mirror of
https://github.com/ethereum/solidity
synced 2023-10-03 13:03:40 +00:00
234 lines
9.5 KiB
C++
234 lines
9.5 KiB
C++
/*
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This file is part of cpp-ethereum.
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cpp-ethereum 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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cpp-ethereum 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 cpp-ethereum. If not, see <http://www.gnu.org/licenses/>.
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*/
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/**
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* @file CommonSubexpressionEliminator.h
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* @author Christian <c@ethdev.com>
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* @date 2015
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* Optimizer step for common subexpression elimination and stack reorganisation.
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*/
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#pragma once
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#include <vector>
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#include <map>
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#include <set>
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#include <tuple>
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#include <ostream>
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#include <libdevcore/CommonIO.h>
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#include <libdevcore/Exceptions.h>
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#include <libevmasm/ExpressionClasses.h>
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#include <libevmasm/SemanticInformation.h>
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namespace dev
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{
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namespace eth
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{
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class AssemblyItem;
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using AssemblyItems = std::vector<AssemblyItem>;
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/**
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* Optimizer step that performs common subexpression elimination and stack reorganisation,
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* i.e. it tries to infer equality among expressions and compute the values of two expressions
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* known to be equal only once.
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*
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* The general workings are that for each assembly item that is fed into the eliminator, an
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* equivalence class is derived from the operation and the equivalence class of its arguments.
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* DUPi, SWAPi and some arithmetic instructions are used to infer equivalences while these
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* classes are determined.
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*
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* When the list of optimized items is requested, they are generated in a bottom-up fashion,
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* adding code for equivalence classes that were not yet computed.
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*/
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class CommonSubexpressionEliminator
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{
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public:
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using Id = ExpressionClasses::Id;
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struct StoreOperation
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{
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enum Target { Memory, Storage };
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StoreOperation(
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Target _target,
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Id _slot,
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unsigned _sequenceNumber,
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Id _expression
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): target(_target), slot(_slot), sequenceNumber(_sequenceNumber), expression(_expression) {}
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Target target;
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Id slot;
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unsigned sequenceNumber;
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Id expression;
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};
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/// Feeds AssemblyItems into the eliminator and @returns the iterator pointing at the first
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/// item that must be fed into a new instance of the eliminator.
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template <class _AssemblyItemIterator>
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_AssemblyItemIterator feedItems(_AssemblyItemIterator _iterator, _AssemblyItemIterator _end);
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/// @returns the resulting items after optimization.
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AssemblyItems getOptimizedItems();
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/// Streams debugging information to @a _out.
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std::ostream& stream(
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std::ostream& _out,
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std::map<int, Id> _initialStack = std::map<int, Id>(),
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std::map<int, Id> _targetStack = std::map<int, Id>()
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) const;
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private:
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/// Feeds the item into the system for analysis.
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void feedItem(AssemblyItem const& _item, bool _copyItem = false);
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/// Tries to optimize the item that breaks the basic block at the end.
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void optimizeBreakingItem();
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/// Simplifies the given item using
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/// Assigns a new equivalence class to the next sequence number of the given stack element.
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void setStackElement(int _stackHeight, Id _class);
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/// Swaps the given stack elements in their next sequence number.
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void swapStackElements(int _stackHeightA, int _stackHeightB, SourceLocation const& _location);
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/// Retrieves the current equivalence class fo the given stack element (or generates a new
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/// one if it does not exist yet).
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Id stackElement(int _stackHeight, SourceLocation const& _location);
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/// @returns the equivalence class id of the special initial stack element at the given height
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/// (must not be positive).
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Id initialStackElement(int _stackHeight, SourceLocation const& _location);
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/// Increments the sequence number, deletes all storage information that might be overwritten
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/// and stores the new value at the given slot.
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void storeInStorage(Id _slot, Id _value, SourceLocation const& _location);
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/// Retrieves the current value at the given slot in storage or creates a new special sload class.
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Id loadFromStorage(Id _slot, SourceLocation const& _location);
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/// Increments the sequence number, deletes all memory information that might be overwritten
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/// and stores the new value at the given slot.
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void storeInMemory(Id _slot, Id _value, SourceLocation const& _location);
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/// Retrieves the current value at the given slot in memory or creates a new special mload class.
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Id loadFromMemory(Id _slot, SourceLocation const& _location);
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/// Finds or creates a new expression that applies the sha3 hash function to the contents in memory.
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Id applySha3(Id _start, Id _length, SourceLocation const& _location);
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/// Current stack height, can be negative.
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int m_stackHeight = 0;
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/// Current stack layout, mapping stack height -> equivalence class
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std::map<int, Id> m_stackElements;
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/// Current sequence number, this is incremented with each modification to storage or memory.
