Optimizer: add an additional DeadObjectElimination at the end of the pipeline

lib/SILOptimizer/PassManager/PassPipeline.cpp

@@ -760,6 +760,9 @@ static void addLowLevelPassPipeline(SILPassPipelinePlan &P) {
   P.addObjectOutliner();
   P.addDeadStoreElimination();
 
+  // dead-store-elimination can expose opportunities for dead object elimination.
+  P.addDeadObjectElimination();
+
   // We've done a lot of optimizations on this function, attempt to FSO.
   P.addFunctionSignatureOpts();
   P.addComputeEscapeEffects();

test/SILOptimizer/dead_alloc.swift

@@ -0,0 +1,47 @@
+// RUN: %target-swift-frontend -O -emit-sil -parse-as-library %s | %FileCheck %s
+
+// REQUIRES: swift_stdlib_no_asserts,optimized_stdlib
+// REQUIRES: swift_in_compiler
+
+protocol E {
+  func f() -> Bool
+}
+
+protocol P {
+  associatedtype A = Int
+}
+
+public struct X : P, E {
+  func f() -> Bool { return true }
+}
+
+func g<T : P>(_ x : T) -> Bool {
+  if let y = x as? E { return y.f() }
+  return false
+}
+
+// Check that this function can be completely constant folded and no alloc_stack remains.
+
+// CHECK-LABEL: sil @$s10dead_alloc0A10AllocStackySbAA1XVF :
+// CHECK:      bb0({{.*}}):
+// CHECK-NEXT:   debug_value
+// CHECK-NEXT:   integer_literal
+// CHECK-NEXT:   struct
+// CHECK-NEXT:   return
+// CHECK-NEXT: } // end sil function '$s10dead_alloc0A10AllocStackySbAA1XVF'
+public func deadAllocStack(_ x: X) -> Bool {
+  return g(x)
+}
+
+public class C<T> {
+    let x: String = "123"
+}
+
+// CHECK-LABEL: sil @$s10dead_alloc0A13ClassInstanceyyF :
+// CHECK:       bb0:
+// CHECK-NEXT:    tuple
+// CHECK-NEXT:    return
+// CHECK-NEXT:  } // end sil function '$s10dead_alloc0A13ClassInstanceyyF'
+public func deadClassInstance() {
+    let _ = C<Int>()
+}