solidity/libyul/optimiser/RedundantAssignEliminator.cpp

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/*
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/>.
*/
/**
* Optimiser component that removes assignments to variables that are not used
* until they go out of scope or are re-assigned.
*/
#include <libyul/optimiser/RedundantAssignEliminator.h>
#include <libyul/optimiser/Semantics.h>
#include <libyul/AsmData.h>
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#include <libdevcore/CommonData.h>
#include <boost/range/algorithm_ext/erase.hpp>
using namespace std;
using namespace dev;
using namespace yul;
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void RedundantAssignEliminator::run(OptimiserStepContext& _context, Block& _ast)
{
RedundantAssignEliminator rae{_context.dialect};
rae(_ast);
AssignmentRemover remover{rae.m_pendingRemovals};
remover(_ast);
}
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void RedundantAssignEliminator::operator()(Identifier const& _identifier)
{
changeUndecidedTo(_identifier.name, State::Used);
}
void RedundantAssignEliminator::operator()(VariableDeclaration const& _variableDeclaration)
{
ASTWalker::operator()(_variableDeclaration);
for (auto const& var: _variableDeclaration.variables)
m_declaredVariables.emplace(var.name);
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}
void RedundantAssignEliminator::operator()(Assignment const& _assignment)
{
visit(*_assignment.value);
for (auto const& var: _assignment.variableNames)
changeUndecidedTo(var.name, State::Unused);
if (_assignment.variableNames.size() == 1)
// Default-construct it in "Undecided" state if it does not yet exist.
m_assignments[_assignment.variableNames.front().name][&_assignment];
}
void RedundantAssignEliminator::operator()(If const& _if)
{
visit(*_if.condition);
TrackedAssignments skipBranch{m_assignments};
(*this)(_if.body);
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merge(m_assignments, move(skipBranch));
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}
void RedundantAssignEliminator::operator()(Switch const& _switch)
{
visit(*_switch.expression);
TrackedAssignments const preState{m_assignments};
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bool hasDefault = false;
vector<TrackedAssignments> branches;
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for (auto const& c: _switch.cases)
{
if (!c.value)
hasDefault = true;
(*this)(c.body);
branches.emplace_back(move(m_assignments));
m_assignments = preState;
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}
if (hasDefault)
{
m_assignments = move(branches.back());
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branches.pop_back();
}
for (auto& branch: branches)
merge(m_assignments, move(branch));
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}
void RedundantAssignEliminator::operator()(FunctionDefinition const& _functionDefinition)
{
std::set<YulString> outerDeclaredVariables;
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std::set<YulString> outerReturnVariables;
TrackedAssignments outerAssignments;
ForLoopInfo forLoopInfo;
swap(m_declaredVariables, outerDeclaredVariables);
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swap(m_returnVariables, outerReturnVariables);
swap(m_assignments, outerAssignments);
swap(m_forLoopInfo, forLoopInfo);
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for (auto const& retParam: _functionDefinition.returnVariables)
m_returnVariables.insert(retParam.name);
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(*this)(_functionDefinition.body);
for (auto const& param: _functionDefinition.parameters)
finalize(param.name, State::Unused);
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for (auto const& retParam: _functionDefinition.returnVariables)
finalize(retParam.name, State::Used);
swap(m_declaredVariables, outerDeclaredVariables);
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swap(m_returnVariables, outerReturnVariables);
swap(m_assignments, outerAssignments);
swap(m_forLoopInfo, forLoopInfo);
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}
void RedundantAssignEliminator::operator()(ForLoop const& _forLoop)
{
ForLoopInfo outerForLoopInfo;
swap(outerForLoopInfo, m_forLoopInfo);
++m_forLoopNestingDepth;
// If the pre block was not empty,
// we would have to deal with more complicated scoping rules.
assertThrow(_forLoop.pre.statements.empty(), OptimizerException, "");
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// We just run the loop twice to account for the back edge.
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// There need not be more runs because we only have three different states.
visit(*_forLoop.condition);
TrackedAssignments zeroRuns{m_assignments};
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(*this)(_forLoop.body);
merge(m_assignments, move(m_forLoopInfo.pendingContinueStmts));
m_forLoopInfo.pendingContinueStmts = {};
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(*this)(_forLoop.post);
visit(*_forLoop.condition);
if (m_forLoopNestingDepth < 6)
{
// Do the second run only for small nesting depths to avoid horrible runtime.
TrackedAssignments oneRun{m_assignments};
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(*this)(_forLoop.body);
merge(m_assignments, move(m_forLoopInfo.pendingContinueStmts));
m_forLoopInfo.pendingContinueStmts.clear();
(*this)(_forLoop.post);
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visit(*_forLoop.condition);
// Order of merging does not matter because "max" is commutative and associative.
merge(m_assignments, move(oneRun));
}
else
{
// Shortcut to avoid horrible runtime:
// Change all assignments that were newly introduced in the for loop to "used".
