From 00f3c42d171cb8c1ae731ad0df0f7cb40726424a Mon Sep 17 00:00:00 2001
From: Alex Beregszaszi <alex@rtfs.hu>
Date: Fri, 13 Dec 2019 00:35:55 +0000
Subject: [PATCH] Use multiprecision::msb() in GasMeter and remove FixedHash
 dependency

---
 libevmasm/GasMeter.cpp | 11 ++++++++---
 libsolutil/FixedHash.h | 16 ----------------
 2 files changed, 8 insertions(+), 19 deletions(-)

diff --git a/libevmasm/GasMeter.cpp b/libevmasm/GasMeter.cpp
index 358d67c3a..6a07e9905 100644
--- a/libevmasm/GasMeter.cpp
+++ b/libevmasm/GasMeter.cpp
@@ -19,8 +19,6 @@
 
 #include <libevmasm/KnownState.h>
 
-#include <libsolutil/FixedHash.h>
-
 using namespace std;
 using namespace solidity;
 using namespace solidity::util;
@@ -182,7 +180,14 @@ GasMeter::GasConsumption GasMeter::estimateMax(AssemblyItem const& _item, bool _
 		case Instruction::EXP:
 			gas = GasCosts::expGas;
 			if (u256 const* value = classes.knownConstant(m_state->relativeStackElement(-1)))
-				gas += GasCosts::expByteGas(m_evmVersion) * (32 - (h256(*value).firstBitSet() / 8));
+			{
+				if (*value)
+				{
+					// Note: msb() counts from 0 and throws on 0 as input.
+					unsigned const significantByteCount  = (boost::multiprecision::msb(*value) + 1 + 7) / 8;
+					gas += GasCosts::expByteGas(m_evmVersion) * significantByteCount;
+				}
+			}
 			else
 				gas += GasCosts::expByteGas(m_evmVersion) * 32;
 			break;
diff --git a/libsolutil/FixedHash.h b/libsolutil/FixedHash.h
index f5a042d4c..4fa5a3072 100644
--- a/libsolutil/FixedHash.h
+++ b/libsolutil/FixedHash.h
@@ -104,22 +104,6 @@ public:
 	/// @returns a copy of the object's data as a byte vector.
 	bytes asBytes() const { return bytes(data(), data() + N); }
 
-	/// Returns the index of the first bit set to one, or size() * 8 if no bits are set.
-	inline unsigned firstBitSet() const
-	{
-		unsigned ret = 0;
-		for (auto d: m_data)
-			if (d)
-			{
-				for (;; ++ret, d <<= 1)
-					if (d & 0x80)
-						return ret;
-			}
-			else
-				ret += 8;
-		return ret;
-	}
-
 private:
 	std::array<uint8_t, N> m_data;		///< The binary data.
 };