Method appendShiftOperatorCode

libsolidity/codegen/ExpressionCompiler.cpp:2646–2715

}

void ExpressionCompiler::appendShiftOperatorCode(Token _operator, Type const& _valueType, Type const& _shiftAmountType)
{
	// stack: shift_amount value_to_shift

	bool c_valueSigned = false;
	if (auto valueType = dynamic_cast<IntegerType const*>(&_valueType))
		c_valueSigned = valueType->isSigned();
	else
		solAssert(dynamic_cast<FixedBytesType const*>(&_valueType), "Only integer and fixed bytes type supported for shifts.");

	// The amount can be a RationalNumberType too.
	if (auto amountType = dynamic_cast<RationalNumberType const*>(&_shiftAmountType))
	{
		// This should be handled by the type checker.
		solAssert(amountType->integerType(), "");
		solAssert(!amountType->integerType()->isSigned(), "");
	}
	else if (auto amountType = dynamic_cast<IntegerType const*>(&_shiftAmountType))
		solAssert(!amountType->isSigned(), "");
	else
		solAssert(false, "Invalid shift amount type.");

	m_context << Instruction::SWAP1;
	// stack: value_to_shift shift_amount

	switch (_operator)
	{
	case Token::SHL:
		if (m_context.evmVersion().hasBitwiseShifting())
			m_context << Instruction::SHL;
		else
			m_context << u256(2) << Instruction::EXP << Instruction::MUL;
		break;
	case Token::SAR:
		if (m_context.evmVersion().hasBitwiseShifting())
			m_context << (c_valueSigned ? Instruction::SAR : Instruction::SHR);
		else
		{
			if (c_valueSigned)
				// In the following assembly snippet, xor_mask will be zero, if value_to_shift is positive.
				// Therefore xor'ing with xor_mask is the identity and the computation reduces to
				// div(value_to_shift, exp(2, shift_amount)), which is correct, since for positive values
				// arithmetic right shift is dividing by a power of two (which, as a bitwise operation, results
				// in discarding bits on the right and filling with zeros from the left).
				// For negative values arithmetic right shift, viewed as a bitwise operation, discards bits to the
				// right and fills in ones from the left. This is achieved as follows:
				// If value_to_shift is negative, then xor_mask will have all bits set, so xor'ing with xor_mask
				// will flip all bits. First all bits in value_to_shift are flipped. As for the positive case,
				// dividing by a power of two using integer arithmetic results in discarding bits to the right
				// and filling with zeros from the left. Flipping all bits in the result again, turns all zeros
				// on the left to ones and restores the non-discarded, shifted bits to their original value (they
				// have now been flipped twice). In summary we now have discarded bits to the right and filled with
				// ones from the left, i.e. we have performed an arithmetic right shift.
				m_context.appendInlineAssembly(R"({
					let xor_mask := sub(0, slt(value_to_shift, 0))
					value_to_shift := xor(div(xor(value_to_shift, xor_mask), exp(2, shift_amount)), xor_mask)
				})", {"value_to_shift", "shift_amount"});
			else