solidity/libyul/optimiser/CommonSubexpressionEliminator.cpp

/*(
	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 replaces expressions known to be the current value of a variable
 * in scope by a reference to that variable.
 */

#include <libyul/optimiser/CommonSubexpressionEliminator.h>

#include <libyul/optimiser/SyntacticalEquality.h>
#include <libyul/optimiser/BlockHasher.h>
#include <libyul/optimiser/CallGraphGenerator.h>
#include <libyul/optimiser/Semantics.h>
#include <libyul/SideEffects.h>
#include <libyul/Exceptions.h>
#include <libyul/AST.h>
#include <libyul/Dialect.h>
#include <libyul/Utilities.h>

using namespace solidity;
using namespace solidity::yul;
using namespace solidity::util;

void CommonSubexpressionEliminator::run(OptimiserStepContext& _context, Block& _ast)
{
	CommonSubexpressionEliminator cse{
		_context.dialect,
		SideEffectsPropagator::sideEffects(_context.dialect, CallGraphGenerator::callGraph(_ast))
	};
	cse(_ast);
}

CommonSubexpressionEliminator::CommonSubexpressionEliminator(
	Dialect const& _dialect,
	std::map<YulString, SideEffects> _functionSideEffects
):
	DataFlowAnalyzer(_dialect, MemoryAndStorage::Ignore, std::move(_functionSideEffects))
{
}

void CommonSubexpressionEliminator::operator()(FunctionDefinition& _fun)
{
	ScopedSaveAndRestore returnVariables(m_returnVariables, {});
	ScopedSaveAndRestore replacementCandidates(m_replacementCandidates, {});

	for (auto const& v: _fun.returnVariables)
		m_returnVariables.insert(v.name);

	DataFlowAnalyzer::operator()(_fun);
}

void CommonSubexpressionEliminator::visit(Expression& _e)
{
	bool descend = true;
	// If this is a function call to a function that requires literal arguments,
	// do not try to simplify there.
	if (std::holds_alternative<FunctionCall>(_e))
	{
		FunctionCall& funCall = std::get<FunctionCall>(_e);

		if (BuiltinFunction const* builtin = m_dialect.builtin(funCall.functionName.name))
		{
			for (size_t i = funCall.arguments.size(); i > 0; i--)
				// We should not modify function arguments that have to be literals
				// Note that replacing the function call entirely is fine,
				// if the function call is movable.
				if (!builtin->literalArgument(i - 1))
					visit(funCall.arguments[i - 1]);

			descend = false;
		}
	}

	// We visit the inner expression first to first simplify inner expressions,
	// which hopefully allows more matches.
	// Note that the DataFlowAnalyzer itself only has code for visiting Statements,
	// so this basically invokes the AST walker directly and thus post-visiting
	// is also fine with regards to data flow analysis.
	if (descend)
		DataFlowAnalyzer::visit(_e);

	if (Identifier const* identifier = std::get_if<Identifier>(&_e))
	{
		YulString identifierName = identifier->name;
		if (AssignedValue const* assignedValue = variableValue(identifierName))
		{
			assertThrow(assignedValue->value, OptimizerException, "");
			if (Identifier const* value = std::get_if<Identifier>(assignedValue->value))
				if (inScope(value->name))
					_e = Identifier{debugDataOf(_e), value->name};
		}
	}
	else if (auto const* candidates = util::valueOrNullptr(m_replacementCandidates, _e))
		for (auto const& variable: *candidates)
			if (AssignedValue const* value = variableValue(variable))
			{
				assertThrow(value->value, OptimizerException, "");
				// Prevent using the default value of return variables
				// instead of literal zeros.
				if (
					m_returnVariables.count(variable) &&
					std::holds_alternative<Literal>(*value->value) &&
					valueOfLiteral(std::get<Literal>(*value->value)) == 0
				)
					continue;
				// We check for syntactic equality again because the value might have changed.
				if (inScope(variable) && SyntacticallyEqual{}(_e, *value->value))
				{
					_e = Identifier{debugDataOf(_e), variable};
					break;
				}
			}
}

void CommonSubexpressionEliminator::assignValue(YulString _variable, Expression const* _value)
{
	if (_value)
		m_replacementCandidates[*_value].insert(_variable);
	DataFlowAnalyzer::assignValue(_variable, _value);
}