Equal store eliminator.

Changelog.md

@@ -5,6 +5,7 @@ Language Features:
 
 
 Compiler Features:
+ * Yul Optimizer: Remove ``mstore`` and ``sstore`` operations if the slot already contains the same value.
 
 
 

docs/internals/optimizer.rst

@@ -290,6 +290,7 @@ on the individual steps and their sequence below.
 - :ref:`conditional-unsimplifier`.
 - :ref:`control-flow-simplifier`.
 - :ref:`dead-code-eliminator`.
+- :ref:`equal-store-eliminator`.
 - :ref:`equivalent-function-combiner`.
 - :ref:`expression-joiner`.
 - :ref:`expression-simplifier`.
@@ -938,6 +939,22 @@ we require ForLoopInitRewriter to run before this step.
 
 Prerequisite: ForLoopInitRewriter, Function Hoister, Function Grouper
 
+.. _equal-store-eliminator:
+
+EqualStoreEliminator
+^^^^^^^^^^^^^^^^^^^^
+
+This steps removes ``mstore(k, v)`` and ``sstore(k, v)`` calls if
+there was a previous call to ``mstore(k, v)`` / ``sstore(k, v)``,
+no other store in between and the values of ``k`` and ``v`` did not change.
+
+This simple step is effective if run after the SSA transform and the
+Common Subexpression Eliminator, because SSA will make sure that the variables
+will not change and the Common Subexpression Eliminator re-uses exactly the same
+variable if the value is known to be the same.
+
+Prerequisites: Disambiguator, ForLoopInitRewriter
+
 .. _unused-pruner:
 
 UnusedPruner

libsolidity/interface/OptimiserSettings.h

@@ -44,7 +44,7 @@ struct OptimiserSettings
 	static char constexpr DefaultYulOptimiserSteps[] =
 		"dhfoDgvulfnTUtnIf"            // None of these can make stack problems worse
 		"["
-			"xa[r]scLM"                // Turn into SSA and simplify
+			"xa[r]EscLM"               // Turn into SSA and simplify
 			"cCTUtTOntnfDIul"          // Perform structural simplification
 			"Lcul"                     // Simplify again
 			"Vcul [j]"                 // Reverse SSA

libyul/CMakeLists.txt

@@ -120,6 +120,8 @@ add_library(yul
 	optimiser/DeadCodeEliminator.h
 	optimiser/Disambiguator.cpp
 	optimiser/Disambiguator.h
+	optimiser/EqualStoreEliminator.cpp
+	optimiser/EqualStoreEliminator.h
 	optimiser/EquivalentFunctionDetector.cpp
 	optimiser/EquivalentFunctionDetector.h
 	optimiser/EquivalentFunctionCombiner.cpp

libyul/optimiser/EqualStoreEliminator.cpp

@@ -0,0 +1,70 @@
+/*
+	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/>.
+*/
+// SPDX-License-Identifier: GPL-3.0
+/**
+ * Optimisation stage that removes mstore and sstore operations if they store the same
+ * value that is already known to be in that slot.
+ */
+
+#include <libyul/optimiser/EqualStoreEliminator.h>
+
+#include <libyul/optimiser/CallGraphGenerator.h>
+#include <libyul/optimiser/OptimizerUtilities.h>
+#include <libyul/optimiser/Semantics.h>
+#include <libyul/AST.h>
+#include <libyul/Utilities.h>
+
+using namespace std;
+using namespace solidity;
+using namespace solidity::util;
+using namespace solidity::evmasm;
+using namespace solidity::yul;
+
+void EqualStoreEliminator::run(OptimiserStepContext const& _context, Block& _ast)
+{
+	EqualStoreEliminator eliminator{
+		_context.dialect,
+		SideEffectsPropagator::sideEffects(_context.dialect, CallGraphGenerator::callGraph(_ast))
+	};
+	eliminator(_ast);
+
+	StatementRemover remover{eliminator.m_pendingRemovals};
+	remover(_ast);
+}
+
+void EqualStoreEliminator::visit(Statement& _statement)
+{
+	// No need to consider potential changes through complex arguments since
+	// isSimpleStore only returns something if the arguments are identifiers.
+	if (ExpressionStatement const* expression = get_if<ExpressionStatement>(&_statement))
+	{
+		if (auto vars = isSimpleStore(StoreLoadLocation::Storage, *expression))
+		{
+			if (auto const* currentValue = valueOrNullptr(m_storage, vars->first))
+				if (*currentValue == vars->second)
+					m_pendingRemovals.insert(&_statement);
+		}
+		else if (auto vars = isSimpleStore(StoreLoadLocation::Memory, *expression))
+		{
+			if (auto const* currentValue = valueOrNullptr(m_memory, vars->first))
+				if (*currentValue == vars->second)
+					m_pendingRemovals.insert(&_statement);
+		}
+	}
+
+	DataFlowAnalyzer::visit(_statement);
+}

