Do not remove potentially reverting returndatacopy cases.

Changelog.md

@@ -9,6 +9,7 @@ Compiler Features:
 
 Bugfixes:
  * ABI Encoder: When encoding an empty string coming from storage do not add a superfluous empty slot for data.
+ * Yul Optimizer: Do not remove ``returndatacopy`` in cases in which it might perform out-of-bounds reads that unconditionally revert as out-of-gas. Previously, any ``returndatacopy`` that wrote to memory that was never read from was removed without accounting for the out-of-bounds condition.
 
 
 ### 0.8.14 (2022-05-17)

libyul/optimiser/UnusedStoreEliminator.cpp

@@ -157,7 +157,25 @@ void UnusedStoreEliminator::visit(Statement const& _statement)
 	yulAssert(isCandidateForRemoval == (isStorageWrite || (!m_ignoreMemory && isMemoryWrite)));
 	if (isCandidateForRemoval)
 	{
-		m_stores[YulString{}].insert({&_statement, State::Undecided});
+		State initialState = State::Undecided;
+		if (*instruction == Instruction::RETURNDATACOPY)
+		{
+			initialState = State::Used;
+			auto startOffset = identifierNameIfSSA(funCall->arguments.at(1));
+			auto length = identifierNameIfSSA(funCall->arguments.at(2));
+			KnowledgeBase knowledge(m_dialect, [this](YulString _var) { return util::valueOrNullptr(m_ssaValues, _var); });
+			if (length && startOffset)
+			{
+				FunctionCall const* lengthCall = get_if<FunctionCall>(m_ssaValues.at(*length).value);
+				if (
+					knowledge.knownToBeZero(*startOffset) &&
+					lengthCall &&
+					toEVMInstruction(m_dialect, lengthCall->functionName.name) == Instruction::RETURNDATASIZE
+				)
+					initialState = State::Undecided;
+			}
+		}
+		m_stores[YulString{}].insert({&_statement, initialState});
 		vector<Operation> operations = operationsFromFunctionCall(*funCall);
 		yulAssert(operations.size() == 1, "");
 		m_storeOperations[&_statement] = move(operations.front());

test/libyul/yulOptimizerTests/unusedStoreEliminator/returndatacopy_fixed_size.yul

@@ -0,0 +1,9 @@
+{
+  returndatacopy(0,0,32)
+}
+// ====
+// EVMVersion: >homestead
+// ----
+// step: unusedStoreEliminator
+//
+// { { returndatacopy(0, 0, 32) } }

test/libyul/yulOptimizerTests/unusedStoreEliminator/returndatacopy_returndatasize.yul

@@ -0,0 +1,15 @@
+{
+  returndatacopy(0,0,returndatasize())
+}
+// ====
+// EVMVersion: >homestead
+// ----
+// step: unusedStoreEliminator
+//
+// {
+//     {
+//         let _1 := returndatasize()
+//         let _2 := 0
+//         let _3 := 0
+//     }
+// }

test/libyul/yulOptimizerTests/unusedStoreEliminator/returndatacopy_returndatasize_nonzero_start.yul

@@ -0,0 +1,13 @@
+{
+  returndatacopy(0,1,returndatasize())
+}
+// ====
+// EVMVersion: >homestead
+// ----
+// step: unusedStoreEliminator
+//
+// {
+//     {
+//         returndatacopy(0, 1, returndatasize())
+//     }
+// }

test/libyul/yulOptimizerTests/unusedStoreEliminator/returndatacopy_returndatasize_var.yul

@@ -0,0 +1,16 @@
+{
+  let s := returndatasize()
+  returndatacopy(0,0,s)
+}
+// ====
+// EVMVersion: >homestead
+// ----
+// step: unusedStoreEliminator
+//
+// {
+//     {
+//         let s := returndatasize()
+//         let _1 := 0
+//         let _2 := 0
+//     }
+// }