A compile-time FFT in C++11

Published: by Creative Commons Licence

Generalised constant expressions are a core language feature introduced to C++ by the recent C++11 standard. A call to a function or method qualified with the new constexpr keyword can now be evaluated at compile-time. Of course there are restrictions: arguments to such a call must also be amenable to compile-time evaluation. The primary restriction is that the body of a constexpr function must comprise of a single return statement. While this initially sounds draconian, the use of recursion, and the ternary operator, can permit essentially any calculation. The language of C++11 generalised constant expressions is that of a functional language.

The code below presents a brief example. Note the call to sqr_int in the second template argument to the C++11 std::array object. Also of interest: the call to sqr_int will prosaically perform floating-point calculations at compile-time. Incidentally, among other restrictions, lambda expressions may not, alas, form part of a C++11 generalised constant expression.

#include <array>
constexpr unsigned sqr_int(double d) { return d*d; }
std::array<bool,sqr_int(4.5)> garr;

Having been impressed by the ctrace and metatrace compile-time ray-tracers, I thought I'd give the fast fourier transform (FFT) a whirl. Seeking an FFT implemented in a functional language, I came across the elegant Single Assignment C (SAC) version shown here. After failing to compile the assembled pieces with the SAC compiler, I converted it to Fortran 9x and verified the results against another Fortran implementation from here.

The next thing I needed was a container data type to stand in for the rank-one, first-class arrays employed by the Fortran. Thoughts of using std::array soon evaporated as I realised that no constexpr element access methods exist; neither operator[] nor std::array::front() comply. Similar issues exist with std::tuple. Furthermore, the FFT only requires a homogeneous container. An implementation using the heterogeneous std::tuple would be inclined to add additional type-checking code. In response, I created the following recursive array container type:

template <typename T, size_t N>
struct recarr {
  constexpr recarr(T x, recarr<T,N-1> a) : m_x(x), m_a(a) {}

  constexpr T             head()   { return m_x; }
  constexpr recarr<T,N-1> tail()   { return m_a; }

  T m_x;
  recarr<T,N-1> m_a;

template <typename T>
struct recarr<T,0> {};

The common restriction on recursive data types: members having the same type as the enclosing class must be pointers, is not applicable here; the m_a member is always a different type; i.e. recarr is not the same type as recarr. The zero-length specialisation of recarr has no members.

The list type used routinely in functional languages such as Haskell is a clear inspiration in the design of std::recarr. A cons function for std::recarr, necessary for the construction of higher order functions such as map and zipWith, may be defined as shown below. Rather than the raw recarr constructor, the stand-alone recarr_cons function can infer its template arguments from the runtime arguments.

template <typename T, size_t N>
recarr<T,N+1> recarr_cons(T x, recarr<T,N> xs) {
  return recarr<T,N+1>(x,xs);

An FFT is an operation on complex numbers. The standard std::complex type, however, delivers another spanner to the works; it too lacks the constexpr methods needed for a compile-time FFT; including the constructors, and arithmetic operators. So, a new complex number class was developed, along with the necessary C++11 generalised constant expression support. Complex number formulae and definitions were drawn from here and here.

GCC and Clang both have excellent support for C++11. Wouldn't it be nice to find out which is the faster compiler for the evaluation of a constexpr FFT? Some more challenges materialise here: Clang rejects all of the cmath library functions, as being non-constexpr. Functions required by the FFT include sin, sinh, cos, cosh, sqrt, log, exp, atan, atan2 and pow. Initially I though that Clang was being a little cranky here, but this is in fact the correct behaviour; n.b. a constexpr function may not be overloaded with a non-constexpr function; and vice-versa. Standard library functions which are to be constexpr must consequently be specified as such by the relevant C++ standard; the functions in cmath are not. After all, functions comprised of a single return statement may not be the most performant! The N3228 and N3231 documents from the C++ Standards Committee's November 2010 mailings elaborate on this theme.

So, constexpr versions of the mathematical functions from cmath necessary for the FFT were developed; with reference to the formulae and guidelines of the Algebra Help e-Book.

I am in fact a big fan of the std::tuple class. In the end I couldn't resist a second, comparative implementation of a constexpr FFT using C++11 tuples; and so the magic of variadic templates. Need I mention that std::tuple is also missing many of the constexpr constructors and support functions necessary for the job? The github code linked below includes a simple recursive version of the std::tuple type, with constexpr constructors; a version of get; a version of make_tuple; and, as the FFT uses concat, a version of tuple_cat. For comparison with recarr_cons above, here is the simple, concise tuple_cons; in constrast to a more verbose definition for tuple_cat provided in the full project.

template <typename T, typename ...Ts>
tuple<T,Ts...> tuple_cons(T x, tuple<Ts...> xs) {
  return tuple_cat(make_tuple(x),xs);

The code is stored in a github repository here.