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Support: Extract ScaledNumbers::matchScale()

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211531 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Duncan P. N. Exon Smith
2014-06-23 20:40:45 +00:00
parent 138222e56f
commit 0da818cdbb
3 changed files with 119 additions and 46 deletions
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51
include/llvm/Support/ScaledNumber.h
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@@ -24,6 +24,7 @@
#include "llvm/Support/MathExtras.h"
#include <algorithm>
#include <cstdint>
#include <limits>
#include <utility>
@@ -263,6 +264,56 @@ int compare(DigitsT LDigits, int16_t LScale, DigitsT RDigits, int16_t RScale) {
return -compareImpl(RDigits, LDigits, LScale - RScale);
}
/// \brief Match scales of two numbers.
///
/// Given two scaled numbers, match up their scales. Change the digits and
/// scales in place. Shift the digits as necessary to form equivalent numbers,
/// losing precision only when necessary.
///
/// If the output value of \c LDigits (\c RDigits) is \c 0, the output value of
/// \c LScale (\c RScale) is unspecified. If both \c LDigits and \c RDigits
/// are \c 0, the output value is one of \c LScale and \c RScale; which is
/// unspecified.
template <class DigitsT>
void matchScales(DigitsT &LDigits, int16_t &LScale, DigitsT &RDigits,
int16_t &RScale) {
static_assert(!std::numeric_limits<DigitsT>::is_signed, "expected unsigned");
if (LScale < RScale) {
// Swap arguments.
matchScales(RDigits, RScale, LDigits, LScale);
return;
}
if (!LDigits || !RDigits || LScale == RScale)
return;
// Now LScale > RScale. Get the difference.
int32_t ScaleDiff = int32_t(LScale) - RScale;
if (ScaleDiff >= 2 * getWidth<DigitsT>()) {
// Don't bother shifting. RDigits will get zero-ed out anyway.
RDigits = 0;
return;
}
// Shift LDigits left as much as possible, then shift RDigits right.
int32_t ShiftL = std::min<int32_t>(countLeadingZeros(LDigits), ScaleDiff);
assert(ShiftL < getWidth<DigitsT>() && "can't shift more than width");
int32_t ShiftR = ScaleDiff - ShiftL;
if (ShiftR >= getWidth<DigitsT>()) {
// Don't bother shifting. RDigits will get zero-ed out anyway.
RDigits = 0;
return;
}
LDigits <<= ShiftL;
RDigits >>= ShiftR;
LScale -= ShiftL;
RScale += ShiftR;
assert(LScale == RScale && "scales should match");
}
} // end namespace ScaledNumbers
} // end namespace llvm