Extend knowledge base.

libyul/optimiser/KnowledgeBase.cpp

@@ -40,9 +40,8 @@ bool KnowledgeBase::knownToBeDifferent(YulString _a, YulString _b)
 	// current values to turn `sub(_a, _b)` into a nonzero constant.
 	// If that fails, try `eq(_a, _b)`.
 
-	Expression expr1 = simplify(FunctionCall{{}, {{}, "sub"_yulstring}, util::make_vector<Expression>(Identifier{{}, _a}, Identifier{{}, _b})});
+	if (optional<u256> difference = differenceIfKnownConstant(_a, _b))
-	if (holds_alternative<Literal>(expr1))
+		return difference != 0;
-		return valueOfLiteral(std::get<Literal>(expr1)) != 0;
 
 	Expression expr2 = simplify(FunctionCall{{}, {{}, "eq"_yulstring}, util::make_vector<Expression>(Identifier{{}, _a}, Identifier{{}, _b})});
 	if (holds_alternative<Literal>(expr2))
@@ -51,21 +50,42 @@ bool KnowledgeBase::knownToBeDifferent(YulString _a, YulString _b)
 	return false;
 }
 
+optional<u256> KnowledgeBase::differenceIfKnownConstant(YulString _a, YulString _b)
+{
+	// Try to use the simplification rules together with the
+	// current values to turn `sub(_a, _b)` into a constant.
+
+	Expression expr1 = simplify(FunctionCall{{}, {{}, "sub"_yulstring}, util::make_vector<Expression>(Identifier{{}, _a}, Identifier{{}, _b})});
+	if (Literal const* value = get_if<Literal>(&expr1))
+		return valueOfLiteral(*value);
+
+	return {};
+}
+
 bool KnowledgeBase::knownToBeDifferentByAtLeast32(YulString _a, YulString _b)
 {
 	// Try to use the simplification rules together with the
 	// current values to turn `sub(_a, _b)` into a constant whose absolute value is at least 32.
 
-	Expression expr1 = simplify(FunctionCall{{}, {{}, "sub"_yulstring}, util::make_vector<Expression>(Identifier{{}, _a}, Identifier{{}, _b})});
+	if (optional<u256> difference = differenceIfKnownConstant(_a, _b))
-	if (holds_alternative<Literal>(expr1))
+		return difference >= 32 && difference <= u256(0) - 32;
-	{
-		u256 val = valueOfLiteral(std::get<Literal>(expr1));
-		return val >= 32 && val <= u256(0) - 32;
-	}
 
 	return false;
 }
 
+bool KnowledgeBase::knownToBeZero(YulString _a)
+{
+	return valueIfKnownConstant(_a) == u256{};
+}
+
+optional<u256> KnowledgeBase::valueIfKnownConstant(YulString _a)
+{
+	if (m_variableValues.count(_a))
+		if (Literal const* literal = get_if<Literal>(m_variableValues.at(_a).value))
+			return valueOfLiteral(*literal);
+	return {};
+}
+
 Expression KnowledgeBase::simplify(Expression _expression)
 {
 	bool startedRecursion = (m_recursionCounter == 0);

libyul/optimiser/KnowledgeBase.h

@@ -24,6 +24,8 @@
 #include <libyul/ASTForward.h>
 #include <libyul/YulString.h>
 
+#include <libsolutil/Common.h>
+
 #include <map>
 
 namespace solidity::yul
@@ -46,8 +48,11 @@ public:
 	{}
 
 	bool knownToBeDifferent(YulString _a, YulString _b);
+	std::optional<u256> differenceIfKnownConstant(YulString _a, YulString _b);
 	bool knownToBeDifferentByAtLeast32(YulString _a, YulString _b);
 	bool knownToBeEqual(YulString _a, YulString _b) const { return _a == _b; }
+	bool knownToBeZero(YulString _a);
+	std::optional<u256> valueIfKnownConstant(YulString _a);
 
 private:
 	Expression simplify(Expression _expression);

test/CMakeLists.txt

@@ -136,6 +136,7 @@ set(libyul_sources
     libyul/FunctionSideEffects.cpp
     libyul/FunctionSideEffects.h
     libyul/Inliner.cpp
+    libyul/KnowledgeBaseTest.cpp
     libyul/Metrics.cpp
     libyul/ObjectCompilerTest.cpp
     libyul/ObjectCompilerTest.h

test/libyul/KnowledgeBaseTest.cpp

@@ -0,0 +1,84 @@
+/*
+    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/>.
+*/
+/**
+ * Unit tests for KnowledgeBase
+ */
+
+#include <test/Common.h>
+
+#include <test/libyul/Common.h>
+
+#include <libyul/Object.h>
+#include <libyul/optimiser/KnowledgeBase.h>
+#include <libyul/optimiser/SSAValueTracker.h>
+#include <libyul/optimiser/DataFlowAnalyzer.h>
+#include <libyul/backends/evm/EVMDialect.h>
+
+#include <liblangutil/ErrorReporter.h>
+
+#include <boost/test/unit_test.hpp>
+
+using namespace std;
+using namespace solidity::langutil;
+
+namespace solidity::yul::test
+{
+
+class KnowledgeBaseTest
+{
+protected:
+	KnowledgeBase constructKnowledgeBase(string const& _source)
+	{
+		ErrorList errorList;
+		shared_ptr<Object> object;
+		shared_ptr<AsmAnalysisInfo> analysisInfo;
+		std::tie(object, analysisInfo) = yul::test::parse(_source, m_dialect, errorList);
+		BOOST_REQUIRE(object && errorList.empty() && object->code);
+
+		SSAValueTracker ssaValues;
+		ssaValues(*object->code);
+		for (auto const& [name, expression]: ssaValues.values())
+			m_values[name].value = expression;
+
+		return KnowledgeBase(m_dialect, m_values);
+	}
+
+	EVMDialect m_dialect{EVMVersion{}, true};
+	map<YulString, AssignedValue> m_values;
+};
+
+BOOST_FIXTURE_TEST_SUITE(KnowledgeBase, KnowledgeBaseTest)
+
+BOOST_AUTO_TEST_CASE(basic)
+{
+	yul::KnowledgeBase kb = constructKnowledgeBase(R"({
+		let a := calldataload(0)
+		let b := calldataload(0)
+		let zero := 0
+		let c := add(b, 0)
+		let d := mul(b, 0)
+		let e := sub(a, b)
+	})");
+
+	BOOST_CHECK(!kb.knownToBeDifferent("a"_yulstring, "b"_yulstring));
+	BOOST_CHECK(kb.valueIfKnownConstant("zero"_yulstring) == u256(0));
+
+}
+
+BOOST_AUTO_TEST_SUITE_END()
+
+}