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BFI: Update ScaledNumber header docs

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211556 91177308-0d34-0410-b5e6-96231b3b80d8
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Duncan P. N. Exon Smith 2014-06-23 23:49:31 +00:00
parent 7c21d709a3
commit 1c4709c618
1 changed files with 17 additions and 13 deletions
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30
include/llvm/Analysis/BlockFrequencyInfoImpl.h
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@ -68,25 +68,26 @@ public:
}
};
/// \brief Simple representation of an unsigned floating point.
/// \brief Simple representation of a scaled number.
///
/// ScaledNumber is a unsigned floating point number. It uses simple
/// saturation arithmetic, and every operation is well-defined for every value.
/// ScaledNumber is a number represented by digits and a scale. It uses simple
/// saturation arithmetic and every operation is well-defined for every value.
/// It's somewhat similar in behaviour to a soft-float, but is *not* a
/// replacement for one. If you're doing numerics, look at \a APFloat instead.
/// Nevertheless, we've found these semantics useful for modelling certain cost
/// metrics.
///
/// The number is split into a signed exponent and unsigned digits. The number
/// The number is split into a signed scale and unsigned digits. The number
/// represented is \c getDigits()*2^getExponent(). In this way, the digits are
/// much like the mantissa in the x87 long double, but there is no canonical
/// form, so the same number can be represented by many bit representations
/// (it's always in "denormal" mode).
/// form so the same number can be represented by many bit representations.
///
/// ScaledNumber is templated on the underlying integer type for digits, which
/// is expected to be one of uint64_t, uint32_t, uint16_t or uint8_t.
///
/// Unlike builtin floating point types, ScaledNumber is portable.
/// is expected to be unsigned.
///
/// Unlike APFloat, ScaledNumber does not model architecture floating point
/// behaviour (this should make it a little faster), and implements most
/// operators (this makes it usable).
/// behaviour -- while this might make it a little faster and easier to reason
/// about, it certainly makes it more dangerous for general numerics.
///
/// ScaledNumber is totally ordered. However, there is no canonical form, so
/// there are multiple representations of most scalars. E.g.:
@ -109,8 +110,11 @@ public:
/// it trivial to add functionality to convert to APFloat (this is already
/// relied on for the implementation of printing).
///
/// The current plan is to gut this and make the necessary parts of it (even
/// more) private to BlockFrequencyInfo.
/// Possible (and conflicting) future directions:
///
/// 1. Turn this into a wrapper around \a APFloat.
/// 2. Share the algorithm implementations with \a APFloat.
/// 3. Allow \a ScaledNumber to represent a signed number.
template <class DigitsT> class ScaledNumber : ScaledNumberBase {
public:
static_assert(!std::numeric_limits<DigitsT>::is_signed,