From 37fb3cd2fd313ece351d2752f7b71fdd63259377 Mon Sep 17 00:00:00 2001
From: chriseth <chris@ethereum.org>
Date: Tue, 14 May 2019 13:12:51 +0200
Subject: [PATCH] Refactor: Pull out single value optimisation.

---
 libevmasm/ConstantOptimiser.cpp | 50 ++++++++++++++++++---------------
 libevmasm/ConstantOptimiser.h   | 13 +++++++--
 2 files changed, 39 insertions(+), 24 deletions(-)

diff --git a/libevmasm/ConstantOptimiser.cpp b/libevmasm/ConstantOptimiser.cpp
index 267daa5e5..68c1e24fb 100644
--- a/libevmasm/ConstantOptimiser.cpp
+++ b/libevmasm/ConstantOptimiser.cpp
@@ -44,39 +44,45 @@ unsigned ConstantOptimisationMethod::optimiseConstants(
 	map<u256, AssemblyItems> pendingReplacements;
 	for (auto it: pushes)
 	{
-		AssemblyItem const& item = it.first;
-		if (item.data() < 0x100)
-			continue;
+		u256 value = it.first.data();
 		Params params;
 		params.multiplicity = it.second;
 		params.isCreation = _isCreation;
 		params.runs = _runs;
 		params.evmVersion = _evmVersion;
-		LiteralMethod lit(params, item.data());
-		bigint literalGas = lit.gasNeeded();
-		CodeCopyMethod copy(params, item.data());
-		bigint copyGas = copy.gasNeeded();
-		ComputeMethod compute(params, item.data());
-		bigint computeGas = compute.gasNeeded();
-		AssemblyItems replacement;
-		if (copyGas < literalGas && copyGas < computeGas)
-		{
-			replacement = copy.execute(_assembly);
-			optimisations++;
-		}
-		else if (computeGas < literalGas && computeGas <= copyGas)
-		{
-			replacement = compute.execute(_assembly);
-			optimisations++;
-		}
-		if (!replacement.empty())
-			pendingReplacements[item.data()] = replacement;
+		AssemblyItems replacement = optimiseSingleConstant(value, params, _assembly);
+		if (replacement.empty())
+			continue;
+		pendingReplacements[value] = move(replacement);
+		optimisations++;
 	}
 	if (!pendingReplacements.empty())
 		replaceConstants(_items, pendingReplacements);
 	return optimisations;
 }
 
+AssemblyItems ConstantOptimisationMethod::optimiseSingleConstant(
+	u256 const& _value,
+	Params const& _params,
+	Assembly& _assembly
+)
+{
+	if (_value < 0x100)
+		return {};
+
+	LiteralMethod lit(_params, _value);
+	bigint literalGas = lit.gasNeeded();
+	CodeCopyMethod copy(_params, _value);
+	bigint copyGas = copy.gasNeeded();
+	ComputeMethod compute(_params, _value);
+	bigint computeGas = compute.gasNeeded();
+	if (copyGas < literalGas && copyGas < computeGas)
+		return copy.execute(_assembly);
+	else if (computeGas < literalGas && computeGas <= copyGas)
+		return compute.execute(_assembly);
+	return {};
+}
+
 bigint ConstantOptimisationMethod::simpleRunGas(AssemblyItems const& _items)
 {
 	bigint gas = 0;
diff --git a/libevmasm/ConstantOptimiser.h b/libevmasm/ConstantOptimiser.h
index be1628c71..a73f94c4e 100644
--- a/libevmasm/ConstantOptimiser.h
+++ b/libevmasm/ConstantOptimiser.h
@@ -57,8 +57,6 @@ public:
 	);
 
 protected:
-	/// This is the public API for the optimiser methods, but it doesn't need to be exposed to the caller.
-
 	struct Params
 	{
 		bool isCreation; ///< Whether this is called during contract creation or runtime.
@@ -67,6 +65,17 @@ protected:
 		langutil::EVMVersion evmVersion; ///< Version of the EVM
 	};
 
+	/// Computes the replacement for a single constant.
+	/// This can already modify @a _assembly by adding a data item,
+	/// but the return value still has to be inserted.
+	static AssemblyItems optimiseSingleConstant(
+		u256 const& _value,
+		Params const& _params,
+		Assembly& _assembly
+	);
+
+	/// This is the public API for the optimiser methods, but it doesn't need to be exposed to the caller.
+
 	explicit ConstantOptimisationMethod(Params const& _params, u256 const& _value):
 		m_params(_params), m_value(_value) {}
 	virtual ~ConstantOptimisationMethod() = default;