solidity/libyul/optimiser/Semantics.h

/*
	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/>.
*/
// SPDX-License-Identifier: GPL-3.0
/**
 * Specific AST walkers that collect semantical facts.
 */

#pragma once

#include <libyul/AST.h>
#include <libyul/Object.h>
#include <libyul/SideEffects.h>
#include <libyul/optimiser/ASTWalker.h>
#include <libyul/optimiser/CallGraphGenerator.h>

#include <set>

namespace solidity::yul
{
struct Dialect;

/**
 * Specific AST walker that determines side-effect free-ness and movability of code.
 * Enters into function definitions.
 */
class SideEffectsCollector: public ASTWalker
{
public:
	explicit SideEffectsCollector(
		Dialect const& _dialect,
		std::map<YulString, SideEffects> const* _functionSideEffects = nullptr
	): m_dialect(_dialect), m_functionSideEffects(_functionSideEffects) {}
	SideEffectsCollector(
		Dialect const& _dialect,
		Expression const& _expression,
		std::map<YulString, SideEffects> const* _functionSideEffects = nullptr
	);
	SideEffectsCollector(Dialect const& _dialect, Statement const& _statement);
	SideEffectsCollector(
		Dialect const& _dialect,
		Block const& _ast,
		std::map<YulString, SideEffects> const* _functionSideEffects = nullptr
	);
	SideEffectsCollector(
		Dialect const& _dialect,
		ForLoop const& _ast,
		std::map<YulString, SideEffects> const* _functionSideEffects = nullptr
	);

	using ASTWalker::operator();
	void operator()(FunctionCall const& _functionCall) override;

	bool movable() const { return m_sideEffects.movable; }

	bool movableRelativeTo(SideEffects const& _other, bool _codeContainsMSize)
	{
		if (!m_sideEffects.cannotLoop)
			return false;

		if (m_sideEffects.movable)
			return true;

		if (
			!m_sideEffects.movableApartFromEffects ||
			m_sideEffects.storage == SideEffects::Write ||
			m_sideEffects.otherState == SideEffects::Write ||
			m_sideEffects.memory == SideEffects::Write
		)
			return false;

		if (m_sideEffects.otherState == SideEffects::Read)
			if (_other.otherState == SideEffects::Write)
				return false;

		if (m_sideEffects.storage == SideEffects::Read)
			if (_other.storage == SideEffects::Write)
				return false;

		if (m_sideEffects.memory == SideEffects::Read)
			if (_codeContainsMSize || _other.memory == SideEffects::Write)
				return false;

		return true;
	}

	bool canBeRemoved(bool _allowMSizeModification = false) const
	{
		if (_allowMSizeModification)
			return m_sideEffects.canBeRemovedIfNoMSize;
		else
			return m_sideEffects.canBeRemoved;
	}
	bool cannotLoop() const { return m_sideEffects.cannotLoop; }
	bool invalidatesStorage() const { return m_sideEffects.storage == SideEffects::Write; }
	bool invalidatesMemory() const { return m_sideEffects.memory == SideEffects::Write; }

	SideEffects sideEffects() { return m_sideEffects; }

private:
	Dialect const& m_dialect;
	std::map<YulString, SideEffects> const* m_functionSideEffects = nullptr;
	SideEffects m_sideEffects;
};

/**
 * This class can be used to determine the side-effects of user-defined functions.
 *
 * It is given a dialect and a mapping that represents the direct calls from user-defined
 * functions to other user-defined functions and built-in functions.
 */
class SideEffectsPropagator
{
public:
	static std::map<YulString, SideEffects> sideEffects(
		Dialect const& _dialect,
		CallGraph const& _directCallGraph
	);
};