libyul/optimiser/EqualStoreEliminator.h

@@ -0,0 +1,60 @@
+/*
+	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/>.
+*/
+// SPDX-License-Identifier: GPL-3.0
+/**
+ * Optimisation stage that removes mstore and sstore operations if they store the same
+ * value that is already known to be in that slot.
+ */
+
+#pragma once
+
+#include <libyul/optimiser/DataFlowAnalyzer.h>
+#include <libyul/optimiser/OptimiserStep.h>
+
+namespace solidity::yul
+{
+
+/**
+ * Optimisation stage that removes mstore and sstore operations if they store the same
+ * value that is already known to be in that slot.
+ *
+ * Works best if the code is in SSA form - without literal arguments.
+ *
+ * Prerequisite: Disambiguator, ForLoopInitRewriter.
+ */
+class EqualStoreEliminator: public DataFlowAnalyzer
+{
+public:
+	static constexpr char const* name{"EqualStoreEliminator"};
+	static void run(OptimiserStepContext const&, Block& _ast);
+
+private:
+	EqualStoreEliminator(
+		Dialect const& _dialect,
+		std::map<YulString, SideEffects> _functionSideEffects
+	):
+		DataFlowAnalyzer(_dialect, std::move(_functionSideEffects))
+	{}
+
+protected:
+	using ASTModifier::visit;
+	void visit(Statement& _statement) override;
+
+	std::set<Statement const*> m_pendingRemovals;
+};
+
+}

libyul/optimiser/OptimizerUtilities.cpp

@@ -57,3 +57,18 @@ optional<evmasm::Instruction> yul::toEVMInstruction(Dialect const& _dialect, Yul
 			return builtin->instruction;
 	return nullopt;
 }
+
+void StatementRemover::operator()(Block& _block)
+{
+	util::iterateReplacing(
+		_block.statements,
+		[&](Statement& _statement) -> std::optional<vector<Statement>>
+		{
+			if (m_toRemove.count(&_statement))
+				return {vector<Statement>{}};
+			else
+				return nullopt;
+		}
+	);
+	ASTModifier::operator()(_block);
+}

libyul/optimiser/OptimizerUtilities.h

@@ -25,6 +25,7 @@
 #include <libyul/ASTForward.h>
 #include <libyul/Dialect.h>
 #include <libyul/YulString.h>
+#include <libyul/optimiser/ASTWalker.h>
 
 #include <optional>
 
@@ -48,4 +49,14 @@ bool isRestrictedIdentifier(Dialect const& _dialect, YulString const& _identifie
 /// Helper function that returns the instruction, if the `_name` is a BuiltinFunction
 std::optional<evmasm::Instruction> toEVMInstruction(Dialect const& _dialect, YulString const& _name);
 
+class StatementRemover: public ASTModifier
+{
+public:
+	explicit StatementRemover(std::set<Statement const*> const& _toRemove): m_toRemove(_toRemove) {}
+
+	void operator()(Block& _block) override;
+private:
+	std::set<Statement const*> const& m_toRemove;
+};
+
 }

libyul/optimiser/Suite.cpp

@@ -32,6 +32,7 @@
 #include <libyul/optimiser/DeadCodeEliminator.h>
 #include <libyul/optimiser/FunctionGrouper.h>
 #include <libyul/optimiser/FunctionHoister.h>
+#include <libyul/optimiser/EqualStoreEliminator.h>
 #include <libyul/optimiser/EquivalentFunctionCombiner.h>
 #include <libyul/optimiser/ExpressionSplitter.h>
 #include <libyul/optimiser/ExpressionJoiner.h>
@@ -204,6 +205,7 @@ map<string, unique_ptr<OptimiserStep>> const& OptimiserSuite::allSteps()
 			ConditionalUnsimplifier,
 			ControlFlowSimplifier,
 			DeadCodeEliminator,
+			EqualStoreEliminator,
 			EquivalentFunctionCombiner,
 			ExpressionInliner,
 			ExpressionJoiner,
@@ -244,6 +246,7 @@ map<string, char> const& OptimiserSuite::stepNameToAbbreviationMap()
 		{ConditionalUnsimplifier::name,       'U'},
 		{ControlFlowSimplifier::name,         'n'},
 		{DeadCodeEliminator::name,            'D'},
+		{EqualStoreEliminator::name,          'E'},
 		{EquivalentFunctionCombiner::name,    'v'},
 		{ExpressionInliner::name,             'e'},
 		{ExpressionJoiner::name,              'j'},

libyul/optimiser/UnusedAssignEliminator.cpp

@@ -23,6 +23,7 @@
 #include <libyul/optimiser/UnusedAssignEliminator.h>
 
 #include <libyul/optimiser/Semantics.h>
+#include <libyul/optimiser/OptimizerUtilities.h>
 #include <libyul/AST.h>
 
