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Improved bitsFromValue implementation.

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This commit is contained in:
chriseth 2019-03-25 18:22:15 +01:00
parent 7fea159d27
commit cfe630c717
1 changed files with 11 additions and 6 deletions
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17
libyul/optimiser/ValueConstraintBasedSimplifier.cpp
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@ -180,15 +180,20 @@ ValueConstraint ValueConstraint::valueFromBits(u256 _minBits, u256 _maxBits)
ValueConstraint ValueConstraint::bitsFromValue(u256 _minValue, u256 _maxValue)
{
if (_minValue == _maxValue)
return constant(_minValue);
int highestBit = highestBitSet(_maxValue);
// Summary of algorithm: Retain the comon higher order bit prefix between
// _minValue and _maxValue and starting from the point of divergence,
// set all following _maxBits bits to 1, and all following _minBits bits to 0.
// Reasoning: If we count up from _minValue to _maxValue, then the common upper
// bits will not change.
int divergence = highestBitSet(_minValue ^ _maxValue);
u256 lowerBits = u256(-1) >> (255 - divergence);
u256 minBits = _minValue & (~lowerBits);
u256 maxBits = _maxValue | lowerBits;
return ValueConstraint{
move(_minValue),
move(_maxValue),
0,
u256(-1) >> (255 - highestBit)
move(minBits),
move(maxBits)
};
}