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Use proper SAR for signed right shifts and emulate on pre-constantinople.

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This commit is contained in:
Daniel Kirchner
2018-06-12 09:32:19 +01:00
committed by Alex Beregszaszi
parent 8999a2f375
commit f33dc92cbd
5 changed files with 152 additions and 70 deletions
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libsolidity/codegen/ExpressionCompiler.cpp
+21 -4
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@@ -1737,11 +1737,28 @@ void ExpressionCompiler::appendShiftOperatorCode(Token::Value _operator, Type co
m_context << u256(2) << Instruction::EXP << Instruction::MUL;
break;
case Token::SAR:
// NOTE: SAR rounds differently than SDIV
if (m_context.evmVersion().hasBitwiseShifting() && !c_valueSigned)
m_context << Instruction::SHR;
if (m_context.evmVersion().hasBitwiseShifting())
m_context << (c_valueSigned ? Instruction::SAR : Instruction::SHR);
else
m_context << u256(2) << Instruction::EXP << Instruction::SWAP1 << (c_valueSigned ? Instruction::SDIV : Instruction::DIV);
{
if (c_valueSigned)
// For negative values xor_mask has all bits set and xor(value_to_shift, xor_mask) will be
// the bitwise complement of value_to_shift, i.e. abs(value_to_shift) - 1. Dividing this by
// exp(2, shift_amount) results in a value that is positive and strictly smaller than the
// absolute value of the desired result. Taking the complement again changes the sign
// back to negative and subtracts one, resulting in rounding towards negative infinity.
// For positive values xor_mask is zero and xor(value_to_shift, xor_mask) is again value_to_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
m_context.appendInlineAssembly(R"({
value_to_shift := div(value_to_shift, exp(2, shift_amount))
})", {"value_to_shift", "shift_amount"});
m_context << Instruction::POP;
}
break;
case Token::SHR:
default: