cnd 2011.2

CnD is a source-to-source translator that makes using n-dimensional arrays in C more pleasant. It will turn this code:

void sgemm(float *a, float *b, float *c, int n)
{
  dimension "fortran" a(n, n);
  dimension "fortran" b(n, n);
  dimension c(n, n);

  for (int i = 1; i <= n; ++i)
    for (int j = 1; j <= n; ++j)
    {
      float tmp = 0;

      for (int k = 1; k <= n; ++k)
        tmp += a(i,k)*b(k,j);

      c(i-1,j-1) = tmp;
    }
}

into this:

void sgemm(float *a, float *b, float *c, int n)
{
  for (int i = 1; i <= n; ++i)
    for (int j = 1; j <= n; ++j)
  {
    float tmp = 0;
    for (int k = 1; k <= n; ++k)
      tmp += a[((k - 1) * ((n - 1) + 1)) + (i - 1)] * b[((j - 1) * ((n - 1) + 1)) + (k - 1)];

    c[((i - 1) * n) + (j - 1)] = tmp;
  }
}

(See the FAQ below if you’re already not liking the round parentheses for array indexing. Turns out C leaves you no choice.)

You may also take a look at a more comprehensive example that shows a few extra bells and whistles.

The only effect of a dimension declaration is to modify the interpretation of the array(idx) subscript operator. dimension declarations obey regular C scoping rules. Note that in order to prevent hard-to-find bugs, multi-dimensional array references using square brackets are considered an error.

I’d also like to note that CnD is a robust, parser-based translator, not a flaky text replacement tool. It understands all of C99.

Each axis specification in a dimension declaration has the following form:

start:end:stride:leading_dimension

start may be omitted. end and stride may also be omitted, but if entries after them are to be specified, their trailing colons must remain in place. For example, the axis specification :5 simply specifies a stride of 5. The stride simply acts as a multiplier on the index. No plausibility checking whatsoever is done on the dimension declaration. You may shoot yourself in the foot any way you like.

If the layout is given as “c” or not given at all, the following things are true:

• The array is laid out in row-major order.

• The end index is taken to be exclusive, if specified.

• The start index defaults to 0.

If the layout is given as “fortran”, the following things are true:

• The array is laid out in column-major order.

• The end index is taken to be inclusive, if specified.

• The start index defaults to 1.

(Most) of the knowledge contained in the dimension declaration may be reobtained programmatically by the follwing functions:

• rankof(a)

• nitemsof(a)

• lboundof(a, axis)

• uboundof(a, axis) (returns the user-specified upper bound)

• puboundof(a, axis) (returns the index just past the end of axis)

• ldimof(a, axis)

• strideof(a, axis)

In each case, axis must be a constant integer (not a constant expression, a plain integer).

$ git clone git://github.com/inducer/cnd.git
$ cd cnd
$ git submodule init
$ git submodule update

$ cd examples
$ ../bin/cndcc gcc -std=c99 basic.c
$ ./a.out

$ cnd -h

#define MY_MACRO(a) /* something rather */

MY_MACRO(array[i,j])