Properly determine whether a variable can be eliminated or not.

This commit is contained in:
chriseth 2019-03-13 17:44:45 +01:00
parent 38cbf8d230
commit aa9a2935ac

View File

@ -39,29 +39,87 @@ 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_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(shared_ptr<Dialect> const& _dialect, ASTNode& _node, size_t _numVariables)
{
SSAValueTracker ssaValues;
ssaValues(_node);
map<YulString, size_t> references = ReferencesCounter::countReferences(_node);
set<pair<size_t, YulString>> rematCosts;
for (auto const& ssa: ssaValues.values())
{
if (!MovableChecker{*_dialect, *ssa.second}.movable())
continue;
size_t numRef = references[ssa.first];
size_t cost = 0;
if (numRef > 1)
cost = CodeCost::codeCost(*ssa.second) * (numRef - 1);
rematCosts.insert(make_pair(cost, ssa.first));
}
RematCandidateSelector selector{*_dialect};
selector(_node);
// Select at most _numVariables
set<YulString> varsToEliminate;
for (auto const& costs: rematCosts)
for (auto const& costs: selector.candidates())
{
if (varsToEliminate.size() >= _numVariables)
break;