Remove side-effect-free statements.

libyul/optimiser/README.md

@@ -135,6 +135,8 @@ If there are two assignments to a variable where the first one is a movable expr
 and the variable is not used between the two assignments (and the second is not inside
 a loop or conditional, the first one is not inside), the first assignment is removed.
 
+This step also removes movable expression statements.
+
 
 ## Function Unifier
 

libyul/optimiser/UnusedPruner.cpp

@@ -85,6 +85,16 @@ void UnusedPruner::operator()(Block& _block)
 					}};
 			}
 		}
+		else if (statement.type() == typeid(ExpressionStatement))
+		{
+			ExpressionStatement& exprStmt = boost::get<ExpressionStatement>(statement);
+			if (MovableChecker(exprStmt.expression).movable())
+			{
+				// pop(x) should be movable!
+				subtractReferences(ReferencesCounter::countReferences(exprStmt.expression));
+				statement = Block{std::move(exprStmt.location), {}};
+			}
+		}
 
 	removeEmptyBlocks(_block);
 

libyul/optimiser/UnusedPruner.h

@@ -32,10 +32,8 @@ namespace yul
 {
 
 /**
- * Optimisation stage that removes unused variables and functions.
- *
- * TODO: Also remove intermediate variable assignments from movable expressions
- * which are not referenced until after the next assignment to the same variable.
+ * Optimisation stage that removes unused variables and functions and also
+ * removes movable expression statements.
  *
  * Note that this does not remove circular references.
  *

test/libyul/yulOptimizerTests/fullSuite/medium.yul

@@ -16,9 +16,7 @@
 //     {
 //         let _18 := 0x20
 //         let allocate__7 := 0x40
-//         let allocate_p_12 := mload(allocate__7)
-//         mstore(allocate__7, add(allocate_p_12, _18))
-//         pop(allocate_p_12)
+//         mstore(allocate__7, add(mload(allocate__7), _18))
 //         let allocate_p_12_31 := mload(allocate__7)
 //         mstore(allocate__7, add(allocate_p_12_31, allocate__7))
 //         mstore(add(allocate_p_12_31, 96), 2)

test/libyul/yulOptimizerTests/unusedPruner/pop.yul

@@ -0,0 +1,8 @@
+{
+    let a := 1
+    pop(a)
+}
+// ----
+// unusedPruner
+// {
+// }