solidity/libyul/optimiser/DataFlowAnalyzer.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/>.
*/
/**
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* 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.
*/
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#include <libyul/optimiser/DataFlowAnalyzer.h>
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#include <libyul/optimiser/NameCollector.h>
#include <libyul/optimiser/Semantics.h>
#include <libyul/Exceptions.h>
#include <libyul/AsmData.h>
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#include <libyul/backends/evm/EVMDialect.h>
#include <libdevcore/CommonData.h>
#include <boost/range/adaptor/reversed.hpp>
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#include <boost/range/algorithm_ext/erase.hpp>
#include <variant>
using namespace std;
using namespace dev;
using namespace yul;
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void DataFlowAnalyzer::operator()(ExpressionStatement& _statement)
{
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if (auto vars = isSimpleStore(dev::eth::Instruction::SSTORE, _statement))
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{
ASTModifier::operator()(_statement);
set<YulString> keysToErase;
for (auto const& item: m_storage.values)
if (!(
m_knowledgeBase.knownToBeDifferent(vars->first, item.first) ||
m_knowledgeBase.knownToBeEqual(vars->second, item.second)
))
keysToErase.insert(item.first);
for (YulString const& key: keysToErase)
m_storage.eraseKey(key);
m_storage.set(vars->first, vars->second);
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}
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else if (auto vars = isSimpleStore(dev::eth::Instruction::MSTORE, _statement))
{
ASTModifier::operator()(_statement);
set<YulString> keysToErase;
for (auto const& item: m_memory.values)
if (!m_knowledgeBase.knownToBeDifferentByAtLeast32(vars->first, item.first))
keysToErase.insert(item.first);
for (YulString const& key: keysToErase)
m_memory.eraseKey(key);
m_memory.set(vars->first, vars->second);
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}
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else
{
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clearKnowledgeIfInvalidated(_statement.expression);
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ASTModifier::operator()(_statement);
}
}
void DataFlowAnalyzer::operator()(Assignment& _assignment)
{
set<YulString> names;
for (auto const& var: _assignment.variableNames)
names.emplace(var.name);
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assertThrow(_assignment.value, OptimizerException, "");
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clearKnowledgeIfInvalidated(*_assignment.value);
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);
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m_variableScopes.back().variables += names;
if (_varDecl.value)
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{
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clearKnowledgeIfInvalidated(*_varDecl.value);
visit(*_varDecl.value);
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}
handleAssignment(names, _varDecl.value.get());
}
void DataFlowAnalyzer::operator()(If& _if)
{
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clearKnowledgeIfInvalidated(*_if.condition);
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InvertibleMap<YulString, YulString> storage = m_storage;
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InvertibleMap<YulString, YulString> memory = m_memory;
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ASTModifier::operator()(_if);
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joinKnowledge(storage, memory);
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Assignments assignments;
assignments(_if.body);
clearValues(assignments.names());
}
void DataFlowAnalyzer::operator()(Switch& _switch)
{
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clearKnowledgeIfInvalidated(*_switch.expression);
visit(*_switch.expression);
set<YulString> assignedVariables;
for (auto& _case: _switch.cases)
{
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InvertibleMap<YulString, YulString> storage = m_storage;
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InvertibleMap<YulString, YulString> memory = m_memory;
(*this)(_case.body);
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joinKnowledge(storage, memory);
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Assignments assignments;
assignments(_case.body);
assignedVariables += assignments.names();
// This is a little too destructive, we could retain the old values.
clearValues(assignments.names());
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clearKnowledgeIfInvalidated(_case.body);
}
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for (auto& _case: _switch.cases)
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clearKnowledgeIfInvalidated(_case.body);
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;
InvertibleRelation<YulString> references;
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InvertibleMap<YulString, YulString> storage;
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InvertibleMap<YulString, YulString> memory;
m_value.swap(value);
swap(m_references, references);
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swap(m_storage, storage);
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swap(m_memory, memory);
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);
handleAssignment({var.name}, nullptr);
}
ASTModifier::operator()(_fun);
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// Note that the contents of return variables, storage and memory at this point
// might be incorrect due to the fact that the DataFlowAnalyzer ignores the ``leave``
// statement.
popScope();
m_value.swap(value);
swap(m_references, references);
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swap(m_storage, storage);
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swap(m_memory, memory);
}
void DataFlowAnalyzer::operator()(ForLoop& _for)
{
// If the pre block was not empty,
// we would have to deal with more complicated scoping rules.
assertThrow(_for.pre.statements.empty(), OptimizerException, "");
AssignmentsSinceContinue assignmentsSinceCont;
assignmentsSinceCont(_for.body);
Assignments assignments;
assignments(_for.body);
assignments(_for.post);
clearValues(assignments.names());
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// break/continue are tricky for storage and thus we almost always clear here.
