diff --git a/libyul/CMakeLists.txt b/libyul/CMakeLists.txt
index b70d813b4..2e04bf9be 100644
--- a/libyul/CMakeLists.txt
+++ b/libyul/CMakeLists.txt
@@ -40,6 +40,8 @@ add_library(yul
 	backends/evm/NoOutputAssembly.cpp
 	backends/wasm/WasmDialect.cpp
 	backends/wasm/WasmDialect.h
+	backends/wasm/WordSizeTransform.cpp
+	backends/wasm/WordSizeTransform.h
 	optimiser/ASTCopier.cpp
 	optimiser/ASTCopier.h
 	optimiser/ASTWalker.cpp
diff --git a/libyul/backends/wasm/WordSizeTransform.cpp b/libyul/backends/wasm/WordSizeTransform.cpp
new file mode 100644
index 000000000..d99d4cc26
--- /dev/null
+++ b/libyul/backends/wasm/WordSizeTransform.cpp
@@ -0,0 +1,238 @@
+/*
+	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/AsmData.h>
+#include <libyul/backends/wasm/WordSizeTransform.h>
+#include <libyul/Utilities.h>
+
+#include <libdevcore/CommonData.h>
+
+#include <array>
+
+using namespace std;
+using namespace dev;
+using namespace yul;
+
+void WordSizeTransform::operator()(FunctionDefinition& _fd)
+{
+	rewriteVarDeclList(_fd.parameters);
+	rewriteVarDeclList(_fd.returnVariables);
+	(*this)(_fd.body);
+}
+
+void WordSizeTransform::operator()(FunctionalInstruction& _ins)
+{
+	rewriteFunctionCallArguments(_ins.arguments);
+}
+
+void WordSizeTransform::operator()(FunctionCall& _fc)
+{
+	rewriteFunctionCallArguments(_fc.arguments);
+}
+
+void WordSizeTransform::operator()(If&)
+{
+	yulAssert(false, "If statement not implemented.");
+}
+
+void WordSizeTransform::operator()(Switch&)
+{
+	yulAssert(false, "Switch statement not implemented.");
+}
+
+void WordSizeTransform::operator()(Block& _block)
+{
+	iterateReplacing(
+		_block.statements,
+		[&](Statement& _s) -> boost::optional<vector<Statement>>
+		{
+			if (_s.type() == typeid(VariableDeclaration))
+			{
+				VariableDeclaration& varDecl = boost::get<VariableDeclaration>(_s);
+				if (
+					!varDecl.value ||
+					varDecl.value->type() == typeid(FunctionalInstruction) ||
+					varDecl.value->type() == typeid(FunctionCall)
+				)
+				{
+					if (varDecl.value) visit(*varDecl.value);
+					rewriteVarDeclList(varDecl.variables);
+					return boost::none;
+				}
+				else if (
+					varDecl.value->type() == typeid(Identifier) ||
+					varDecl.value->type() == typeid(Literal)
+				)
+				{
+					yulAssert(varDecl.variables.size() == 1, "");
+					auto newRhs = expandValue(*varDecl.value);
+					auto newLhs = generateU64IdentifierNames(varDecl.variables[0].name);
+					vector<Statement> ret;
+					for (int i = 0; i < 4; i++)
+						ret.push_back(
+							VariableDeclaration{
+								varDecl.location,
+								{TypedName{varDecl.location, newLhs[i], "u64"_yulstring}},
+								std::move(newRhs[i])
+							}
+						);
+					return ret;
+				}
+				else
+					yulAssert(false, "");
+			}
+			else if (_s.type() == typeid(Assignment))
+			{
+				Assignment& assignment = boost::get<Assignment>(_s);
+				yulAssert(assignment.value, "");
+				if (
+					assignment.value->type() == typeid(FunctionalInstruction) ||
+					assignment.value->type() == typeid(FunctionCall)
+				)
+				{
+					if (assignment.value) visit(*assignment.value);
+					rewriteIdentifierList(assignment.variableNames);
+					return boost::none;
+				}
+				else if (
+					assignment.value->type() == typeid(Identifier) ||
+					assignment.value->type() == typeid(Literal)
+				)
+				{
+					yulAssert(assignment.variableNames.size() == 1, "");
+					auto newRhs = expandValue(*assignment.value);
+					YulString lhsName = assignment.variableNames[0].name;
+					vector<Statement> ret;
+					for (int i = 0; i < 4; i++)
+						ret.push_back(
+							Assignment{
+								assignment.location,
+								{Identifier{assignment.location, m_variableMapping.at(lhsName)[i]}},
+								std::move(newRhs[i])
+							}
+						);
+					return ret;
+				}
+				else
+					yulAssert(false, "");
+			}
+			else
+				visit(_s);
+			return boost::none;
+		}
+	);
+}
+
+void WordSizeTransform::run(Block& _ast, NameDispenser& _nameDispenser)
+{
+	WordSizeTransform{_nameDispenser}(_ast);
+}
+
+void WordSizeTransform::rewriteVarDeclList(TypedNameList& _nameList)
+{
+	iterateReplacing(
+		_nameList,
+		[&](TypedName const& _n) -> boost::optional<TypedNameList>
+		{
+			TypedNameList ret;
+			yulAssert(m_variableMapping.find(_n.name) == m_variableMapping.end(), "");
+			for (int i = 0; i < 4; i++)
+			{
+				auto newName = m_nameDispenser.newName(_n.name);
+				m_variableMapping[_n.name][i] = newName;
+				ret.push_back(TypedName{_n.location, newName, "u64"_yulstring});
