[YulOpt] Implement loop-invariant code motion

libyul/optimiser/LoopInvariantCodeMotion.h

@@ -0,0 +1,67 @@
+/*
+	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/>.
+*/
+#pragma once
+
+#include <libyul/optimiser/ASTWalker.h>
+#include <libyul/optimiser/Semantics.h>
+#include <libyul/optimiser/OptimiserStep.h>
+
+namespace yul
+{
+
+/**
+ * Loop-invariant code motion.
+ *
+ * This optimization moves movable SSA variable declarations outside the loop.
+ *
+ * Only statements at the top level in a loop's body or post block are considered, i.e variable
+ * declarations inside conditional branches will not be moved out of the loop.
+ *
+ * Requirements:
+ * - The Disambiguator, ForLoopInitRewriter and FunctionHoister must be run upfront.
+ * - Expression splitter and SSA transform should be run upfront to obtain better result.
+ */
+
+class LoopInvariantCodeMotion: public ASTModifier
+{
+public:
+	static constexpr char const* name{"LoopInvariantCodeMotion"};
+	static void run(OptimiserStepContext& _context, Block& _ast);
+
+	void operator()(Block& _block) override;
+
+private:
+	explicit LoopInvariantCodeMotion(
+		Dialect const& _dialect,
+		std::set<YulString> const& _ssaVariables,
+		std::map<YulString, SideEffects> const& _functionSideEffects
+	):
+		m_dialect(_dialect),
+		m_ssaVariables(_ssaVariables),
+		m_functionSideEffects(_functionSideEffects)
+	{ }
+
+	/// @returns true if the given variable declaration can be moved to in front of the loop.
+	bool canBePromoted(VariableDeclaration const& _varDecl, std::set<YulString> const& _varsDefinedInCurrentScope) const;
+	std::optional<std::vector<Statement>> rewriteLoop(ForLoop& _for);
+
+	Dialect const& m_dialect;
+	std::set<YulString> const& m_ssaVariables;
+	std::map<YulString, SideEffects> const& m_functionSideEffects;
+};
+
+}