Two major off-by-one errors are fixed in this patch. The first is in
float_to_string.h with length_for_num, which wasn't accounting for the
implicit leading bit when calculating the length of a number, causing
a missing digit on 80 bit float max. The other off-by-one is the
ryu_long_double_constants.h (a.k.a the Mega Table) not having any
entries for the last POW10_OFFSET in POW10_SPLIT. This was also found on
80 bit float max. Finally, the integer calculation mode was using a
slightly too short integer, again on 80 bit float max, not accounting
for the mantissa width. All of these are fixed in this patch.
This patch adds three options for printf decimal long doubles, and these
can also apply to doubles.
1. Use a giant table which is fast and accurate, but takes up ~5MB).
2. Use dyadic floats for approximations, which only gives ~50 digits of
accuracy but is very fast.
3. Use large integers for approximations, which is accurate but very
slow.
Reviewed By: sivachandra, lntue
Differential Revision: https://reviews.llvm.org/D150399
