C++11: g++-4.7 internal compiler error

Question

The following code:

#include <iostream>
#include <array>
using namespace std;

constexpr int N = 1000000;
constexpr int f(int x) { return x*2; }

typedef array<int, N> A;

template<int... i> struct F { static constexpr A f() { return A{{ ::f(i)... }}; } };

template<class A, class B> struct C {};
template<int... i, int... j> struct C<F<i...>, F<j...>> : F<i..., (sizeof...(i)+j)...>
{
        using T = F<i..., (sizeof...(i)+j)...>;
};

template<int n> struct S : C<typename S<n/2>::T, typename S<n-n/2>::T> {};
template<> struct S<1> : F<0> { using T = F<0>; };

constexpr auto X = S<N>::f();

int main()
{
        cout << X[3] << endl;
}

Produces an internal compiler error in GCC 4.7 in -std=gnu++11 mode.

$ g++ -std=gnu++11 test.cpp
g++-4.7.real: internal compiler error: Killed (program cc1plus)

What is going wrong?

Answer 1

An internal error means that you hit a compiler bug.

Answer 2

It seems that your program requires an unreasonable amount of memory (perhaps because of too many template expansions).

Using a recent g++-trunk :

gcc version 4.8.0 20121026 (experimental) [trunk revision 192860] (GCC) 

with the following zsh limits:

   % limit          
   cputime         unlimited
   filesize        unlimited
   datasize        15000MB
   stacksize       8MB
   coredumpsize    400MB
   memoryuse       15000MB
   maxproc         128166
   descriptors     1024
   memorylocked    64kB
   addressspace    16000MB
   maxfilelocks    unlimited
   sigpending      128166
   msgqueue        819200
   nice            0
   rt_priority     0
   rt_time         unlimited

(this on Debian/Sid/AMD64 with i3770K intel processor & 16Gb RAM)

I am getting:

  % time g++-trunk -std=gnu++11 andrew.cc -o andrew
  virtual memory exhausted: Cannot allocate memory
  g++-trunk -std=gnu++11 andrew.cc -o andrew :
  108.25s user 3.28s system 89% cpu 2:03.98 total

So it seems that template expansion requires so much memory that you program is not reasonable.

I'm not sure if this will be accepted as a GCC bug. The macro expansion for C++ templates is known to be Turing complete and you just hit the wall. And the GCC trunk does report a fatal, but understandable error.

The moral of the story might be to setrlimit(2) appropriately (with limits compatible with your system and hardware), perhaps using limit zsh built-in or ulimit bash built-in.