 #include <libsolutil/CommonData.h>

libyul/optimiser/UnusedStoreBase.cpp

@@ -156,18 +156,3 @@ void UnusedStoreBase::merge(TrackedStores& _target, vector<TrackedStores>&& _sou
 		merge(_target, move(ts));
 	_source.clear();
 }
-
-void StatementRemover::operator()(Block& _block)
-{
-	util::iterateReplacing(
-		_block.statements,
-		[&](Statement& _statement) -> std::optional<vector<Statement>>
-		{
-			if (m_toRemove.count(&_statement))
-				return {vector<Statement>{}};
-			else
-				return nullopt;
-		}
-	);
-	ASTModifier::operator()(_block);
-}

libyul/optimiser/UnusedStoreBase.h

@@ -105,14 +105,4 @@ class UnusedStoreBase: public ASTWalker
 	size_t m_forLoopNestingDepth = 0;
 };
 
-class StatementRemover: public ASTModifier
-{
-public:
-	explicit StatementRemover(std::set<Statement const*> const& _toRemove): m_toRemove(_toRemove) {}
-
-	void operator()(Block& _block) override;
-private:
-	std::set<Statement const*> const& m_toRemove;
-};
-
 }

test/libsolidity/semanticTests/externalContracts/deposit_contract.sol

@@ -178,7 +178,7 @@ contract DepositContract is IDepositContract, ERC165 {
 // compileViaYul: also
 // ----
 // constructor()
-// gas irOptimized: 1558001
+// gas irOptimized: 1557137
 // gas legacy: 2436584
 // gas legacyOptimized: 1776483
 // supportsInterface(bytes4): 0x0 -> 0

test/libsolidity/semanticTests/functionCall/mapping_array_internal_argument.sol

@@ -20,7 +20,7 @@ contract test {
 // compileViaYul: also
 // ----
 // set(uint8,uint8,uint8,uint8,uint8): 1, 21, 22, 42, 43 -> 0, 0, 0, 0
-// gas irOptimized: 111965
+// gas irOptimized: 111896
 // gas legacy: 113806
 // gas legacyOptimized: 111781
 // get(uint8): 1 -> 21, 22, 42, 43

test/libsolidity/semanticTests/structs/struct_copy.sol

@@ -38,12 +38,12 @@ contract c {
 // compileViaYul: also
 // ----
 // set(uint256): 7 -> true
-// gas irOptimized: 110011
+// gas irOptimized: 110119
 // gas legacy: 110616
 // gas legacyOptimized: 110006
 // retrieve(uint256): 7 -> 1, 3, 4, 2
 // copy(uint256,uint256): 7, 8 -> true
-// gas irOptimized: 118707
+// gas irOptimized: 118698
 // gas legacy: 119166
 // gas legacyOptimized: 118622
 // retrieve(uint256): 7 -> 1, 3, 4, 2

test/libyul/YulOptimizerTestCommon.cpp

@@ -28,6 +28,7 @@
 #include <libyul/optimiser/ConditionalUnsimplifier.h>
 #include <libyul/optimiser/ConditionalSimplifier.h>
 #include <libyul/optimiser/CommonSubexpressionEliminator.h>
+#include <libyul/optimiser/EqualStoreEliminator.h>
 #include <libyul/optimiser/EquivalentFunctionCombiner.h>
 #include <libyul/optimiser/ExpressionSplitter.h>
 #include <libyul/optimiser/FunctionGrouper.h>
@@ -236,6 +237,11 @@ YulOptimizerTestCommon::YulOptimizerTestCommon(
 			ForLoopInitRewriter::run(*m_context, *m_ast);
 			UnusedAssignEliminator::run(*m_context, *m_ast);
 		}},
+		{"equalStoreEliminator", [&]() {
+			disambiguate();
+			ForLoopInitRewriter::run(*m_context, *m_ast);
+			EqualStoreEliminator::run(*m_context, *m_ast);
+		}},
 		{"ssaPlusCleanup", [&]() {
 			disambiguate();
 			ForLoopInitRewriter::run(*m_context, *m_ast);

test/libyul/yulOptimizerTests/equalStoreEliminator/branching.yul

@@ -0,0 +1,25 @@
+{
+    let a := calldataload(0)
+    let b := 20
+    sstore(a, b)
+    if calldataload(32) {
+        sstore(a, b)
+        pop(staticcall(0, 0, 0, 0, 0, 0))
+        sstore(a, b)
+    }
+    sstore(a, b)
+}
+// ====
+// EVMVersion: >=byzantium
+// ----
+// step: equalStoreEliminator
+//
+// {
+//     let a := calldataload(0)
+//     let b := 20
+//     sstore(a, b)
+//     if calldataload(32)
+//     {
+//         pop(staticcall(0, 0, 0, 0, 0, 0))
+//     }
+// }

test/libyul/yulOptimizerTests/equalStoreEliminator/forloop.yul

@@ -0,0 +1,20 @@
+{
+    let x := calldataload(0)
+    let y := calldataload(1)
+
+    sstore(x, y)
+    for {let a := 1} lt(a, 10) {a := add(a, 1) } {
+        sstore(x, y)
+    }
+}
+// ----
+// step: equalStoreEliminator
+//
+// {
+//     let x := calldataload(0)
+//     let y := calldataload(1)
+//     sstore(x, y)
+//     let a := 1
+//     for { } lt(a, 10) { a := add(a, 1) }
+//     { sstore(x, y) }
+// }