mirror of
https://github.com/ethereum/solidity
synced 2023-10-03 13:03:40 +00:00
98 lines
3.2 KiB
C++
98 lines
3.2 KiB
C++
/*
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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/AsmDataForward.h>
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#include <libyul/optimiser/ASTWalker.h>
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#include <libyul/optimiser/OptimiserStep.h>
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#include <liblangutil/SourceLocation.h>
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#include <vector>
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namespace solidity::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. We compensate that by appending a declaration
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* of the form of "let a_1 := a" right after the location where control flow joins so
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* variable references can use the SSA variable. The only exception to this rule are
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* for loop conditions, as we cannot insert a variable declaration there.
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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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* The transform is implemented in three stages. All stages are only concerned
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* with variables that are assigned somewhere in the code (excluding declarations).
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* The first stage inserts new SSA variables for each declaration and assignment of
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* such variables.
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* The second stage inserts new SSA variables at control flow joins.
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* The last stage replaces references to variables that are assigned to somewhere in the
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* code by their current SSA variable.
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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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* Prerequisite: Disambiguator, ForLoopInitRewriter.
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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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static constexpr char const* name{"SSATransform"};
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static void run(OptimiserStepContext& _context, Block& _ast);
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};
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}
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