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unsigned m_sequenceNumber = 1;
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/// Knowledge about storage content.
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std::map<Id, Id> m_storageContent;
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/// Knowledge about memory content. Keys are memory addresses, note that the values overlap
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/// and are not contained here if they are not completely known.
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std::map<Id, Id> m_memoryContent;
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/// Keeps record of all sha3 hashes that are computed.
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std::map<std::vector<Id>, Id> m_knownSha3Hashes;
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/// Keeps information about which storage or memory slots were written to at which sequence
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/// number with what instruction.
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std::vector<StoreOperation> m_storeOperations;
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/// Structure containing the classes of equivalent expressions.
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ExpressionClasses m_expressionClasses;
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/// The item that breaks the basic block, can be nullptr.
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/// It is usually appended to the block but can be optimized in some cases.
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AssemblyItem const* m_breakingItem = nullptr;
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};
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/**
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* Unit that generates code from current stack layout, target stack layout and information about
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* the equivalence classes.
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*/
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class CSECodeGenerator
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{
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public:
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using StoreOperation = CommonSubexpressionEliminator::StoreOperation;
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using StoreOperations = std::vector<StoreOperation>;
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using Id = ExpressionClasses::Id;
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/// Initializes the code generator with the given classes and store operations.
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/// The store operations have to be sorted by sequence number in ascending order.
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CSECodeGenerator(ExpressionClasses& _expressionClasses, StoreOperations const& _storeOperations);
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/// @returns the assembly items generated from the given requirements
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/// @param _initialStack current contents of the stack (up to stack height of zero)
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/// @param _targetStackContents final contents of the stack, by stack height relative to initial
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/// @note should only be called once on each object.
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AssemblyItems generateCode(
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std::map<int, Id> const& _initialStack,
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std::map<int, Id> const& _targetStackContents
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);
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private:
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/// Recursively discovers all dependencies to @a m_requests.
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void addDependencies(Id _c);
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/// Produce code that generates the given element if it is not yet present.
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/// @returns the stack position of the element or c_invalidPosition if it does not actually
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/// generate a value on the stack.
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/// @param _allowSequenced indicates that sequence-constrained operations are allowed
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int generateClassElement(Id _c, bool _allowSequenced = false);
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/// @returns the position of the representative of the given id on the stack.
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/// @note throws an exception if it is not on the stack.
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int classElementPosition(Id _id) const;
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/// @returns true if @a _element can be removed - in general or, if given, while computing @a _result.
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bool canBeRemoved(Id _element, Id _result = Id(-1));
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/// Appends code to remove the topmost stack element if it can be removed.
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bool removeStackTopIfPossible();
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/// Appends a dup instruction to m_generatedItems to retrieve the element at the given stack position.
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void appendDup(int _fromPosition, SourceLocation const& _location);
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/// Appends a swap instruction to m_generatedItems to retrieve the element at the given stack position.
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/// @note this might also remove the last item if it exactly the same swap instruction.
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void appendOrRemoveSwap(int _fromPosition, SourceLocation const& _location);
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/// Appends the given assembly item.
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void appendItem(AssemblyItem const& _item);
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static const int c_invalidPosition = -0x7fffffff;
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AssemblyItems m_generatedItems;
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/// Current height of the stack relative to the start.
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int m_stackHeight = 0;
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/// If (b, a) is in m_requests then b is needed to compute a.
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std::multimap<Id, Id> m_neededBy;
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/// Current content of the stack.
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std::map<int, Id> m_stack;
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/// Current positions of equivalence classes, equal to c_invalidPosition if already deleted.
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std::map<Id, int> m_classPositions;
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/// The actual eqivalence class items and how to compute them.
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ExpressionClasses& m_expressionClasses;
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/// Keeps information about which storage or memory slots were written to by which operations.
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/// The operations are sorted ascendingly by sequence number.
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std::map<std::pair<StoreOperation::Target, Id>, StoreOperations> m_storeOperations;
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/// The set of equivalence classes that should be present on the stack at the end.
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std::set<Id> m_finalClasses;
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};
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template <class _AssemblyItemIterator>
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_AssemblyItemIterator CommonSubexpressionEliminator::feedItems(
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_AssemblyItemIterator _iterator,
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_AssemblyItemIterator _end
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)
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{
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for (; _iterator != _end && !SemanticInformation::breaksCSEAnalysisBlock(*_iterator); ++_iterator)
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feedItem(*_iterator);
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if (_iterator != _end)
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m_breakingItem = &(*_iterator++);
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return _iterator;
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}
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}
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}
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