I just noticed two missing defines. Sorry about that. Fixed below.

I did some testing and seems like this generates much less code at least with clang (with -O3 -fomit-frame-pointer) than geocar's much briefer solution!

  #define VA_NUM_ARGS(...) \
    VA_NUM_ARGS_IMPL(__VA_ARGS__,5,4,3,2,1)
  #define VA_NUM_ARGS_IMPL(_1,_2,_3,_4,_5,N,...) N
  #define mkmacro(func, ...) \
    mkmacro_(func, VA_NUM_ARGS(__VA_ARGS__))
  #define mkmacro_(func, nargs) mkmacro__(func, nargs)
  #define mkmacro__(func, nargs) func ## nargs

  #define DO(...) mkmacro(DO, __VA_ARGS__)(__VA_ARGS__)
  #define DO1(n) for(I i=0;i<(n); i++)
  #define DO2(m,n) DO1(m) for(I j=0;j<(n);j++)
  #define DO3(l,m,n) DO2(l,m) for(I k=0;k<(n);k++)