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clearKnowledgeIfInvalidated(*_for.condition);
clearKnowledgeIfInvalidated(_for.post);
clearKnowledgeIfInvalidated(_for.body);
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visit(*_for.condition);
(*this)(_for.body);
clearValues(assignmentsSinceCont.names());
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clearKnowledgeIfInvalidated(_for.body);
(*this)(_for.post);
clearValues(assignments.names());
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clearKnowledgeIfInvalidated(*_for.condition);
clearKnowledgeIfInvalidated(_for.post);
clearKnowledgeIfInvalidated(_for.body);
}
void DataFlowAnalyzer::operator()(Block& _block)
{
size_t numScopes = m_variableScopes.size();
pushScope(false);
ASTModifier::operator()(_block);
popScope();
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assertThrow(numScopes == m_variableScopes.size(), OptimizerException, "");
}
void DataFlowAnalyzer::handleAssignment(set<YulString> const& _variables, Expression* _value)
{
clearValues(_variables);
MovableChecker movableChecker{m_dialect, &m_functionSideEffects};
if (_value)
movableChecker.visit(*_value);
else
for (auto const& var: _variables)
m_value[var] = &m_zero;
if (_value && _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 (movableChecker.movable() && !movableChecker.referencedVariables().count(name))
m_value[name] = _value;
}
auto const& referencedVariables = movableChecker.referencedVariables();
for (auto const& name: _variables)
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{
m_references.set(name, referencedVariables);
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// assignment to slot denoted by "name"
m_storage.eraseKey(name);
// assignment to slot contents denoted by "name"
m_storage.eraseValue(name);
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// assignment to slot denoted by "name"
m_memory.eraseKey(name);
// assignment to slot contents denoted by "name"
m_memory.eraseValue(name);
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}
}
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)
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// First clear storage knowledge, because we do not have to clear
// storage knowledge of variables whose expression has changed,
// since the value is still unchanged.
for (auto const& name: _variables)
{
// clear slot denoted by "name"
m_storage.eraseKey(name);
// clear slot contents denoted by "name"
m_storage.eraseValue(name);
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// assignment to slot denoted by "name"
m_memory.eraseKey(name);
// assignment to slot contents denoted by "name"
m_memory.eraseValue(name);
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}
// Also clear variables that reference variables to be cleared.
for (auto const& name: _variables)
for (auto const& ref: m_references.backward[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)
m_references.eraseKey(name);
}
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void DataFlowAnalyzer::clearKnowledgeIfInvalidated(Block const& _block)
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{
SideEffectsCollector sideEffects(m_dialect, _block, &m_functionSideEffects);
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if (sideEffects.invalidatesStorage())
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m_storage.clear();
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if (sideEffects.invalidatesMemory())
m_memory.clear();
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}
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void DataFlowAnalyzer::clearKnowledgeIfInvalidated(Expression const& _expr)
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{
SideEffectsCollector sideEffects(m_dialect, _expr, &m_functionSideEffects);
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if (sideEffects.invalidatesStorage())
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m_storage.clear();
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if (sideEffects.invalidatesMemory())
m_memory.clear();
}
void DataFlowAnalyzer::joinKnowledge(
InvertibleMap<YulString, YulString> const& _olderStorage,
InvertibleMap<YulString, YulString> const& _olderMemory
)
{
joinKnowledgeHelper(m_storage, _olderStorage);
joinKnowledgeHelper(m_memory, _olderMemory);
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}
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void DataFlowAnalyzer::joinKnowledgeHelper(
InvertibleMap<YulString, YulString>& _this,
InvertibleMap<YulString, YulString> const& _older
)
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{
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// We clear if the key does not exist in the older map or if the value is different.
// This also works for memory because _older is an "older version"
// of m_memory and thus any overlapping write would have cleared the keys
// that are not known to be different inside m_memory already.
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set<YulString> keysToErase;
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for (auto const& item: _this.values)
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{
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auto it = _older.values.find(item.first);
if (it == _older.values.end() || it->second != item.second)
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keysToErase.insert(item.first);
}
for (auto const& key: keysToErase)
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_this.eraseKey(key);
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}
bool DataFlowAnalyzer::inScope(YulString _variableName) const
{
for (auto const& scope: m_variableScopes | boost::adaptors::reversed)
{
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if (scope.variables.count(_variableName))
return true;
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if (scope.isFunction)
return false;
}
return false;
}
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std::optional<pair<YulString, YulString>> DataFlowAnalyzer::isSimpleStore(
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dev::eth::Instruction _store,
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ExpressionStatement const& _statement
) const
{
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yulAssert(
_store == dev::eth::Instruction::MSTORE ||
_store == dev::eth::Instruction::SSTORE,
""
);
if (holds_alternative<FunctionCall>(_statement.expression))
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{
FunctionCall const& funCall = std::get<FunctionCall>(_statement.expression);
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if (EVMDialect const* dialect = dynamic_cast<EVMDialect const*>(&m_dialect))
if (auto const* builtin = dialect->builtin(funCall.functionName.name))
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if (builtin->instruction == _store)
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if (
holds_alternative<Identifier>(funCall.arguments.at(0)) &&
holds_alternative<Identifier>(funCall.arguments.at(1))
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)
{
YulString key = std::get<Identifier>(funCall.arguments.at(0)).name;
YulString value = std::get<Identifier>(funCall.arguments.at(1)).name;
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return make_pair(key, value);
}
}
return {};
}