Merge pull request #12762 from ethereum/emptyReturnRevert

Optimize ``return(x,0) -> pop(x) return(0,0)`` (and also for ``revert``).

Author: Marenz

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/cmdlineTests/debug_info_in_yul_and_evm_asm_print_all/output

@@ -194,7 +194,7 @@ object "C_6" {
                     {
                         if callvalue() { revert(_1, _1) }
                         if slt(add(calldatasize(), not(3)), _1) { revert(_1, _1) }
-                        return(memoryguard(0x80), _1)
+                        return(_1, _1)
                     }
                 }
                 revert(0, 0)

test/cmdlineTests/debug_info_in_yul_and_evm_asm_print_location_only/output

@@ -193,7 +193,7 @@ object "C_6" {
                     {
                         if callvalue() { revert(_1, _1) }
                         if slt(add(calldatasize(), not(3)), _1) { revert(_1, _1) }
-                        return(memoryguard(0x80), _1)
+                        return(_1, _1)
                     }
                 }
                 revert(0, 0)

test/cmdlineTests/debug_info_in_yul_and_evm_asm_print_none/output

@@ -182,7 +182,7 @@ object "C_6" {
                     {
                         if callvalue() { revert(_1, _1) }
                         if slt(add(calldatasize(), not(3)), _1) { revert(_1, _1) }
-                        return(memoryguard(0x80), _1)
+                        return(_1, _1)
                     }
                 }
                 revert(0, 0)

test/cmdlineTests/ir_compiler_subobjects/output

@@ -71,7 +71,7 @@ object "D_16" {
                             returndatacopy(pos, _2, returndatasize())
                             revert(pos, returndatasize())
                         }
-                        return(mload(64), _2)
+                        return(_2, _2)
                     }
                 }
                 revert(0, 0)

test/cmdlineTests/ir_with_assembly_no_memoryguard_creation/output

@@ -22,7 +22,7 @@ object "D_12" {
                     {
                         if callvalue() { revert(_1, _1) }
                         if slt(add(calldatasize(), not(3)), _1) { revert(_1, _1) }
-                        return(memoryguard(0x80), _1)
+                        return(_1, _1)
                     }
                 }
                 revert(0, 0)