solidity/libyul/optimiser/StackToMemoryMover.h

/*
	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/>.
*/
/**
 * Optimisation stage that moves Yul variables from stack to memory.
 */

#pragma once

#include <libyul/optimiser/ASTWalker.h>
#include <libyul/optimiser/OptimiserStep.h>
#include <libyul/ASTForward.h>

#include <liblangutil/SourceLocation.h>
#include <libsolutil/Common.h>

namespace solidity::yul
{

/**
 * Optimisation stage that moves Yul variables from stack to memory.
 * It takes a map from functions names and variable names to memory offsets.
 * It then transforms the AST as follows:
 *
 * Single variable declarations are replaced by mstore's as follows:
 *   If a is in the map, replace
 *     let a
 *   by
 *     mstore(<memory offset for a>, 0)
 *   respectively, replace
 *     let a := expr
 *   by
 *     mstore(<memory offset for a>, expr)
 *
 * In a multi-variable declaration, variables to be moved are replaced by fresh variables and then moved to memory:
 *   If b and d are in the map, replace
 *     let a, b, c, d := f()
 *   by
 *     let _1, _2, _3, _4 := f()
 *     mstore(<memory offset for d>, _4)
 *     mstore(<memory offset for b>, _2)
 *     let c := _3
 *     let a := _1
 *
 * In case f has return parameters that are moved to memory, fewer variables are returned and the return values read
 * from memory instead. Assume the third return parameter of f (i.e. c) has to be moved to memory:
 *     let a, b, c, d := f()
 *   then it is replaced by
 *     let _1, _2, _4 := f()
 *     mstore(<memory offset for d>, _4)
 *     mstore(<memory offset for b>, _2)
 *     let c := mload(<memory offset of third return parameter of f>)
 *     let a := _1
 *
 * Assignments to single variables are replaced by mstore's:
 *   If a is in the map, replace
 *     a := expr
 *   by
 *     mstore(<memory offset for a>, expr)
 *
 * Assignments to multiple variables are split up similarly to multi-variable declarations:
 *   If b and d are in the map, replace
 *     a, b, c, d := f()
 *   by
 *     let _1, _2, _3, _4 := f()
 *     mstore(<memory offset for d>, _4)
 *     mstore(<memory offset for b>, _2)
 *     c := _3
 *     a := _1
 *
 * Replace all references to a variable ``a`` in the map by ``mload(<memory offset for a>)``.
 *
 * Function arguments are moved at the beginning of a function body:
 *   If a1 is in the map, replace
 *     function f(a1, a2, ..., a17)
 *     {
 *       ...
 *       sstore(a1, a17)
 *     }
 *   by
 *     function f(a1, a2, ..., a17)
 *     {
 *       mstore(<memory offset for a1>, a1)
 *       ...
 *       sstore(mload(<memory offset for a1>, a17)
 *     }
 * This relies on the code transform popping arguments that are no longer used, if they are on the stack top.
 *
 * Functions with only one return argument that has to be moved are encapsulated in a wrapper function as follows:
 *   Suppose b and r need to be moved in:
 *     function f(a, b) -> r
 *     {
 *       ...body of f...
 *       r := b
 *       ...body of f continued...
 *     }
 *   then replace by:
 *     function f(a, b) -> r
 *     {
 *       mstore(<memory offset of b>, b)
 *       mstore(<memory offset of r>, 0)
 *       f_1(a)
 *       r := mload(<memory offset of r>)
 *     }
 *     function f_1(a)
 *     {
 *       ...body of f...
 *       mstore(<memory offset of r>, mload(<memory offset of b>))
 *       ...body of f continued...
 *     }
 *
 * Prerequisite: Disambiguator, ForLoopInitRewriter, FunctionHoister.
 */
class StackToMemoryMover: ASTModifier
{
public:
	/**
	 * Runs the stack to memory mover.
	 * @param _reservedMemory Is the amount of previously reserved memory,
	 *                        i.e. the lowest memory offset to which variables can be moved.
	 * @param _memorySlots A map from variables to a slot in memory. Based on the slot a unique offset in the memory range
	 *                     between _reservedMemory and _reservedMemory + 32 * _numRequiredSlots is calculated for each
	 *                     variable.
	 * @param _numRequiredSlots The number of slots required in total. The maximum value that may occur in @a _memorySlots.
	 */
	static void run(
		OptimiserStepContext& _context,
		u256 _reservedMemory,
		std::map<YulString, uint64_t> const& _memorySlots,
		uint64_t _numRequiredSlots,
		Block& _block
	);
	using ASTModifier::operator();

	void operator()(FunctionDefinition& _functionDefinition) override;
	void operator()(Block& _block) override;
	using ASTModifier::visit;
	void visit(Expression& _expression) override;
private:
	class VariableMemoryOffsetTracker
	{
	public:
		VariableMemoryOffsetTracker(
			u256 _reservedMemory,
			std::map<YulString, uint64_t> const& _memorySlots,
			uint64_t _numRequiredSlots
		): m_reservedMemory(_reservedMemory), m_memorySlots(_memorySlots), m_numRequiredSlots(_numRequiredSlots)
		{}

		/// @returns a YulString containing the memory offset to be assigned to @a _variable as number literal
		/// or std::nullopt if the variable should not be moved.
		std::optional<YulString> operator()(YulString _variable) const;
		/// @returns a YulString containing the memory offset to be assigned to @a _variable as number literal
		/// or std::nullopt if the variable should not be moved.
		std::optional<YulString> operator()(TypedName const& _variable) const;
		/// @returns a YulString containing the memory offset to be assigned to @a _variable as number literal
		/// or std::nullopt if the variable should not be moved.
		std::optional<YulString> operator()(Identifier const& _variable) const;

	private:
		u256 m_reservedMemory;
		std::map<YulString, uint64_t> const& m_memorySlots;
		uint64_t m_numRequiredSlots = 0;
	};
	struct FunctionMoveInfo
	{
		std::vector<std::optional<YulString>> returnVariableSlots;
	};

	StackToMemoryMover(
		OptimiserStepContext& _context,
		VariableMemoryOffsetTracker const& _memoryOffsetTracker,
		std::map<YulString, std::vector<TypedName>> _functionReturnVariables
	);

	OptimiserStepContext& m_context;
	VariableMemoryOffsetTracker const& m_memoryOffsetTracker;
	NameDispenser& m_nameDispenser;
	/// Map from function names to the return variables of the function with that name.
	std::map<YulString, std::vector<TypedName>> m_functionReturnVariables;
	/// List of functions generated while running this step that are to be appended to the code in the end.
	std::list<Statement> m_newFunctionDefinitions;
};

}