path: root/libyul/optimiser/DataFlowAnalyzer.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/>.
*/
/**
 * Base class to perform data flow analysis during AST walks.
 * Tracks assignments and is used as base class for both Rematerialiser and
 * Common Subexpression Eliminator.
 */

#include <libyul/optimiser/DataFlowAnalyzer.h>

#include <libyul/optimiser/NameCollector.h>
#include <libyul/optimiser/Semantics.h>
#include <libyul/Exceptions.h>
#include <libyul/AsmData.h>

#include <libdevcore/CommonData.h>

#include <boost/range/adaptor/reversed.hpp>

using namespace std;
using namespace dev;
using namespace yul;

void DataFlowAnalyzer::operator()(Assignment& _assignment)
{
    set<YulString> names;
    for (auto const& var: _assignment.variableNames)
        names.emplace(var.name);
    assertThrow(_assignment.value, OptimizerException, "");
    visit(*_assignment.value);
    handleAssignment(names, _assignment.value.get());
}

void DataFlowAnalyzer::operator()(VariableDeclaration& _varDecl)
{
    set<YulString> names;
    for (auto const& var: _varDecl.variables)
        names.emplace(var.name);
    m_variableScopes.back().variables += names;
    if (_varDecl.value)
        visit(*_varDecl.value);
    handleAssignment(names, _varDecl.value.get());
}

void DataFlowAnalyzer::operator()(If& _if)
{
    ASTModifier::operator()(_if);

    Assignments assignments;
    assignments(_if.body);
    clearValues(assignments.names());
}

void DataFlowAnalyzer::operator()(Switch& _switch)
{
    visit(*_switch.expression);
    set<YulString> assignedVariables;
    for (auto& _case: _switch.cases)
    {
        (*this)(_case.body);
        Assignments assignments;
        assignments(_case.body);
        assignedVariables += assignments.names();
        // This is a little too destructive, we could retain the old values.
        clearValues(assignments.names());
    }
    clearValues(assignedVariables);
}

void DataFlowAnalyzer::operator()(FunctionDefinition& _fun)
{
    // Save all information. We might rather reinstantiate this class,
    // but this could be difficult if it is subclassed.
    map<YulString, Expression const*> value;
    map<YulString, set<YulString>> references;
    map<YulString, set<YulString>> referencedBy;
    m_value.swap(value);
    m_references.swap(references);
    m_referencedBy.swap(referencedBy);
    pushScope(true);

    for (auto const& parameter: _fun.parameters)
        m_variableScopes.back().variables.emplace(parameter.name);
    for (auto const& var: _fun.returnVariables)
        m_variableScopes.back().variables.emplace(var.name);
    ASTModifier::operator()(_fun);

    popScope();
    m_value.swap(value);
    m_references.swap(references);
    m_referencedBy.swap(referencedBy);
}

void DataFlowAnalyzer::operator()(ForLoop& _for)
{
    // Special scope handling of the pre block.
    pushScope(false);
    for (auto& statement: _for.pre.statements)
        visit(statement);

    Assignments assignments;
    assignments(_for.body);
    assignments(_for.post);
    clearValues(assignments.names());

    visit(*_for.condition);
    (*this)(_for.body);
    (*this)(_for.post);

    clearValues(assignments.names());
    popScope();
}

void DataFlowAnalyzer::operator()(Block& _block)
{
    size_t numScopes = m_variableScopes.size();
    pushScope(false);
    ASTModifier::operator()(_block);
    popScope();
    assertThrow(numScopes == m_variableScopes.size(), OptimizerException, "");
}

void DataFlowAnalyzer::handleAssignment(set<YulString> const& _variables, Expression* _value)
{
    clearValues(_variables);

    MovableChecker movableChecker;
    if (_value)
        movableChecker.visit(*_value);
    if (_variables.size() == 1)
    {
        YulString name = *_variables.begin();
        // Expression has to be movable and cannot contain a reference
        // to the variable that will be assigned to.
        if (_value && movableChecker.movable() && !movableChecker.referencedVariables().count(name))
            m_value[name] = _value;
    }

    auto const& referencedVariables = movableChecker.referencedVariables();
    for (auto const& name: _variables)
    {
        m_references[name] = referencedVariables;
        for (auto const& ref: referencedVariables)
            m_referencedBy[ref].emplace(name);
    }
}

void DataFlowAnalyzer::pushScope(bool _functionScope)
{
    m_variableScopes.emplace_back(_functionScope);
}

void DataFlowAnalyzer::popScope()
{
    clearValues(std::move(m_variableScopes.back().variables));
    m_variableScopes.pop_back();
}

void DataFlowAnalyzer::clearValues(set<YulString> _variables)
{
    // All variables that reference variables to be cleared also have to be
    // cleared, but not recursively, since only the value of the original
    // variables changes. Example:
    // let a := 1
    // let b := a
    // let c := b
    // let a := 2
    // add(b, c)
    // In the last line, we can replace c by b, but not b by a.
    //
    // This cannot be easily tested since the substitutions will be done
    // one by one on the fly, and the last line will just be add(1, 1)

    // Clear variables that reference variables to be cleared.
    for (auto const& name: _variables)
        for (auto const& ref: m_referencedBy[name])
            _variables.emplace(ref);

    // Clear the value and update the reference relation.
    for (auto const& name: _variables)
        m_value.erase(name);
    for (auto const& name: _variables)
    {
        for (auto const& ref: m_references[name])
            m_referencedBy[ref].erase(name);
        m_references[name].clear();
    }
}

bool DataFlowAnalyzer::inScope(YulString _variableName) const
{
    for (auto const& scope: m_variableScopes | boost::adaptors::reversed)
    {
        if (scope.variables.count(_variableName))
            return true;
        if (scope.isFunction)
            return false;
    }
    return false;
}