[Sol - Yul] Implement checked multiplication.

libsolidity/codegen/YulUtilFunctions.cpp

@@ -346,6 +346,34 @@ string YulUtilFunctions::overflowCheckedUIntAddFunction(size_t _bits)
 	});
 }
 
+string YulUtilFunctions::overflowCheckedUIntMulFunction(size_t _bits)
+{
+	solAssert(0 < _bits && _bits <= 256 && _bits % 8 == 0, "");
+	string functionName = "checked_mul_uint_" + to_string(_bits);
+	return m_functionCollector->createFunction(functionName, [&]() {
+		return
+			// - The current overflow check *before* the multiplication could
+			//   be replaced by the following check *after* the multiplication:
+			//   if and(iszero(iszero(x)), iszero(eq(div(product, x), y))) { revert(0, 0) }
+			// - The case the x equals 0 could be treated separately and directly return zero.
+			Whiskers(R"(
+			function <functionName>(x, y) -> product {
+				if and(iszero(iszero(x)), lt(div(<mask>, x), y)) { revert(0, 0) }
+				<?shortType>
+					product := mulmod(x, y, <powerOfTwo>)
+				<!shortType>
+					product := mul(x, y)
+				</shortType>
+			}
+			)")
+				("shortType", _bits < 256)
+				("functionName", functionName)
+				("powerOfTwo", toCompactHexWithPrefix(u256(1) << _bits))
+				("mask", toCompactHexWithPrefix((u256(1) << _bits) - 1))
+				.render();
+	});
+}
+
 string YulUtilFunctions::overflowCheckedUIntSubFunction()
 {
 	string functionName = "checked_sub_uint";

libsolidity/codegen/YulUtilFunctions.h

@@ -86,6 +86,8 @@ public:
 
 	std::string overflowCheckedUIntAddFunction(size_t _bits);
 
+	std::string overflowCheckedUIntMulFunction(size_t _bits);
+
 	/// @returns computes the difference between two values.
 	/// Assumes the input to be in range for the type.
 	std::string overflowCheckedUIntSubFunction();

libsolidity/codegen/ir/IRGeneratorForStatements.cpp

@@ -304,6 +304,8 @@ bool IRGeneratorForStatements::visit(BinaryOperation const& _binOp)
 				fun = m_utils.overflowCheckedUIntAddFunction(type->numBits());
 			else if (_binOp.getOperator() == Token::Sub)
 				fun = m_utils.overflowCheckedUIntSubFunction();
+			else if (_binOp.getOperator() == Token::Mul)
+				fun = m_utils.overflowCheckedUIntMulFunction(type->numBits());
 			else
 				solUnimplementedAssert(false, "");
 			defineExpression(_binOp) << fun << "(" << left << ", " << right << ")\n";