Add option divModWithSlacks

libsolidity/formal/ModelCheckerSettings.h

@@ -112,6 +112,13 @@ struct ModelCheckerTargets
 struct ModelCheckerSettings
 {
 	ModelCheckerContracts contracts = ModelCheckerContracts::Default();
+	/// Currently division and modulo are replaced by multiplication with slack vars, such that
+	/// a / b <=> a = b * k + m
+	/// where k and m are slack variables.
+	/// This is the default because Spacer prefers that over precise / and mod.
+	/// This option allows disabling this mechanism since other solvers
+	/// might prefer the precise encoding.
+	bool divModNoSlacks = false;
 	ModelCheckerEngine engine = ModelCheckerEngine::None();
 	bool showUnproved = false;
 	smtutil::SMTSolverChoice solvers = smtutil::SMTSolverChoice::All();
@@ -123,6 +130,7 @@ struct ModelCheckerSettings
 	{
 		return
 			contracts == _other.contracts &&
+			divModNoSlacks == _other.divModNoSlacks &&
 			engine == _other.engine &&
 			showUnproved == _other.showUnproved &&
 			solvers == _other.solvers &&

libsolidity/formal/SMTEncoder.cpp

@@ -1916,6 +1916,9 @@ pair<smtutil::Expression, smtutil::Expression> SMTEncoder::divModWithSlacks(
 	IntegerType const& _type
 )
 {
+	if (m_settings.divModNoSlacks)
+		return {_left / _right, _left % _right};
+
 	IntegerType const* intType = &_type;
 	string suffix = "div_mod_" + to_string(m_context.newUniqueId());
 	smt::SymbolicIntVariable dSymb(intType, intType, "d_" + suffix, m_context);