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Merge pull request #4277 from ethereum/signedRightShift

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Signed Right Shift: Additional test and more explanation.
This commit is contained in:
Daniel Kirchner
2018-06-12 17:19:20 +02:00
committed by GitHub
parent 2c8eca5dcf 510f227bd7
commit e2f4a9fcf4
2 changed files with 40 additions and 2 deletions
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libsolidity/ast/Types.cpp
+7 -2
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@@ -1088,8 +1088,13 @@ TypePointer RationalNumberType::binaryOperatorResult(Token::Value _operator, Typ
else
{
if (m_value.numerator() < 0)
// add 1 to the negative value before dividing to get a result that is strictly too large
// subtract 1 afterwards to round towards negative infinity
// Add 1 to the negative value before dividing to get a result that is strictly too large,
// then subtract 1 afterwards to round towards negative infinity.
// This is the same algorithm as used in ExpressionCompiler::appendShiftOperatorCode(...).
// To see this note that for negative x, xor(x,all_ones) = (-x-1) and
// therefore xor(div(xor(x,all_ones), exp(2, shift_amount)), all_ones) is
// -(-x - 1) / 2^shift_amount - 1, which is the same as
// (x + 1) / 2^shift_amount - 1.
value = rational((m_value.numerator() + 1) / boost::multiprecision::pow(bigint(2), exponent) - bigint(1), 1);
else
value = rational(m_value.numerator() / boost::multiprecision::pow(bigint(2), exponent), 1);