Tutorial part 3 Loops and variables<2> (libgccjit Documentation)

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3.1.3 Tutorial part 3: Loops and variables ¶

Consider this C function:

int loop_test (int n)
{
  int sum = 0;
  for (int i = 0; i < n; i++)
    sum += i * i;
  return sum;
}

This example demonstrates some more features of libgccjit, with local variables and a loop.

To break this down into libgccjit terms, it’s usually easier to reword the for loop as a while loop, giving:

int loop_test (int n)
{
  int sum = 0;
  int i = 0;
  while (i < n)
  {
    sum += i * i;
    i++;
  }
  return sum;
}

Here’s what the final control flow graph will look like:

Figure

As before, we include the libgccjit++ header and make a gccjit;;context.

#include <libgccjit++.h>

void test (void)
{
  gccjit::context ctxt;
  ctxt = gccjit::context::acquire ();

The function works with the C int type.

In the previous tutorial we acquired this via

gccjit::type the_type = ctxt.get_type (ctxt, GCC_JIT_TYPE_INT);

though we could equally well make it work on, say, double:

gccjit::type the_type = ctxt.get_type (ctxt, GCC_JIT_TYPE_DOUBLE);

For integer types we can use gccjit::context::get_int_type to directly bind a specific type:

gccjit::type the_type = ctxt.get_int_type <int> ();

Let’s build the function:

gcc_jit_param n = ctxt.new_param (the_type, "n");
std::vector<gccjit::param> params;
params.push_back (n);
gccjit::function func =
  ctxt.new_function (GCC_JIT_FUNCTION_EXPORTED,
                     return_type,
                     "loop_test",
                     params, 0);