From f86c5da20289c444af8f583eab656af8860dc582 Mon Sep 17 00:00:00 2001
From: Daniel Kirchner <daniel@ekpyron.org>
Date: Thu, 12 Sep 2019 18:17:13 +0200
Subject: [PATCH] Fix SSA reverser in special case of declaration followed by
 self-assignment.

---
 libyul/optimiser/SSAReverser.cpp              | 30 +++++++++++--------
 libyul/optimiser/SSAReverser.h                |  7 +++++
 .../ssaReverser/self_assign.yul               |  8 +++++
 3 files changed, 33 insertions(+), 12 deletions(-)
 create mode 100644 test/libyul/yulOptimizerTests/ssaReverser/self_assign.yul

diff --git a/libyul/optimiser/SSAReverser.cpp b/libyul/optimiser/SSAReverser.cpp
index a9a49fdb6..5bc6ab5c6 100644
--- a/libyul/optimiser/SSAReverser.cpp
+++ b/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
diff --git a/libyul/optimiser/SSAReverser.h b/libyul/optimiser/SSAReverser.h
index 67abeb568..7845bff5d 100644
--- a/libyul/optimiser/SSAReverser.h
+++ b/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
diff --git a/test/libyul/yulOptimizerTests/ssaReverser/self_assign.yul b/test/libyul/yulOptimizerTests/ssaReverser/self_assign.yul
new file mode 100644
index 000000000..48b9d39cf
--- /dev/null
+++ b/test/libyul/yulOptimizerTests/ssaReverser/self_assign.yul
@@ -0,0 +1,8 @@
+{
+    let a := calldataload(0)
+    a := a
+}
+// ====
+// step: ssaReverser
+// ----
+// { let a := calldataload(0) }