10 files changed, 196 insertions, 0 deletions
diff --git a/libyul/CMakeLists.txt b/libyul/CMakeLists.txt
index 74a5703c..9dee5348 100644
--- a/libyul/CMakeLists.txt
+++ b/libyul/CMakeLists.txt
@@ -44,5 +44,6 @@ add_library(yul
 	optimiser/UnusedPruner.cpp
 	optimiser/Utilities.cpp
 	optimiser/VarDeclPropagator.cpp
+	optimiser/VarDeclInitializer.cpp
 )
 target_link_libraries(yul PUBLIC evmasm devcore langutil)
diff --git a/libyul/optimiser/Suite.cpp b/libyul/optimiser/Suite.cpp
index ad22bfa3..16df6d3c 100644
--- a/libyul/optimiser/Suite.cpp
+++ b/libyul/optimiser/Suite.cpp
@@ -21,6 +21,7 @@
 #include <libyul/optimiser/Suite.h>
 
 #include <libyul/optimiser/Disambiguator.h>
+#include <libyul/optimiser/VarDeclInitializer.h>
 #include <libyul/optimiser/FunctionGrouper.h>
 #include <libyul/optimiser/FunctionHoister.h>
 #include <libyul/optimiser/ExpressionSplitter.h>
@@ -56,6 +57,7 @@ void OptimiserSuite::run(
 
 	Block ast = boost::get<Block>(Disambiguator(_analysisInfo, reservedIdentifiers)(_ast));
 
+	(VarDeclInitializer{})(ast);
 	(FunctionHoister{})(ast);
 	(FunctionGrouper{})(ast);
 	(ForLoopInitRewriter{})(ast);
diff --git a/libyul/optimiser/VarDeclInitializer.cpp b/libyul/optimiser/VarDeclInitializer.cpp
new file mode 100644
index 00000000..4a26757f
--- /dev/null
+++ b/libyul/optimiser/VarDeclInitializer.cpp
@@ -0,0 +1,56 @@
+/*
+	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/>.
+*/
+
+#include <libyul/optimiser/VarDeclInitializer.h>
+#include <libyul/AsmData.h>
+
+#include <libdevcore/CommonData.h>
+#include <libdevcore/Visitor.h>
+
+using namespace std;
+using namespace dev;
+using namespace yul;
+
+void VarDeclInitializer::operator()(Block& _block)
+{
+	ASTModifier::operator()(_block);
+
+	static Expression const zero{Literal{{}, LiteralKind::Number, YulString{"0"}, {}}};
+
+	using OptionalStatements = boost::optional<vector<Statement>>;
+	GenericFallbackReturnsVisitor<OptionalStatements, VariableDeclaration> visitor{
+		[](VariableDeclaration& _varDecl) -> OptionalStatements
+		{
+			if (_varDecl.value)
+				return {};
+			else if (_varDecl.variables.size() == 1)
+			{
+				_varDecl.value = make_shared<Expression>(zero);
+				return {};
+			}
+			else
+			{
+				OptionalStatements ret{vector<Statement>{}};
+				langutil::SourceLocation loc{std::move(_varDecl.location)};
+				for (auto& var: _varDecl.variables)
+					ret->push_back(VariableDeclaration{loc, {std::move(var)}, make_shared<Expression>(zero)});
+				return ret;
+			}
+		}
+	};
+	iterateReplacing(_block.statements, boost::apply_visitor(visitor));
+}
diff --git a/libyul/optimiser/VarDeclInitializer.h b/libyul/optimiser/VarDeclInitializer.h
new file mode 100644
index 00000000..41d0917c
--- /dev/null
+++ b/libyul/optimiser/VarDeclInitializer.h
@@ -0,0 +1,38 @@
+/*
+	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/>.
+*/
+
+#pragma once
+
+#include <libyul/AsmDataForward.h>
+#include <libyul/optimiser/ASTWalker.h>
+
+namespace yul
+{
+
+/**
+ * Rewrites variable declarations so that all of them are initialized.
+ * Declarations like ``let x, y`` are split into multiple declaration
+ * statements.
+ * Only supports initializing with the zero literal for now.
+ */
+class VarDeclInitializer: public ASTModifier
+{
+public:
+	void operator()(Block& _block) override;
+};
+
+}
diff --git a/test/libyul/YulOptimizerTest.cpp b/test/libyul/YulOptimizerTest.cpp
index 68226e33..68de563d 100644
--- a/test/libyul/YulOptimizerTest.cpp
+++ b/test/libyul/YulOptimizerTest.cpp
@@ -23,6 +23,7 @@
 