// We do not have to do that with the "break" or "continue" paths, because
// they will be joined later anyway.
// TODO parallel traversal might be more efficient here.
for (auto& var: m_assignments)
for (auto& assignment: var.second)
{
auto zeroIt = zeroRuns.find(var.first);
if (zeroIt != zeroRuns.end() && zeroIt->second.count(assignment.first))
continue;
assignment.second = State::Value::Used;
}
}
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// Order of merging does not matter because "max" is commutative and associative.
merge(m_assignments, move(zeroRuns));
merge(m_assignments, move(m_forLoopInfo.pendingBreakStmts));
m_forLoopInfo.pendingBreakStmts.clear();
// Restore potential outer for-loop states.
swap(m_forLoopInfo, outerForLoopInfo);
--m_forLoopNestingDepth;
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}
void RedundantAssignEliminator::operator()(Break const&)
{
m_forLoopInfo.pendingBreakStmts.emplace_back(move(m_assignments));
m_assignments.clear();
}
void RedundantAssignEliminator::operator()(Continue const&)
{
m_forLoopInfo.pendingContinueStmts.emplace_back(move(m_assignments));
m_assignments.clear();
}
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void RedundantAssignEliminator::operator()(Leave const&)
{
for (YulString name: m_returnVariables)
changeUndecidedTo(name, State::Used);
}
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void RedundantAssignEliminator::operator()(Block const& _block)
{
set<YulString> outerDeclaredVariables;
swap(m_declaredVariables, outerDeclaredVariables);
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ASTWalker::operator()(_block);
for (auto const& var: m_declaredVariables)
finalize(var, State::Unused);
swap(m_declaredVariables, outerDeclaredVariables);
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}
template <class K, class V, class F>
void joinMap(std::map<K, V>& _a, std::map<K, V>&& _b, F _conflictSolver)
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{
// TODO Perhaps it is better to just create a sorted list
// and then use insert(begin, end)
auto ita = _a.begin();
auto aend = _a.end();
auto itb = _b.begin();
auto bend = _b.end();
for (; itb != bend; ++ita)
{
if (ita == aend)
ita = _a.insert(ita, std::move(*itb++));
else if (ita->first < itb->first)
continue;
else if (itb->first < ita->first)
ita = _a.insert(ita, std::move(*itb++));
else
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{
_conflictSolver(ita->second, std::move(itb->second));
++itb;
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}
}
}
void RedundantAssignEliminator::merge(TrackedAssignments& _target, TrackedAssignments&& _other)
{
joinMap(_target, move(_other), [](
map<Assignment const*, State>& _assignmentHere,
map<Assignment const*, State>&& _assignmentThere
)
{
return joinMap(_assignmentHere, move(_assignmentThere), State::join);
});
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}
void RedundantAssignEliminator::merge(TrackedAssignments& _target, vector<TrackedAssignments>&& _source)
{
for (TrackedAssignments& ts: _source)
merge(_target, move(ts));
_source.clear();
}
void RedundantAssignEliminator::changeUndecidedTo(YulString _variable, RedundantAssignEliminator::State _newState)
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{
for (auto& assignment: m_assignments[_variable])
if (assignment.second == State::Undecided)
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assignment.second = _newState;
}
void RedundantAssignEliminator::finalize(YulString _variable, RedundantAssignEliminator::State _finalState)
{
finalize(m_assignments, _variable, _finalState);
for (auto& assignments: m_forLoopInfo.pendingBreakStmts)
finalize(assignments, _variable, _finalState);
for (auto& assignments: m_forLoopInfo.pendingContinueStmts)
finalize(assignments, _variable, _finalState);
}
void RedundantAssignEliminator::finalize(
TrackedAssignments& _assignments,
YulString _variable,
RedundantAssignEliminator::State _finalState
)
{
for (auto const& assignment: _assignments[_variable])
{
State const state = assignment.second == State::Undecided ? _finalState : assignment.second;
if (state == State::Unused && SideEffectsCollector{*m_dialect, *assignment.first->value}.movable())
// TODO the only point where we actually need this
// to be a set is for the for loop
m_pendingRemovals.insert(assignment.first);
}
_assignments.erase(_variable);
}
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void AssignmentRemover::operator()(Block& _block)
{
boost::range::remove_erase_if(_block.statements, [=](Statement const& _statement) -> bool {
return holds_alternative<Assignment>(_statement) && m_toRemove.count(&std::get<Assignment>(_statement));
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});
ASTModifier::operator()(_block);
}