Accuracy and the power function

[Werner]

Truncation (and keeping track of lost non-zero digits with the Sticky Bit) in the 15-digit routines is not only common practice, it's a necessity if you want to be able to produce a correctly rounded 12-digit result for simple calculations.
Suppose the 15-digit routines rounded as well, then:
5.02000000121e15 + 4995 (exact result: 5.020000001214995e15) would be rounded to 15 digits as 5.02000000121500e15
5.02000000121500e15 rounded to even to 12 digits is 5.02000000122e15
yet the 16-digit number rounded to 12 digits is 5.02000000121e15

Werner