssa
;; Function factorial (factorial, funcdef_no=0, decl_uid=53, symbol_order=0)

factorial (signed int arg)
{
  signed int stack[8];
  signed int stack_depth;
  signed int x;
  signed int y;
  <unnamed type> _20;
  signed int _21;
  signed int _38;
  signed int _44;
  signed int _51;
  signed int _56;

initial:
  stack_depth_3 = 0;
  stack[stack_depth_3] = arg_5(D);
  stack_depth_7 = stack_depth_3 + 1;
  stack_depth_8 = stack_depth_7 + -1;
  x_9 = stack[stack_depth_8];
  stack[stack_depth_8] = x_9;
  stack_depth_11 = stack_depth_8 + 1;
  stack[stack_depth_11] = x_9;
  stack_depth_13 = stack_depth_11 + 1;
  stack[stack_depth_13] = 2;
  stack_depth_15 = stack_depth_13 + 1;
  stack_depth_16 = stack_depth_15 + -1;
  y_17 = stack[stack_depth_16];
  stack_depth_18 = stack_depth_16 + -1;
  x_19 = stack[stack_depth_18];
  _20 = x_19 < y_17;
  _21 = (signed int) _20;
  stack[stack_depth_18] = _21;
  stack_depth_23 = stack_depth_18 + 1;
  stack_depth_24 = stack_depth_23 + -1;
  x_25 = stack[stack_depth_24];
  if (x_25 != 0)
    goto <bb 4> (instr9);
  else
    goto <bb 3> (instr4);

instr4:
/* DUP */:
  stack_depth_26 = stack_depth_24 + -1;
  x_27 = stack[stack_depth_26];
  stack[stack_depth_26] = x_27;
  stack_depth_29 = stack_depth_26 + 1;
  stack[stack_depth_29] = x_27;
  stack_depth_31 = stack_depth_29 + 1;
  stack[stack_depth_31] = 1;
  stack_depth_33 = stack_depth_31 + 1;
  stack_depth_34 = stack_depth_33 + -1;
  y_35 = stack[stack_depth_34];
  stack_depth_36 = stack_depth_34 + -1;
  x_37 = stack[stack_depth_36];
  _38 = x_37 - y_35;
  stack[stack_depth_36] = _38;
  stack_depth_40 = stack_depth_36 + 1;
  stack_depth_41 = stack_depth_40 + -1;
  x_42 = stack[stack_depth_41];
  _44 = factorial (x_42);
  stack[stack_depth_41] = _44;
  stack_depth_46 = stack_depth_41 + 1;
  stack_depth_47 = stack_depth_46 + -1;
  y_48 = stack[stack_depth_47];
  stack_depth_49 = stack_depth_47 + -1;
  x_50 = stack[stack_depth_49];
  _51 = x_50 * y_48;
  stack[stack_depth_49] = _51;
  stack_depth_53 = stack_depth_49 + 1;

  # stack_depth_1 = PHI <stack_depth_24(2), stack_depth_53(3)>
instr9:
/* RETURN */:
  stack_depth_54 = stack_depth_1 + -1;
  x_55 = stack[stack_depth_54];
  _56 = x_55;
  stack ={v} {CLOBBER};
  return _56;

}

Note in the above how all the gccjit::block instances we created have been consolidated into just 3 blocks in GCC’s internal representation: initial, instr4 and instr9.

Optimizing away stack manipulation

Recall our simple implementation of stack operations. Let’s examine how the stack operations are optimized away.

