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https://github.com/ethereum/solidity
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SSA transform - first step.
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chriseth
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/*
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This file is part of solidity.
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solidity is free software: you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation, either version 3 of the License, or
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(at your option) any later version.
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solidity is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with solidity. If not, see <http://www.gnu.org/licenses/>.
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*/
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/**
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* Optimiser component that turns subsequent assignments to variable declarations
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* and assignments.
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*/
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#include <libyul/optimiser/SSATransform.h>
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#include <libyul/optimiser/NameCollector.h>
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#include <libyul/optimiser/NameDispenser.h>
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#include <libsolidity/inlineasm/AsmData.h>
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#include <libdevcore/CommonData.h>
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using namespace std;
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using namespace dev;
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using namespace dev::yul;
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using namespace dev::solidity;
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void SSATransform::operator()(Identifier& _identifier)
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{
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if (m_currentVariableValues.count(_identifier.name))
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_identifier.name = m_currentVariableValues[_identifier.name];
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}
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void SSATransform::operator()(ForLoop& _for)
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{
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// This will clear the current value in case of a reassignment inside the
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// init part, although the new variable would still be in scope inside the whole loop.
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// This small inefficiency is fine if we move the pre part of all for loops out
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// of the for loop.
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(*this)(_for.pre);
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Assignments assignments;
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assignments(_for.body);
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assignments(_for.post);
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for (auto const& var: assignments.names())
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m_currentVariableValues.erase(var);
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visit(*_for.condition);
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(*this)(_for.body);
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(*this)(_for.post);
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}
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void SSATransform::operator()(Block& _block)
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{
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set<string> variablesToClearAtEnd;
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// Creates a new variable (and returns its declaration) with value _value
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// and replaces _value by a reference to that new variable.
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auto replaceByNew = [&](SourceLocation _loc, string _varName, string _type, shared_ptr<Expression>& _value) -> VariableDeclaration
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{
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string newName = m_nameDispenser.newName(_varName);
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m_currentVariableValues[_varName] = newName;
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variablesToClearAtEnd.insert(_varName);
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shared_ptr<Expression> v = make_shared<Expression>(Identifier{_loc, newName});
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_value.swap(v);
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return VariableDeclaration{_loc, {TypedName{_loc, std::move(newName), std::move(_type)}}, std::move(v)};
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};
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iterateReplacing(
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_block.statements,
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[&](Statement& _s) -> boost::optional<vector<Statement>>
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{
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if (_s.type() == typeid(VariableDeclaration))
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{
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VariableDeclaration& varDecl = boost::get<VariableDeclaration>(_s);
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if (varDecl.value)
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visit(*varDecl.value);
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if (varDecl.variables.size() != 1 || !m_variablesToReplace.count(varDecl.variables.front().name))
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return {};
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// Replace "let a := v" by "let a_1 := v let a := v"
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VariableDeclaration newVarDecl = replaceByNew(
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varDecl.location,
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varDecl.variables.front().name,
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varDecl.variables.front().type,
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varDecl.value
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);
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return vector<Statement>{std::move(newVarDecl), std::move(varDecl)};
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}
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else if (_s.type() == typeid(Assignment))
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{
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Assignment& assignment = boost::get<Assignment>(_s);
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visit(*assignment.value);
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if (assignment.variableNames.size() != 1)
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return {};
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assertThrow(m_variablesToReplace.count(assignment.variableNames.front().name), OptimizerException, "");
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// Replace "a := v" by "let a_1 := v a := v"
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VariableDeclaration newVarDecl = replaceByNew(
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assignment.location,
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assignment.variableNames.front().name,
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{}, // TODO determine type
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assignment.value
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);
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return vector<Statement>{std::move(newVarDecl), std::move(assignment)};
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}
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else
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visit(_s);
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return {};
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}
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);
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for (auto const& var: variablesToClearAtEnd)
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m_currentVariableValues.erase(var);
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}
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void SSATransform::run(Block& _ast, NameDispenser& _nameDispenser)
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{
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Assignments assignments;
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assignments(_ast);
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SSATransform{_nameDispenser, assignments.names()}(_ast);
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}
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@@ -0,0 +1,98 @@
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/*
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This file is part of solidity.
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solidity is free software: you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation, either version 3 of the License, or
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(at your option) any later version.
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solidity is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with solidity. If not, see <http://www.gnu.org/licenses/>.
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*/
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/**
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* Optimiser component that turns subsequent assignments to variable declarations
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* and assignments.
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*/
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#pragma once
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#include <libyul/ASTDataForward.h>
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#include <libyul/optimiser/ASTWalker.h>
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#include <vector>
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namespace dev
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{
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namespace yul
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{
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class NameDispenser;
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/**
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* Optimizer stage that tries to replace repeated assignments to
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* existing variables by declarations of new variables as much as
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* possible.
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* The reassignments are still there, but all references to the
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* reassigned variables are replaced by the newly declared variables.
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*
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* Example:
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* {
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* let a := 1
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* mstore(a, 2)
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* a := 3
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* }
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* is transformed to
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* {
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* let a_1 := 1
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* let a := a_1
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* mstore(a_1, 2)
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* let a_3 := 3
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* a := a_3
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* }
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*
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* Exact semantics:
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*
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* For any variable a that is assigned to somewhere in the code (assignment with
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* declaration does not count) perform the following transforms:
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* - replace "let a := v" by "let a_1 := v let a := a_1"
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* - replace "a := v" by "let a_1 := v a := a_1"
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* Furthermore, always note the current variable/value assigned to a and replace each
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* reference to a by this variable.
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* The current value mapping is cleared for a variable a at the end of each block
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* in which it was assigned and just after the for loop init block if it is assigned
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* inside the for loop.
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*
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* After this stage, redundantAssignmentRemover is recommended to remove the unnecessary
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* intermediate assignments.
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*
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* This stage provides best results if CSE is run right before it, because
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* then it does not generate excessive amounts of variables.
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*
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* TODO Which transforms are required to keep this idempotent?
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*/
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class SSATransform: public ASTModifier
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{
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public:
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void operator()(Identifier&) override;
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void operator()(ForLoop&) override;
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void operator()(Block& _block) override;
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static void run(Block& _ast, NameDispenser& _nameDispenser);
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private:
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explicit SSATransform(NameDispenser& _nameDispenser, std::set<std::string> const& _variablesToReplace):
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m_nameDispenser(_nameDispenser), m_variablesToReplace(_variablesToReplace)
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{ }
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NameDispenser& m_nameDispenser;
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std::set<std::string> const& m_variablesToReplace;
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std::map<std::string, std::string> m_currentVariableValues;
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};
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}
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}
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