Actually, 2^53 different mantissas, plus a few different exponents, depending on the function under test. Also, there's a huge number of NaNs, all of which are equivalent. For ceil/floor, it wouldn't make sense to test exponents larger than 54.
There are a huge number of double-precision NaNs in the absolute sense -- there are 2^54-2 of them. But they are a small portion of all of the doubles (roughly 1/2048) so omitting them from the tests does not help significantly with saving time.
And, skipping testing of large numbers and NaNs can lead to missing bugs. One of the fixed ceil functions handled everything except NaNs correctly. Implementations that failed on exponents beyond 63 or 64 are easy to imagine.
But, testing doubles requires compromises. Testing of special values plus random testing plus exhaustive testing of the smallest ten trillion or so numbers should be sufficient -- better than the current status quo at least.
