1 files changed, 29 insertions, 4 deletions
diff --git a/libsolidity/codegen/ExpressionCompiler.cpp b/libsolidity/codegen/ExpressionCompiler.cpp
index 93d440c8..0470c3ec 100644
--- a/libsolidity/codegen/ExpressionCompiler.cpp
+++ b/libsolidity/codegen/ExpressionCompiler.cpp
@@ -1737,11 +1737,36 @@ 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)
+				// In the following assembly snippet, xor_mask will be zero, if value_to_shift is positive.
+				// Therefor 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
+				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: