From 81d011335a9032d0224ca21502e0d651432c24e2 Mon Sep 17 00:00:00 2001
From: Alex Beregszaszi <alex@rtfs.hu>
Date: Thu, 21 Feb 2019 23:50:37 +0000
Subject: [PATCH] Support shifts in the constant optimiser when Constantinople
 is targeted

---
 Changelog.md                    |  1 +
 libevmasm/ConstantOptimiser.cpp | 25 +++++++++++++++++++++++--
 libevmasm/ConstantOptimiser.h   |  2 +-
 3 files changed, 25 insertions(+), 3 deletions(-)

diff --git a/Changelog.md b/Changelog.md
index 5a528ff49..b6b96964c 100644
--- a/Changelog.md
+++ b/Changelog.md
@@ -7,6 +7,7 @@ Compiler Features:
  * SMTChecker: Do not report underflow/overflow if they always revert. This removes false positives when using ``SafeMath``.
  * Static Analyzer: Warn about expressions with custom types when they have no effect.
  * Optimizer: Add rule for shifts with constants for Constantinople.
+ * Optimizer: Support shifts in the constant optimiser for Constantinople.
 
 
 Bugfixes:
diff --git a/libevmasm/ConstantOptimiser.cpp b/libevmasm/ConstantOptimiser.cpp
index 7e9616321..ae3039a6c 100644
--- a/libevmasm/ConstantOptimiser.cpp
+++ b/libevmasm/ConstantOptimiser.cpp
@@ -210,7 +210,10 @@ AssemblyItems ComputeMethod::findRepresentation(u256 const& _value)
 			AssemblyItems newRoutine;
 			if (lowerPart != 0)
 				newRoutine += findRepresentation(u256(abs(lowerPart)));
-			newRoutine += AssemblyItems{u256(bits), u256(2), Instruction::EXP};
+			if (m_params.evmVersion.hasBitwiseShifting())
+				newRoutine += AssemblyItems{u256(1), u256(bits), Instruction::SHL};
+			else
+				newRoutine += AssemblyItems{u256(bits), u256(2), Instruction::EXP};
 			if (upperPart != 1)
 				newRoutine += findRepresentation(upperPart) + AssemblyItems{Instruction::MUL};
 			if (lowerPart > 0)
@@ -231,7 +234,7 @@ AssemblyItems ComputeMethod::findRepresentation(u256 const& _value)
 	}
 }
 
-bool ComputeMethod::checkRepresentation(u256 const& _value, AssemblyItems const& _routine)
+bool ComputeMethod::checkRepresentation(u256 const& _value, AssemblyItems const& _routine) const
 {
 	// This is a tiny EVM that can only evaluate some instructions.
 	vector<u256> stack;
@@ -263,6 +266,24 @@ bool ComputeMethod::checkRepresentation(u256 const& _value, AssemblyItems const&
 			case Instruction::NOT:
 				sp[0] = ~sp[0];
 				break;
+			case Instruction::SHL:
+				assertThrow(
+					m_params.evmVersion.hasBitwiseShifting(),
+					OptimizerException,
+					"Shift generated for invalid EVM version."
+				);
+				assertThrow(sp[0] <= u256(255), OptimizerException, "Invalid shift generated.");
+				sp[-1] = u256(bigint(sp[-1]) << unsigned(sp[0]));
+				break;
+			case Instruction::SHR:
+				assertThrow(
+					m_params.evmVersion.hasBitwiseShifting(),
+					OptimizerException,
+					"Shift generated for invalid EVM version."
+				);
+				assertThrow(sp[0] <= u256(255), OptimizerException, "Invalid shift generated.");
+				sp[-1] = sp[-1] >> unsigned(sp[0]);
+				break;
 			default:
 				return false;
 			}
diff --git a/libevmasm/ConstantOptimiser.h b/libevmasm/ConstantOptimiser.h
index bd87c4dce..92436db15 100644
--- a/libevmasm/ConstantOptimiser.h
+++ b/libevmasm/ConstantOptimiser.h
@@ -155,7 +155,7 @@ protected:
 	/// Tries to recursively find a way to compute @a _value.
 	AssemblyItems findRepresentation(u256 const& _value);
 	/// Recomputes the value from the calculated representation and checks for correctness.
-	static bool checkRepresentation(u256 const& _value, AssemblyItems const& _routine);
+	bool checkRepresentation(u256 const& _value, AssemblyItems const& _routine) const;
 	bigint gasNeeded(AssemblyItems const& _routine) const;
 
 	/// Counter for the complexity of optimization, will stop when it reaches zero.