/*
This file is part of cpp-ethereum.
cpp-ethereum 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.
cpp-ethereum 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 cpp-ethereum. If not, see <http://www.gnu.org/licenses/>.
*/
/**
* @file CommonSubexpressionEliminator.h
* @author Christian <c@ethdev.com>
* @date 2015
* Optimizer step for common subexpression elimination and stack reorganisation.
*/
#pragma once
#include <vector>
#include <map>
#include <set>
#include <tuple>
#include <ostream>
#include <libdevcore/CommonIO.h>
#include <libdevcore/Exceptions.h>
#include <libevmasm/ExpressionClasses.h>
#include <libevmasm/SemanticInformation.h>
#include <libevmasm/KnownState.h>
namespace dev
{
namespace eth
{
class AssemblyItem;
using AssemblyItems = std::vector<AssemblyItem>;
/**
* Optimizer step that performs common subexpression elimination and stack reorganisation,
* i.e. it tries to infer equality among expressions and compute the values of two expressions
* known to be equal only once.
*
* The general workings are that for each assembly item that is fed into the eliminator, an
* equivalence class is derived from the operation and the equivalence class of its arguments.
* DUPi, SWAPi and some arithmetic instructions are used to infer equivalences while these
* classes are determined.
*
* When the list of optimized items is requested, they are generated in a bottom-up fashion,
* adding code for equivalence classes that were not yet computed.
*/
class CommonSubexpressionEliminator
{
public:
using Id = ExpressionClasses::Id;
using StoreOperation = KnownState::StoreOperation;
CommonSubexpressionEliminator(KnownState const& _state): m_initialState(_state), m_state(_state) {}
/// Feeds AssemblyItems into the eliminator and @returns the iterator pointing at the first
/// item that must be fed into a new instance of the eliminator.
template <class _AssemblyItemIterator>
_AssemblyItemIterator feedItems(_AssemblyItemIterator _iterator, _AssemblyItemIterator _end);
/// @returns the resulting items after optimization.
AssemblyItems getOptimizedItems();
private:
/// Feeds the item into the system for analysis.
void feedItem(AssemblyItem const& _item, bool _copyItem = false);
/// Tries to optimize the item that breaks the basic block at the end.
void optimizeBreakingItem();
KnownState m_initialState;
KnownState m_state;
/// Keeps information about which storage or memory slots were written to at which sequence
/// number with what instruction.
std::vector<StoreOperation> m_storeOperations;
/// The item that breaks the basic block, can be nullptr.
/// It is usually appended to the block but can be optimized in some cases.
AssemblyItem const* m_breakingItem = nullptr;
};
/**
* Unit that generates code from current stack layout, target stack layout and information about
* the equivalence classes.
*/
class CSECodeGenerator
{
public:
using StoreOperation = CommonSubexpressionEliminator::StoreOperation;
using StoreOperations = std::vector<StoreOperation>;
using Id = ExpressionClasses::Id;
/// Initializes the code generator with the given classes and store operations.
/// The store operations have to be sorted by sequence number in ascending order.
CSECodeGenerator(ExpressionClasses& _expressionClasses, StoreOperations const& _storeOperations);
/// @returns the assembly items generated from the given requirements
/// @param _initialSequenceNumber starting sequence number, do not generate sequenced operations
/// before this number.
/// @param _initialStack current contents of the stack (up to stack height of zero)
/// @param _targetStackContents final contents of the stack, by stack height relative to initial
/// @note should only be called once on each object.
AssemblyItems generateCode(
unsigned _initialSequenceNumber,
int _initialStackHeight,
std::map<int, Id> const& _initialStack,
std::map<int, Id> const& _targetStackContents
);
private:
/// Recursively discovers all dependencies to @a m_requests.
void addDependencies(Id _c);
/// Produce code that generates the given element if it is not yet present.
/// @param _allowSequenced indicates that sequence-constrained operations are allowed
void generateClassElement(Id _c, bool _allowSequenced = false);
/// @returns the position of the representative of the given id on the stack.
/// @note throws an exception if it is not on the stack.
int classElementPosition(Id _id) const;
/// @returns true if the copy of @a _element can be removed from stack position _fromPosition
/// - in general or, if given, while computing @a _result.
bool canBeRemoved(Id _element, Id _result = Id(-1), int _fromPosition = c_invalidPosition);
/// Appends code to remove the topmost stack element if it can be removed.
bool removeStackTopIfPossible();
/// Appends a dup instruction to m_generatedItems to retrieve the element at the given stack position.
void appendDup(int _fromPosition, SourceLocation const& _location);
/// Appends a swap instruction to m_generatedItems to retrieve the element at the given stack position.
/// @note this might also remove the last item if it exactly the same swap instruction.
void appendOrRemoveSwap(int _fromPosition, SourceLocation const& _location);
/// Appends the given assembly item.
void appendItem(AssemblyItem const& _item);
static const int c_invalidPosition = -0x7fffffff;
AssemblyItems m_generatedItems;
/// Current height of the stack relative to the start.
int m_stackHeight;
/// If (b, a) is in m_requests then b is needed to compute a.
std::multimap<Id, Id> m_neededBy;
/// Current content of the stack.
std::map<int, Id> m_stack;
/// Current positions of equivalence classes, equal to the empty set if already deleted.
std::map<Id, std::set<int>> m_classPositions;
/// The actual eqivalence class items and how to compute them.
ExpressionClasses& m_expressionClasses;
/// Keeps information about which storage or memory slots were written to by which operations.
/// The operations are sorted ascendingly by sequence number.
std::map<std::pair<StoreOperation::Target, Id>, StoreOperations> m_storeOperations;
/// The set of equivalence classes that should be present on the stack at the end.
std::set<Id> m_finalClasses;
std::map<int, Id> m_targetStack;
};
template <class _AssemblyItemIterator>
_AssemblyItemIterator CommonSubexpressionEliminator::feedItems(
_AssemblyItemIterator _iterator,
_AssemblyItemIterator _end
)
{
assertThrow(!m_breakingItem, OptimizerException, "Invalid use of CommonSubexpressionEliminator.");
for (; _iterator != _end && !SemanticInformation::breaksCSEAnalysisBlock(*_iterator); ++_iterator)
feedItem(*_iterator);
if (_iterator != _end)
m_breakingItem = &(*_iterator++);
return _iterator;
}
}
}