After a pass of constant-propagation, the depth of the stack at each opcode can be determined at compile-time:

$ less /tmp/libgccjit-1Hywc0/fake.c.021t.ccp1
;; Function factorial (factorial, funcdef_no=0, decl_uid=53, symbol_order=0)

factorial (signed int arg)
{
  signed int stack[8];
  signed int stack_depth;
  signed int x;
  signed int y;
  <unnamed type> _20;
  signed int _21;
  signed int _38;
  signed int _44;
  signed int _51;

initial:
  stack[0] = arg_5(D);
  x_9 = stack[0];
  stack[0] = x_9;
  stack[1] = x_9;
  stack[2] = 2;
  y_17 = stack[2];
  x_19 = stack[1];
  _20 = x_19 < y_17;
  _21 = (signed int) _20;
  stack[1] = _21;
  x_25 = stack[1];
  if (x_25 != 0)
    goto <bb 4> (instr9);
  else
    goto <bb 3> (instr4);

instr4:
/* DUP */:
  x_27 = stack[0];
  stack[0] = x_27;
  stack[1] = x_27;
  stack[2] = 1;
  y_35 = stack[2];
  x_37 = stack[1];
  _38 = x_37 - y_35;
  stack[1] = _38;
  x_42 = stack[1];
  _44 = factorial (x_42);
  stack[1] = _44;
  y_48 = stack[1];
  x_50 = stack[0];
  _51 = x_50 * y_48;
  stack[0] = _51;

instr9:
/* RETURN */:
  x_55 = stack[0];
  x_56 = x_55;
  stack ={v} {CLOBBER};
  return x_56;

}

Note how, in the above, all those stack_depth values are now just constants: we’re accessing specific stack locations at each opcode.

The “esra” pass (“Early Scalar Replacement of Aggregates”) breaks out our “stack” array into individual elements:

$ less /tmp/libgccjit-1Hywc0/fake.c.024t.esra
;; Function factorial (factorial, funcdef_no=0, decl_uid=53, symbol_order=0)

Created a replacement for stack offset: 0, size: 32: stack$0
Created a replacement for stack offset: 32, size: 32: stack$1
Created a replacement for stack offset: 64, size: 32: stack$2

Symbols to be put in SSA form
{ D.89 D.90 D.91 }
Incremental SSA update started at block: 0
Number of blocks in CFG: 5
Number of blocks to update: 4 ( 80%)


factorial (signed int arg)
{
  signed int stack$2;
  signed int stack$1;
  signed int stack$0;
  signed int stack[8];
  signed int stack_depth;
  signed int x;
  signed int y;
  <unnamed type> _20;
  signed int _21;
  signed int _38;
  signed int _44;
  signed int _51;

initial:
  stack$0_45 = arg_5(D);
  x_9 = stack$0_45;
  stack$0_39 = x_9;
  stack$1_32 = x_9;
  stack$2_30 = 2;
  y_17 = stack$2_30;
  x_19 = stack$1_32;
  _20 = x_19 < y_17;
  _21 = (signed int) _20;
  stack$1_28 = _21;
  x_25 = stack$1_28;
  if (x_25 != 0)
    goto <bb 4> (instr9);
  else
    goto <bb 3> (instr4);

instr4:
/* DUP */:
  x_27 = stack$0_39;
  stack$0_22 = x_27;
  stack$1_14 = x_27;
  stack$2_12 = 1;
  y_35 = stack$2_12;
  x_37 = stack$1_14;
  _38 = x_37 - y_35;
  stack$1_10 = _38;
  x_42 = stack$1_10;
  _44 = factorial (x_42);
  stack$1_6 = _44;
  y_48 = stack$1_6;
  x_50 = stack$0_22;
  _51 = x_50 * y_48;
  stack$0_1 = _51;

  # stack$0_52 = PHI <stack$0_39(2), stack$0_1(3)>
instr9:
/* RETURN */:
  x_55 = stack$0_52;
  x_56 = x_55;
  stack ={v} {CLOBBER};
  return x_56;

}

Hence at this point, all those pushes and pops of the stack are now simply assignments to specific temporary variables.

