Yul Optimizer: Simplify start offset of zero-length operations.

Author: ekpyron

Changelog.md

@@ -8,6 +8,7 @@ Language Features:
 
 Compiler Features:
  * Code Generator: More efficient overflow checks for multiplication.
+ * Yul Optimizer: Simplify the starting offset of zero-length operations to zero.
 
 
 Bugfixes:

libyul/optimiser/DataFlowAnalyzer.cpp

@@ -411,7 +411,7 @@ bool DataFlowAnalyzer::inScope(YulString _variableName) const
 	return false;
 }
 
-optional<u256> DataFlowAnalyzer::valueOfIdentifier(YulString const& _name)
+optional<u256> DataFlowAnalyzer::valueOfIdentifier(YulString const& _name) const
 {
 	if (AssignedValue const* value = variableValue(_name))
 		if (Literal const* literal = get_if<Literal>(value->value))

libyul/optimiser/DataFlowAnalyzer.h

@@ -148,7 +148,7 @@ class DataFlowAnalyzer: public ASTModifier
 	bool inScope(YulString _variableName) const;
 
 	/// Returns the literal value of the identifier, if it exists.
-	std::optional<u256> valueOfIdentifier(YulString const& _name);
+	std::optional<u256> valueOfIdentifier(YulString const& _name) const;
 
 	enum class StoreLoadLocation {
 		Memory = 0,

libyul/optimiser/ExpressionSimplifier.cpp

@@ -23,7 +23,11 @@
 
 #include <libyul/optimiser/SimplificationRules.h>
 #include <libyul/optimiser/OptimiserStep.h>
+#include <libyul/optimiser/OptimizerUtilities.h>
 #include <libyul/AST.h>
+#include <libyul/Utilities.h>
+
+#include <libevmasm/SemanticInformation.h>
 
 using namespace std;
 using namespace solidity;
@@ -44,4 +48,29 @@ void ExpressionSimplifier::visit(Expression& _expression)
 		[this](YulString _var) { return variableValue(_var); }
 	))
 		_expression = match->action().toExpression(debugDataOf(_expression));
+
+	if (auto* functionCall = get_if<FunctionCall>(&_expression))
+		if (optional<evmasm::Instruction> instruction = toEVMInstruction(m_dialect, functionCall->functionName.name))
+			for (auto op: evmasm::SemanticInformation::readWriteOperations(*instruction))
+				if (op.startParameter && op.lengthParameter)
+				{
+					Expression& startArgument = functionCall->arguments.at(*op.startParameter);
+					Expression const& lengthArgument = functionCall->arguments.at(*op.lengthParameter);
+					if (
+						knownToBeZero(lengthArgument) &&
+						!knownToBeZero(startArgument) &&
+						!holds_alternative<FunctionCall>(startArgument)
+					)
+						startArgument = Literal{debugDataOf(startArgument), LiteralKind::Number, "0"_yulstring, {}};
+				}
+}
+
+bool ExpressionSimplifier::knownToBeZero(Expression const& _expression) const
+{
+	if (auto const* literal = get_if<Literal>(&_expression))
+		return valueOfLiteral(*literal) == 0;
+	else if (auto const* identifier = get_if<Identifier>(&_expression))
+		return valueOfIdentifier(identifier->name) == 0;
+	else
+		return false;
 }

libyul/optimiser/ExpressionSimplifier.h

@@ -54,6 +54,7 @@ class ExpressionSimplifier: public DataFlowAnalyzer
 	explicit ExpressionSimplifier(Dialect const& _dialect):
 		DataFlowAnalyzer(_dialect, MemoryAndStorage::Ignore)
 	{}
+	bool knownToBeZero(Expression const& _expression) const;
 };
 
 }

test/libyul/yulOptimizerTests/expressionSimplifier/side_effects_in_for_condition.yul

@@ -1,5 +1,5 @@
 {
-  for {} div(create(0, 1, 0), shl(msize(), 1)) {}
+  for {} div(create(0, 1, 1), shl(msize(), 1)) {}
   {
   }
 }
@@ -10,7 +10,7 @@
 //
 // {
 //     {
-//         for { } div(create(0, 1, 0), shl(msize(), 1)) { }
+//         for { } div(create(0, 1, 1), shl(msize(), 1)) { }
 //         { }
 //     }
 // }

test/libyul/yulOptimizerTests/expressionSimplifier/zero_length_read.yul

@@ -0,0 +1,21 @@
+{
+	revert(calldataload(0), 0)
+	revert(call(0,0,0,0,0,0,0), 0)
+	calldatacopy(calldataload(1), calldataload(2), 0)
+	return(calldataload(3), 0)
+	codecopy(calldataload(4), calldataload(5), sub(42,42))
+}
+// ----
+// step: expressionSimplifier
+//
+// {
+//     {
+//         let _1 := 0
+//         revert(0, _1)
+//         pop(call(_1, _1, _1, _1, _1, _1, _1))
+//         revert(0, _1)
+//         calldatacopy(0, calldataload(2), _1)
+//         return(0, _1)
+//         codecopy(0, calldataload(5), 0)
+//     }
+// }