Faster dec128 fast

[mborland]

Closes: #656

Provides 2.375x speedup vs develop and 3.236x speedup vs decimal128

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 98.5%. Comparing base (c7aed9d) to head (3c87b81).

Additional details and impacted files

[](https://app.codecov.io/gh/cppalliance/decimal/pull/658?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=cppalliance) ```diff @@ Coverage Diff @@ ## develop #658 +/- ## ========================================= - Coverage 98.5% 98.5% -0.0% ========================================= Files 221 221 Lines 15696 15661 -35 Branches 1723 1722 -1 ========================================= - Hits 15452 15416 -36 - Misses 244 245 +1 ``` | [Files](https://app.codecov.io/gh/cppalliance/decimal/pull/658?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=cppalliance) | Coverage Δ | | |---|---|---| | [include/boost/decimal/decimal128\_fast.hpp](https://app.codecov.io/gh/cppalliance/decimal/pull/658?src=pr&el=tree&filepath=include%2Fboost%2Fdecimal%2Fdecimal128_fast.hpp&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=cppalliance#diff-aW5jbHVkZS9ib29zdC9kZWNpbWFsL2RlY2ltYWwxMjhfZmFzdC5ocHA=) | `97.0% <100.0%> (-0.7%)` | :arrow_down: | | [include/boost/decimal/detail/add\_impl.hpp](https://app.codecov.io/gh/cppalliance/decimal/pull/658?src=pr&el=tree&filepath=include%2Fboost%2Fdecimal%2Fdetail%2Fadd_impl.hpp&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=cppalliance#diff-aW5jbHVkZS9ib29zdC9kZWNpbWFsL2RldGFpbC9hZGRfaW1wbC5ocHA=) | `100.0% <100.0%> (ø)` | | ------ [Continue to review full report in Codecov by Sentry](https://app.codecov.io/gh/cppalliance/decimal/pull/658?dropdown=coverage&src=pr&el=continue&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=cppalliance). > **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=cppalliance) > `Δ = absolute (impact)`, `ø = not affected`, `? = missing data` > Powered by [Codecov](https://app.codecov.io/gh/cppalliance/decimal/pull/658?dropdown=coverage&src=pr&el=footer&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=cppalliance). Last update [c7aed9d...3c87b81](https://app.codecov.io/gh/cppalliance/decimal/pull/658?dropdown=coverage&src=pr&el=lastupdated&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=cppalliance). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=cppalliance).