 #include <libyul/optimiser/BlockFlattener.h>
 #include <libyul/optimiser/VarDeclPropagator.h>
+#include <libyul/optimiser/VarDeclInitializer.h>
 #include <libyul/optimiser/Disambiguator.h>
 #include <libyul/optimiser/CommonSubexpressionEliminator.h>
 #include <libyul/optimiser/NameCollector.h>
@@ -112,6 +113,8 @@ bool YulOptimizerTest::run(ostream& _stream, string const& _linePrefix, bool con
 		disambiguate();
 		VarDeclPropagator{}(*m_ast);
 	}
+	else if (m_optimizerStep == "varDeclInitializer")
+		VarDeclInitializer{}(*m_ast);
 	else if (m_optimizerStep == "forLoopInitRewriter")
 	{
 		disambiguate();
diff --git a/test/libyul/yulOptimizerTests/varDeclInitializer/ambiguous.yul b/test/libyul/yulOptimizerTests/varDeclInitializer/ambiguous.yul
new file mode 100644
index 00000000..5e2d60c2
--- /dev/null
+++ b/test/libyul/yulOptimizerTests/varDeclInitializer/ambiguous.yul
@@ -0,0 +1,29 @@
+{
+  // This component does not need the disambiguator
+  function f() -> x, y {
+    let a, b
+    mstore(a, b)
+    let d
+    d := 2
+  }
+  let a
+  a := 4
+  let b := 2
+  let x, y := f()
+}
+// ----
+// varDeclInitializer
+// {
+//     function f() -> x, y
+//     {
+//         let a := 0
+//         let b := 0
+//         mstore(a, b)
+//         let d := 0
+//         d := 2
+//     }
+//     let a := 0
+//     a := 4
+//     let b := 2
+//     let x, y := f()
+// }
diff --git a/test/libyul/yulOptimizerTests/varDeclInitializer/inside_func.yul b/test/libyul/yulOptimizerTests/varDeclInitializer/inside_func.yul
new file mode 100644
index 00000000..16428d7e
--- /dev/null
+++ b/test/libyul/yulOptimizerTests/varDeclInitializer/inside_func.yul
@@ -0,0 +1,24 @@
+{
+  function f() -> x, y {
+    let a, b
+    mstore(a, b)
+    let d
+    d := 2
+  }
+  let r
+  r := 4
+}
+// ----
+// varDeclInitializer
+// {
+//     function f() -> x, y
+//     {
+//         let a := 0
+//         let b := 0
+//         mstore(a, b)
+//         let d := 0
+//         d := 2
+//     }
+//     let r := 0
+//     r := 4
+// }
diff --git a/test/libyul/yulOptimizerTests/varDeclInitializer/multi.yul b/test/libyul/yulOptimizerTests/varDeclInitializer/multi.yul
new file mode 100644
index 00000000..02d731af
--- /dev/null
+++ b/test/libyul/yulOptimizerTests/varDeclInitializer/multi.yul
@@ -0,0 +1,14 @@
+{
+  let x, y, z
+  let a
+  let b
+}
+// ----
+// varDeclInitializer
+// {
+//     let x := 0
+//     let y := 0
+//     let z := 0
+//     let a := 0
+//     let b := 0
+// }
diff --git a/test/libyul/yulOptimizerTests/varDeclInitializer/multi_assign.yul b/test/libyul/yulOptimizerTests/varDeclInitializer/multi_assign.yul
new file mode 100644
index 00000000..2e14fe70
--- /dev/null
+++ b/test/libyul/yulOptimizerTests/varDeclInitializer/multi_assign.yul
@@ -0,0 +1,21 @@
+{
+  function f() -> x, y {
+    let a, b := f()
+	let u
+  }
+  let r
+  let s := 3
+  let t
+}
+// ----
+// varDeclInitializer
+// {
+//     function f() -> x, y
+//     {
+//         let a, b := f()
+//         let u := 0
+//     }
+//     let r := 0
+//     let s := 3
+//     let t := 0
+// }
diff --git a/test/libyul/yulOptimizerTests/varDeclInitializer/simple.yul b/test/libyul/yulOptimizerTests/varDeclInitializer/simple.yul
new file mode 100644
index 00000000..2a9bbe42
--- /dev/null
+++ b/test/libyul/yulOptimizerTests/varDeclInitializer/simple.yul
@@ -0,0 +1,8 @@
+{
+  let a
+}
+// ----
+// varDeclInitializer
+// {
+//     let a := 0
+// }