Merge pull request #7418 from ethereum/ssaReverseFix

Fix SSA reverser for declaration + self-assignment

libyul/optimiser/SSAReverser.cpp

@@ -56,18 +56,24 @@ void SSAReverser::operator()(Block& _block)
 					identifier &&
 					identifier->name == varDecl->variables.front().name
 				)
-					return make_vector<Statement>(
-						Assignment{
-							std::move(assignment->location),
-							assignment->variableNames,
-							std::move(varDecl->value)
-						},
-						VariableDeclaration{
-							std::move(varDecl->location),
-							std::move(varDecl->variables),
-							std::make_unique<Expression>(std::move(assignment->variableNames.front()))
-						}
-					);
+				{
+					// in the special case a == a_1, just remove the assignment
+					if (assignment->variableNames.front().name == identifier->name)
+						return make_vector<Statement>(std::move(_stmt1));
+					else
+						return make_vector<Statement>(
+							Assignment{
+								std::move(assignment->location),
+								assignment->variableNames,
+								std::move(varDecl->value)
+							},
+							VariableDeclaration{
+								std::move(varDecl->location),
+								std::move(varDecl->variables),
+								std::make_unique<Expression>(std::move(assignment->variableNames.front()))
+							}
+						);
+				}
 			}
 			// Replaces
 			//   let a_1 := E

libyul/optimiser/SSAReverser.h

@@ -40,6 +40,13 @@ class AssignmentCounter;
  * 		a := E
  * 		let a_1 := a
  *
+ * 	In the special case
+ * 		let a := E
+ * 		a := a
+ *
+ * 	the redundant assignment "a := a" is removed.
+ *
+ *
  * Secondly, the SSA transform will rewrite
  *
  * 		let a := E

test/libyul/yulOptimizerTests/ssaReverser/self_assign.yul

@@ -0,0 +1,8 @@
+{
+    let a := calldataload(0)
+    a := a
+}
+// ====
+// step: ssaReverser
+// ----
+// { let a := calldataload(0) }