/**
 * Class that can be used to find out if certain code contains the MSize instruction
 * or a verbatim bytecode builtin (which is always assumed that it could contain MSize).
 *
 * Note that this is a purely syntactic property meaning that even if this is false,
 * the code can still contain calls to functions that contain the msize instruction.
 *
 * The only safe way to determine this is by passing the full AST.
 */
class MSizeFinder: public ASTWalker
{
public:
	static bool containsMSize(Dialect const& _dialect, Block const& _ast);
	static bool containsMSize(Dialect const& _dialect, Object const& _object);

	using ASTWalker::operator();
	void operator()(FunctionCall const& _funCall) override;

private:
	MSizeFinder(Dialect const& _dialect): m_dialect(_dialect) {}
	Dialect const& m_dialect;
	bool m_msizeFound = false;
};

/**
 * Class that can be used to find out if the given function contains the ``leave`` statement.
 *
 * Returns true even in the case where the function definition contains another function definition
 * that contains the leave statement.
 */
class LeaveFinder: public ASTWalker
{
public:
	static bool containsLeave(FunctionDefinition const& _fun)
	{
		LeaveFinder f;
		f(_fun);
		return f.m_leaveFound;
	}

	using ASTWalker::operator();
	void operator()(Leave const&) override { m_leaveFound = true; }

private:
	LeaveFinder() = default;

	bool m_leaveFound = false;
};

/**
 * Specific AST walker that determines whether an expression is movable
 * and collects the referenced variables.
 * Can only be used on expressions.
 */
class MovableChecker: public SideEffectsCollector
{
public:
	explicit MovableChecker(
		Dialect const& _dialect,
		std::map<YulString, SideEffects> const* _functionSideEffects = nullptr
	): SideEffectsCollector(_dialect, _functionSideEffects) {}
	MovableChecker(Dialect const& _dialect, Expression const& _expression);

	void operator()(Identifier const& _identifier) override;

	/// Disallow visiting anything apart from Expressions (this throws).
	void visit(Statement const&) override;
	using ASTWalker::visit;

	std::set<YulString> const& referencedVariables() const { return m_variableReferences; }

private:
	/// Which variables the current expression references.
	std::set<YulString> m_variableReferences;
};

struct ControlFlowSideEffects;

/**
 * Helper class to find "irregular" control flow.
 * This includes termination, break, continue and leave.
 * In general, it is applied only to "simple" statements. The control-flow
 * of loops, switches and if statements is always "FlowOut" with the assumption
 * that the caller will descend into them.
 */
class TerminationFinder
{
public:
	/// "Terminate" here means that there is no continuing control-flow.
	/// If this is applied to a function that can revert or stop, but can also
	/// exit regularly, the property is set to "FlowOut".
	enum class ControlFlow { FlowOut, Break, Continue, Terminate, Leave };

	TerminationFinder(
		Dialect const& _dialect,
		std::map<YulString, ControlFlowSideEffects> const* _functionSideEffects = nullptr
	): m_dialect(_dialect), m_functionSideEffects(_functionSideEffects) {}

	/// @returns the index of the first statement in the provided sequence
	/// that is an unconditional ``break``, ``continue``, ``leave`` or a
	/// call to a terminating function.
	/// If control flow can continue at the end of the list,
	/// returns `FlowOut` and ``size_t(-1)``.
	/// The function might return ``FlowOut`` even though control
	/// flow cannot actually continue.
	std::pair<ControlFlow, size_t> firstUnconditionalControlFlowChange(
		std::vector<Statement> const& _statements
	);

	/// @returns the control flow type of the given statement.
	/// This function could return FlowOut even if control flow never continues.
	ControlFlow controlFlowKind(Statement const& _statement);

	/// @returns true if the expression contains a
	/// call to a terminating function, i.e. a function that does not have
	/// a regular "flow out" control-flow (it might also be recursive).
	bool containsNonContinuingFunctionCall(Expression const& _expr);

private:
	Dialect const& m_dialect;
	std::map<YulString, ControlFlowSideEffects> const* m_functionSideEffects;
};

}