The term containing the `pow` invocation computes the
alternating sequence -1, 1, -1, ..., which can be replaced by
e.g.
```
immutable int [2] sign = [-1, 1];
n += sign [i & 1] / (i * 2.0 - 1.0);
```
This saves the expensive call to the pow function.
I used the loop:
```d
for (int i = 1; i < iter; i++)
n += ((i%2) ? -1.0 : 1.0) / (i * 2.0 + 1.0);
```
in both C and D, with gcc and gdc and got average execution times:
--- C -----
original: .... loop replacement: .... -O2:
0.009989 .... 0.003198 ........... 0.001335
--- D -----
original: .... loop replacement: .... -O2:
0.230346 .... 0.003083 ........... 0.001309
almost no difference.
But the D binary is much larger on my Linux:
4600920 bytes instead of 15504 bytes for the C version.
Are there some simple switches / settings to get a smaller binary?
