Merge pull request #4068 from ethereum/gasEstimatorPerformance

Gas estimator performance

Changelog.md

@@ -3,6 +3,7 @@
 Features:
  * Build System: Update internal dependency of jsoncpp to 1.8.4, which introduces more strictness and reduces memory usage.
  * Code Generator: Use native shift instructions on target Constantinople.
+ * Gas Estimator: Only explore paths with higher gas costs. This reduces accuracy but greatly improves the speed of gas estimation.
  * Optimizer: Remove unnecessary masking of the result of known short instructions (``ADDRESS``, ``CALLER``, ``ORIGIN`` and ``COINBASE``).
  * Type Checker: Deprecate the ``years`` unit denomination and raise a warning for it (or an error as experimental 0.5.0 feature).
  * Type Checker: Make literals (without explicit type casting) an error for tight packing as experimental 0.5.0 feature.

libevmasm/PathGasMeter.cpp

@@ -43,7 +43,7 @@ GasMeter::GasConsumption PathGasMeter::estimateMax(
 	auto path = unique_ptr<GasPath>(new GasPath());
 	path->index = _startIndex;
 	path->state = _state->copy();
-	m_queue.push_back(move(path));
+	queue(move(path));
 
 	GasMeter::GasConsumption gas;
 	while (!m_queue.empty() && !gas.isInfinite)
@@ -51,12 +51,23 @@ GasMeter::GasConsumption PathGasMeter::estimateMax(
 	return gas;
 }
 
+void PathGasMeter::queue(std::unique_ptr<GasPath>&& _newPath)
+{
+	if (
+		m_highestGasUsagePerJumpdest.count(_newPath->index) &&
+		_newPath->gas < m_highestGasUsagePerJumpdest.at(_newPath->index)
+	)
+		return;
+	m_highestGasUsagePerJumpdest[_newPath->index] = _newPath->gas;
+	m_queue[_newPath->index] = move(_newPath);
+}
+
 GasMeter::GasConsumption PathGasMeter::handleQueueItem()
 {
 	assertThrow(!m_queue.empty(), OptimizerException, "");
 
-	unique_ptr<GasPath> path = move(m_queue.back());
+	unique_ptr<GasPath> path = move(m_queue.rbegin()->second);
-	m_queue.pop_back();
+	m_queue.erase(--m_queue.end());
 
 	shared_ptr<KnownState> state = path->state;
 	GasMeter meter(state, m_evmVersion, path->largestMemoryAccess);
@@ -117,7 +128,7 @@ GasMeter::GasConsumption PathGasMeter::handleQueueItem()
 			newPath->largestMemoryAccess = meter.largestMemoryAccess();
 			newPath->state = state->copy();
 			newPath->visitedJumpdests = path->visitedJumpdests;
-			m_queue.push_back(move(newPath));
+			queue(move(newPath));
 		}
 
 		if (branchStops)

libevmasm/PathGasMeter.h

@@ -58,9 +58,17 @@ public:
 	GasMeter::GasConsumption estimateMax(size_t _startIndex, std::shared_ptr<KnownState> const& _state);
 
 private:
+	/// Adds a new path item to the queue, but only if we do not already have
+	/// a higher gas usage at that point.
+	/// This is not exact as different state might influence higher gas costs at a later
+	/// point in time, but it greatly reduces computational overhead.
+	void queue(std::unique_ptr<GasPath>&& _newPath);
 	GasMeter::GasConsumption handleQueueItem();
 
-	std::vector<std::unique_ptr<GasPath>> m_queue;
+	/// Map of jumpdest -> gas path, so not really a queue. We only have one queued up
+	/// item per jumpdest, because of the behaviour of `queue` above.
+	std::map<size_t, std::unique_ptr<GasPath>> m_queue;
+	std::map<size_t, GasMeter::GasConsumption> m_highestGasUsagePerJumpdest;
 	std::map<u256, size_t> m_tagPositions;
 	AssemblyItems const& m_items;
 	solidity::EVMVersion m_evmVersion;

test/libsolidity/GasMeter.cpp

@@ -307,6 +307,46 @@ BOOST_AUTO_TEST_CASE(regular_functions_exclude_fallback)
 	testCreationTimeGas(sourceCode);
 	testRunTimeGas("x()", vector<bytes>{encodeArgs()});
 }
+
+BOOST_AUTO_TEST_CASE(complex_control_flow)
+{
+	// This crashed the gas estimator previously (or took a very long time).
+	// Now we do not follow branches if they start out with lower gas costs than the ones
+	// we previously considered. This of course reduces accuracy.
+	char const* sourceCode = R"(
+		contract log {
+			function ln(int128 x) constant returns (int128 result) {
+				int128 t = x / 256;
+				int128 y = 5545177;
+				x = t;
+				t = x * 16; if (t <= 1000000) { x = t; y = y - 2772588; }
+				t = x * 4; if (t <= 1000000) { x = t; y = y - 1386294; }
+				t = x * 2; if (t <= 1000000) { x = t; y = y - 693147; }
+				t = x + x / 2; if (t <= 1000000) { x = t; y = y - 405465; }
+				t = x + x / 4; if (t <= 1000000) { x = t; y = y - 223144; }
+				t = x + x / 8; if (t <= 1000000) { x = t; y = y - 117783; }
+				t = x + x / 16; if (t <= 1000000) { x = t; y = y - 60624; }
+				t = x + x / 32; if (t <= 1000000) { x = t; y = y - 30771; }
+				t = x + x / 64; if (t <= 1000000) { x = t; y = y - 15504; }
+				t = x + x / 128; if (t <= 1000000) { x = t; y = y - 7782; }
+				t = x + x / 256; if (t <= 1000000) { x = t; y = y - 3898; }
+				t = x + x / 512; if (t <= 1000000) { x = t; y = y - 1951; }
+				t = x + x / 1024; if (t <= 1000000) { x = t; y = y - 976; }
+				t = x + x / 2048; if (t <= 1000000) { x = t; y = y - 488; }
+				t = x + x / 4096; if (t <= 1000000) { x = t; y = y - 244; }
+				t = x + x / 8192; if (t <= 1000000) { x = t; y = y - 122; }
+				t = x + x / 16384; if (t <= 1000000) { x = t; y = y - 61; }
+				t = x + x / 32768; if (t <= 1000000) { x = t; y = y - 31; }
+				t = x + x / 65536; if (t <= 1000000) { y = y - 15; }
+				return y;
+			}
+		}
+	)";
+	testCreationTimeGas(sourceCode);
+	// max gas is used for small x
+	testRunTimeGas("ln(int128)", vector<bytes>{encodeArgs(0), encodeArgs(10), encodeArgs(105), encodeArgs(30000)});
+}
+
 BOOST_AUTO_TEST_SUITE_END()
 
 }