Mark Dickinson <report@bugs.python.org> wrote:
> Whoops;  that *was* string to float.  How about float to string?

These are the results for format_float_short. The first one is quite funny:

If the control word is left as is (64-bit prec on Linux), _Py_dg_dtoa
apparently requires more iterations (but still produces the correct
result in this case).

import timeit

s = "str(38210.0)"
t = timeit.Timer(stmt=s)

4.266580820083618
>>> t.timeit(number=10000000)
4.3112568855285645
>>> t.timeit(number=10000000)
4.313380002975464
>>>

3.7413151264190674
>>> t.timeit(number=10000000)
3.7296619415283203
>>> t.timeit(number=10000000)
3.737776041030884

3.756165027618408
>>> t.timeit(number=10000000)
3.775428056716919
>>> t.timeit(number=10000000)
3.775447130203247

In general, execution times for _Py_dg_dtoa vary quite a bit depending on
the number that is converted:

5.097490072250366

2.417008876800537

Judging from these numbers, I think the macros could be called by default without
any real effect on performance. How about setting the rounding mode as well (you
suggested that in #9009)?