The new calculation should have less problems with integer overflows.
Consider the following sample values with a tcount of 2000000100 and a
tc_frequency of 2000000000 (2GHz). The scale is 9223372036. Then scale *
tcount is 18446744994337203600 which is larger than
UINT64_MAX (18446744073709551615). The result is 49.
In the new calculation, 1000000000 * tcount is 2000000100000000000 which is
less than UINT64_MAX. This yields after the division with tc_frequency the
correct result of 1000000050.