Fast Types

Now that we have seen the three basic types as specified in IEEE-754 there are three additional adjacent types: decimal_fast32_t, decimal_fast64_t, and decimal_fast128_t. These types yield identical computational results but with faster performance. These types make the classic tradeoff of space for time as they require more storage width than the IEEE 754 conformant type. For example decimal32_t is fundamentally:

class decimal32_t final
{
    std::uint32_t bits_;
};

Where the value of decimal32_t is encoded into bits_ per IEEE 754 BID encoding pattern. Conversely, the internals of decimal_fast32_t are:

class decimal_fast32_t final
{
    std::uint32_t signficand_;
    std::uint8_t exponent_;
    bool sign_;
};

Instead of having to decode the bits at every operation, decimal_fast32_t is able to skip this step. The other major difference is that decimal32_t has cohorts as previously discussed. This requires normalization to ensure correct results for the comparison:

decimal32_t a {20};
decimal32_t b {2, 1};

assert(a == b);

In this example a and b both have different encodings (cohorts), so they must be normalized before comparison. decimal_fast32_t and the other fast types store the significand and exponent in a normalized fashion, so once again we can skip that step for every operation.

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