+			}
+			return ret;
+		}
+	);
+}
+
+void WordSizeTransform::rewriteIdentifierList(vector<Identifier>& _ids)
+{
+	iterateReplacing(
+		_ids,
+		[&](Identifier const& _id) -> boost::optional<vector<Identifier>>
+		{
+			vector<Identifier> ret;
+			for (auto newId: m_variableMapping.at(_id.name))
+				ret.push_back(Identifier{_id.location, newId});
+			return ret;
+		}
+	);
+}
+
+void WordSizeTransform::rewriteFunctionCallArguments(vector<Expression>& _args)
+{
+	iterateReplacing(
+		_args,
+		[&](Expression& _e) -> boost::optional<vector<Expression>>
+		{
+			// ExpressionSplitter guarantees arguments to be Identifier or Literal
+			yulAssert(_e.type() == typeid(Identifier) || _e.type() == typeid(Literal), "");
+			vector<Expression> ret;
+			for (auto& v: expandValue(_e))
+				ret.push_back(*v);
+			return ret;
+		}
+	);
+}
+
+array<YulString, 4> WordSizeTransform::generateU64IdentifierNames(YulString const& _s)
+{
+	yulAssert(m_variableMapping.find(_s) == m_variableMapping.end(), "");
+	array<YulString, 4> ret;
+	for (int i = 0; i < 4; i++)
+	{
+		auto newName = m_nameDispenser.newName(_s);
+		m_variableMapping[_s][i] = newName;
+		ret[i] = newName;
+	}
+	return ret;
+}
+
+array<unique_ptr<Expression>, 4> WordSizeTransform::expandValue(Expression const& _e)
+{
+	array<unique_ptr<Expression>, 4> ret;
+	if (_e.type() == typeid(Identifier))
+	{
+		Identifier const& id = boost::get<Identifier>(_e);
+		for (int i = 0; i < 4; i++)
+			ret[i] = make_unique<Expression>(Identifier{id.location, m_variableMapping.at(id.name)[i]});
+	}
+	else if (_e.type() == typeid(Literal))
+	{
+		Literal const& lit = boost::get<Literal>(_e);
+		u256 val = valueOfLiteral(lit);
+		for (int i = 3; i >= 0; i--)
+		{
+			u256 currentVal = val & std::numeric_limits<uint64_t>::max();
+			val >>= 64;
+			ret[i] = make_unique<Expression>(
+				Literal{
+					lit.location,
+					LiteralKind::Number,
+					YulString(currentVal.str()),
+					"u64"_yulstring
+				}
+			);
+		}
+	}
+	else
+		yulAssert(false, "");
+	return ret;
+}
+
diff --git a/libyul/backends/wasm/WordSizeTransform.h b/libyul/backends/wasm/WordSizeTransform.h
new file mode 100644
index 000000000..a9f480551
--- /dev/null
+++ b/libyul/backends/wasm/WordSizeTransform.h
@@ -0,0 +1,81 @@
+/*
+	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/>.
+*/
+/**
+ * Replace every u256 variable with four u64 variables.
+ */
+
+#pragma once
+
+#include <libyul/optimiser/ASTWalker.h>
+#include <libyul/optimiser/NameDispenser.h>
+
+#include <array>
+#include <vector>
+
+namespace yul
+{
+
+/**
+ * A stage that replace every u256 variable with four u64 variables.
+ * This transformation stage is required because values in EVM are 256 bits,
+ * but wasm only supports values up to 64 bits, so we use four u64 values to simulate
+ * one u256 value.
+ *
+ * For FunctionalInstruction that accepts or returns u256 values, they accepts or returns
+ * four times the number of values after this transformation, with the order of significance,
+ * from the most significant to the least significant.
+ *
+ * For example, the FunctionalInstruction MUL supplied by code generator
+ * should take 8 arguments and return 4 values (instead of 2 and 1) after this transformation.
+ *
+ * mul(a1, a2, a3, a4, b1, b2, b3, b4) -> c1, c2, c3, c4
+ *
+ * the value of c4 should be
+ *	((a1*(2^192) + a2*(2^128) + a3(2^64) + a4) * (b1*(2^192) + b2*(2^128) + b3(2^64) + b4)) & ((1<<64)-1)
+ *
+ * Prerequisite: Disambiguator, ExpressionSplitter
+ */
+class WordSizeTransform: public ASTModifier
+{
+public:
+	void operator()(FunctionDefinition&) override;
+	void operator()(FunctionalInstruction&) override;
+	void operator()(FunctionCall&) override;
+	void operator()(If&) override;
+	void operator()(Switch&) override;
+	void operator()(Block& _block) override;
+
+	static void run(Block& _ast, NameDispenser& _nameDispenser);
+
+private:
+	explicit WordSizeTransform(NameDispenser& _nameDispenser):
+		m_nameDispenser(_nameDispenser)
+	{ }
+
+	void rewriteVarDeclList(std::vector<TypedName>&);
+	void rewriteIdentifierList(std::vector<Identifier>&);
+	void rewriteFunctionCallArguments(std::vector<Expression>&);
+
+	std::array<YulString, 4> generateU64IdentifierNames(YulString const& _s);
+	std::array<std::unique_ptr<Expression>, 4> expandValue(Expression const& _e);
+
+	NameDispenser& m_nameDispenser;
+	/// maps original u256 variable's name to corresponding u64 variables' names
+	std::map<YulString, std::array<YulString, 4>> m_variableMapping;
+};
+
+}