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blockfreq: Defer to BranchProbability::scale() 

`BlockMass` can now defer to `BranchProbability::scale()`.

llvm-svn: 207547
This commit is contained in:
Duncan P. N. Exon Smith 2014-04-29 16:20:05 +00:00
parent 4ac56cf249
commit d22bea7dad
2 changed files with 4 additions and 51 deletions
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29
llvm/include/llvm/Analysis/BlockFrequencyInfoImpl.h
View File

@ -758,31 +758,10 @@ public:
return *this; return *this;
} }
/// \brief Multiply by a branch probability. BlockMass &operator*=(const BranchProbability &P) {
/// Mass = P.scale(Mass);
/// Multiply by P. Guarantees full precision. return *this;
/// }
/// This could be naively implemented by multiplying by the numerator and
/// dividing by the denominator, but in what order? Multiplying first can
/// overflow, while dividing first will lose precision (potentially, changing
/// a non-zero mass to zero).
///
/// The implementation mixes the two methods. Since \a BranchProbability
/// uses 32-bits and \a BlockMass 64-bits, shift the mass as far to the left
/// as there is room, then divide by the denominator to get a quotient.
/// Multiplying by the numerator and right shifting gives a first
/// approximation.
///
/// Calculate the error in this first approximation by calculating the
/// opposite mass (multiply by the opposite numerator and shift) and
/// subtracting both from teh original mass.
///
/// Add to the first approximation the correct fraction of this error value.
/// This time, multiply first and then divide, since there is no danger of
/// overflow.
///
/// \pre P represents a fraction between 0.0 and 1.0.
BlockMass &operator*=(const BranchProbability &P);
bool operator==(const BlockMass &X) const { return Mass == X.Mass; } bool operator==(const BlockMass &X) const { return Mass == X.Mass; }
bool operator!=(const BlockMass &X) const { return Mass != X.Mass; } bool operator!=(const BlockMass &X) const { return Mass != X.Mass; }

26
llvm/lib/Analysis/BlockFrequencyInfoImpl.cpp
View File

@ -311,32 +311,6 @@ std::pair<uint64_t, int16_t> UnsignedFloatBase::multiply64(uint64_t L,
// BlockMass implementation. // BlockMass implementation.
// //
//===----------------------------------------------------------------------===// //===----------------------------------------------------------------------===//
BlockMass &BlockMass::operator*=(const BranchProbability &P) {
uint32_t N = P.getNumerator(), D = P.getDenominator();
assert(D && "divide by 0");
assert(N <= D && "fraction greater than 1");
// Fast path for multiplying by 1.0.
if (!Mass || N == D)
return *this;
// Get as much precision as we can.
int Shift = countLeadingZeros(Mass);
uint64_t ShiftedQuotient = (Mass << Shift) / D;
uint64_t Product = ShiftedQuotient * N >> Shift;
// Now check for what's lost.
uint64_t Left = ShiftedQuotient * (D - N) >> Shift;
uint64_t Lost = Mass - Product - Left;
// TODO: prove this assertion.
assert(Lost <= UINT32_MAX);
// Take the product plus a portion of the spoils.
Mass = Product + Lost * N / D;
return *this;
}
UnsignedFloat<uint64_t> BlockMass::toFloat() const { UnsignedFloat<uint64_t> BlockMass::toFloat() const {
if (isFull()) if (isFull())
return UnsignedFloat<uint64_t>(1, 0); return UnsignedFloat<uint64_t>(1, 0);