::pow(T, n) vs std::pow(T, n) for non-constant n

[Paolo Carlini]

... however, for the very same loop that I used at the beginning:

  for (int i = 0; i < 100000000; ++i)
    {
      double a = M_PI * i;
      int n = i % 10;
      std::pow(a, n);
      //::pow(a, n);
    }

With -O2 -ffast-math <- IMPORTANT

::pow
-----
0.090u 0.000s 0:00.09 100.0%    0+0k 0+0io 149pf+0w

std::pow
--------
1.540u 0.000s 0:01.54 100.0%    0+0k 0+0io 149pf+0w

Therefore, in the situation displayed above, gcc + builtin pow,
when requested to do so (-ffast-math) is able to establish the
constness of the argument and use the fast optimized code!!!

For sure, in other, more complex, situations this will not happen,
but when, exactly?

The more I try out the behavior of gcc + builtin pow, the more I
become convinced that, for -ffast-math, is so faster than the current
v3 binary algorithm, otherwise, more accurate.

Paolo.