solidity/libyul/optimiser/StackCompressor.cpp
2019-05-20 17:32:56 +02:00

171 lines
4.8 KiB
C++

/*(
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 aggressively rematerializes certain variables ina a function to free
* space on the stack until it is compilable.
*/
#include <libyul/optimiser/StackCompressor.h>
#include <libyul/optimiser/SSAValueTracker.h>
#include <libyul/optimiser/NameCollector.h>
#include <libyul/optimiser/Rematerialiser.h>
#include <libyul/optimiser/UnusedPruner.h>
#include <libyul/optimiser/Metrics.h>
#include <libyul/optimiser/Semantics.h>
#include <libyul/CompilabilityChecker.h>
#include <libyul/AsmData.h>
using namespace std;
using namespace dev;
using namespace yul;
namespace
{
/**
* Class that discovers all variables that can be fully eliminated by rematerialization,
* and the corresponding approximate costs.
*/
class RematCandidateSelector: public DataFlowAnalyzer
{
public:
explicit RematCandidateSelector(Dialect const& _dialect): DataFlowAnalyzer(_dialect) {}
/// @returns a set of pairs of rematerialisation costs and variable to rematerialise.
/// Note that this set is sorted by cost.
set<pair<size_t, YulString>> candidates()
{
set<pair<size_t, YulString>> cand;
for (auto const& codeCost: m_expressionCodeCost)
{
size_t numRef = m_numReferences[codeCost.first];
cand.emplace(make_pair(codeCost.second * numRef, codeCost.first));
}
return cand;
}
using DataFlowAnalyzer::operator();
void operator()(VariableDeclaration& _varDecl) override
{
DataFlowAnalyzer::operator()(_varDecl);
if (_varDecl.variables.size() == 1)
{
YulString varName = _varDecl.variables.front().name;
if (m_value.count(varName))
m_expressionCodeCost[varName] = CodeCost::codeCost(m_dialect, *m_value[varName]);
}
}
void operator()(Assignment& _assignment) override
{
for (auto const& var: _assignment.variableNames)
rematImpossible(var.name);
DataFlowAnalyzer::operator()(_assignment);
}
// We use visit(Expression) because operator()(Identifier) would also
// get called on left-hand-sides of assignments.
void visit(Expression& _e) override
{
if (_e.type() == typeid(Identifier))
{
YulString name = boost::get<Identifier>(_e).name;
if (m_expressionCodeCost.count(name))
{
if (!m_value.count(name))
rematImpossible(name);
else
++m_numReferences[name];
}
}
DataFlowAnalyzer::visit(_e);
}
/// Remove the variable from the candidate set.
void rematImpossible(YulString _variable)
{
m_numReferences.erase(_variable);
m_expressionCodeCost.erase(_variable);
}
/// Candidate variables and the code cost of their value.
map<YulString, size_t> m_expressionCodeCost;
/// Number of references to each candidate variable.
map<YulString, size_t> m_numReferences;
};
template <typename ASTNode>
void eliminateVariables(Dialect const& _dialect, ASTNode& _node, size_t _numVariables)
{
RematCandidateSelector selector{_dialect};
selector(_node);
// Select at most _numVariables
set<YulString> varsToEliminate;
for (auto const& costs: selector.candidates())
{
if (varsToEliminate.size() >= _numVariables)
break;
varsToEliminate.insert(costs.second);
}
Rematerialiser::run(_dialect, _node, std::move(varsToEliminate));
UnusedPruner::runUntilStabilised(_dialect, _node);
}
}
bool StackCompressor::run(
Dialect const& _dialect,
Block& _ast,
bool _optimizeStackAllocation,
size_t _maxIterations
)
{
yulAssert(
_ast.statements.size() > 0 && _ast.statements.at(0).type() == typeid(Block),
"Need to run the function grouper before the stack compressor."
);
for (size_t iterations = 0; iterations < _maxIterations; iterations++)
{
map<YulString, int> stackSurplus = CompilabilityChecker::run(_dialect, _ast, _optimizeStackAllocation);
if (stackSurplus.empty())
return true;
if (stackSurplus.count(YulString{}))
{
yulAssert(stackSurplus.at({}) > 0, "Invalid surplus value.");
eliminateVariables(_dialect, boost::get<Block>(_ast.statements.at(0)), stackSurplus.at({}));
}
for (size_t i = 1; i < _ast.statements.size(); ++i)
{
FunctionDefinition& fun = boost::get<FunctionDefinition>(_ast.statements[i]);
if (!stackSurplus.count(fun.name))
continue;
yulAssert(stackSurplus.at(fun.name) > 0, "Invalid surplus value.");
eliminateVariables(_dialect, fun, stackSurplus.at(fun.name));
}
}
return false;
}