1 files changed, 117 insertions, 0 deletions
diff --git a/libyul/optimiser/UnusedPruner.cpp b/libyul/optimiser/UnusedPruner.cpp
new file mode 100644
index 00000000..10b1c171
--- /dev/null
+++ b/libyul/optimiser/UnusedPruner.cpp
@@ -0,0 +1,117 @@
+/*(
+	This file is part of solidity.
+
+	solidity is free software: you can redistribute it and/or modify
+	it under the terms of the GNU General Public License as published by
+	the Free Software Foundation, either version 3 of the License, or
+	(at your option) any later version.
+
+	solidity is distributed in the hope that it will be useful,
+	but WITHOUT ANY WARRANTY; without even the implied warranty of
+	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+	GNU General Public License for more details.
+
+	You should have received a copy of the GNU General Public License
+	along with solidity.  If not, see <http://www.gnu.org/licenses/>.
+*/
+/**
+ * Optimisation stage that removes unused variables and functions.
+ */
+
+#include <libyul/optimiser/UnusedPruner.h>
+
+#include <libyul/optimiser/NameCollector.h>
+#include <libyul/optimiser/Semantics.h>
+#include <libyul/optimiser/Utilities.h>
+#include <libyul/Exceptions.h>
+
+#include <libsolidity/inlineasm/AsmData.h>
+
+#include <boost/algorithm/cxx11/none_of.hpp>
+
+using namespace std;
+using namespace dev;
+using namespace dev::julia;
+
+UnusedPruner::UnusedPruner(Block& _ast)
+{
+	ReferencesCounter counter;
+	counter(_ast);
+
+	m_references = counter.references();
+}
+
+void UnusedPruner::operator()(Block& _block)
+{
+	for (auto&& statement: _block.statements)
+		if (statement.type() == typeid(FunctionDefinition))
+		{
+			FunctionDefinition& funDef = boost::get<FunctionDefinition>(statement);
+			if (!used(funDef.name))
+			{
+				subtractReferences(ReferencesCounter::countReferences(funDef.body));
+				statement = Block{std::move(funDef.location), {}};
+			}
+		}
+		else if (statement.type() == typeid(VariableDeclaration))
+		{
+			VariableDeclaration& varDecl = boost::get<VariableDeclaration>(statement);
+			// Multi-variable declarations are special. We can only remove it
+			// if all vairables are unused and the right-hand-side is either
+			// movable or it return a single value. In the latter case, we
+			// replace `let a := f()` by `pop(f())` (in pure Yul, this will be
+			// `drop(f())`).
+			if (boost::algorithm::none_of(
+				varDecl.variables,
+				[=](TypedName const& _typedName) { return used(_typedName.name); }
+			))
+			{
+				if (!varDecl.value)
+					statement = Block{std::move(varDecl.location), {}};
+				else if (MovableChecker(*varDecl.value).movable())
+				{
+					subtractReferences(ReferencesCounter::countReferences(*varDecl.value));
+					statement = Block{std::move(varDecl.location), {}};
+				}
+				else if (varDecl.variables.size() == 1)
+					// In pure Yul, this should be replaced by a function call to `drop`
+					// instead of `pop`.
+					statement = ExpressionStatement{varDecl.location, FunctionalInstruction{
+						varDecl.location,
+						solidity::Instruction::POP,
+						{*std::move(varDecl.value)}
+					}};
+			}
+		}
+
+	removeEmptyBlocks(_block);
+
+	ASTModifier::operator()(_block);
+}
+
+void UnusedPruner::runUntilStabilised(Block& _ast)
+{
+	while (true)
+	{
+		UnusedPruner pruner(_ast);
+		pruner(_ast);
+		if (!pruner.shouldRunAgain())
+			return;
+	}
+}
+
+bool UnusedPruner::used(string const& _name) const
+{
+	return m_references.count(_name) && m_references.at(_name) > 0;
+}
+
+void UnusedPruner::subtractReferences(map<string, size_t> const& _subtrahend)
+{
+	for (auto const& ref: _subtrahend)
+	{
+		assertThrow(m_references.count(ref.first), OptimizerException, "");
+		assertThrow(m_references.at(ref.first) >= ref.second, OptimizerException, "");
+		m_references[ref.first] -= ref.second;
+		m_shouldRunAgain = true;
+	}
+}