After some copy propagation, the stack manipulation has been completely optimized away:

$ less /tmp/libgccjit-1Hywc0/fake.c.026t.copyprop1
;; Function factorial (factorial, funcdef_no=0, decl_uid=53, symbol_order=0)

factorial (signed int arg)
{
  signed int stack$2;
  signed int stack$1;
  signed int stack$0;
  signed int stack[8];
  signed int stack_depth;
  signed int x;
  signed int y;
  <unnamed type> _20;
  signed int _21;
  signed int _38;
  signed int _44;
  signed int _51;

initial:
  stack$0_39 = arg_5(D);
  _20 = arg_5(D) <= 1;
  _21 = (signed int) _20;
  if (_21 != 0)
    goto <bb 4> (instr9);
  else
    goto <bb 3> (instr4);

instr4:
/* DUP */:
  _38 = arg_5(D) + -1;
  _44 = factorial (_38);
  _51 = arg_5(D) * _44;
  stack$0_1 = _51;

  # stack$0_52 = PHI <arg_5(D)(2), _51(3)>
instr9:
/* RETURN */:
  stack ={v} {CLOBBER};
  return stack$0_52;

}

Later on, another pass finally eliminated stack_depth local and the unused parts of the stack` array altogether:

$ less /tmp/libgccjit-1Hywc0/fake.c.036t.release_ssa
;; Function factorial (factorial, funcdef_no=0, decl_uid=53, symbol_order=0)

Released 44 names, 314.29%, removed 44 holes
factorial (signed int arg)
{
  signed int stack$0;
  signed int mult_acc_1;
  <unnamed type> _5;
  signed int _6;
  signed int _7;
  signed int mul_tmp_10;
  signed int mult_acc_11;
  signed int mult_acc_13;

  # arg_9 = PHI <arg_8(D)(0)>
  # mult_acc_13 = PHI <1(0)>
initial:

  <bb 5>:
  # arg_4 = PHI <arg_9(2), _7(3)>
  # mult_acc_1 = PHI <mult_acc_13(2), mult_acc_11(3)>
  _5 = arg_4 <= 1;
  _6 = (signed int) _5;
  if (_6 != 0)
    goto <bb 4> (instr9);
  else
    goto <bb 3> (instr4);

instr4:
/* DUP */:
  _7 = arg_4 + -1;
  mult_acc_11 = mult_acc_1 * arg_4;
  goto <bb 5>;

  # stack$0_12 = PHI <arg_4(5)>
instr9:
/* RETURN */:
  mul_tmp_10 = mult_acc_1 * stack$0_12;
  return mul_tmp_10;

}

Elimination of tail recursion

Another significant optimization is the detection that the call to factorial is tail recursion, which can be eliminated in favor of an iteration:

$ less /tmp/libgccjit-1Hywc0/fake.c.030t.tailr1
;; Function factorial (factorial, funcdef_no=0, decl_uid=53, symbol_order=0)


Symbols to be put in SSA form
{ D.88 }
Incremental SSA update started at block: 0
Number of blocks in CFG: 5
Number of blocks to update: 4 ( 80%)


factorial (signed int arg)
{
  signed int stack$2;
  signed int stack$1;
  signed int stack$0;
  signed int stack[8];
  signed int stack_depth;
  signed int x;
  signed int y;
  signed int mult_acc_1;
  <unnamed type> _20;
  signed int _21;
  signed int _38;
  signed int mul_tmp_44;
  signed int mult_acc_51;

  # arg_5 = PHI <arg_39(D)(0), _38(3)>
  # mult_acc_1 = PHI <1(0), mult_acc_51(3)>
initial:
  _20 = arg_5 <= 1;
  _21 = (signed int) _20;
  if (_21 != 0)
    goto <bb 4> (instr9);
  else
    goto <bb 3> (instr4);

instr4:
/* DUP */:
  _38 = arg_5 + -1;
  mult_acc_51 = mult_acc_1 * arg_5;
  goto <bb 2> (initial);

  # stack$0_52 = PHI <arg_5(2)>
instr9:
/* RETURN */:
  stack ={v} {CLOBBER};
  mul_tmp_44 = mult_acc_1 * stack$0_52;
  return mul_tmp_44;

}