[SILOpt] Put DestroyHoisting behind flag.

Tested that import enum case is optimized appropriately without the
classic DestroyHoisting pass.

Author: nate-chandler

lib/SILOptimizer/PassManager/PassPipeline.cpp

@@ -66,6 +66,10 @@ llvm::cl::opt<bool> SILDisableLateOMEByDefault(
     llvm::cl::desc(
         "Disable late OME for non-transparent functions by default"));
 
+llvm::cl::opt<bool>
+    EnableDestroyHoisting("enable-destroy-hoisting", llvm::cl::init(true),
+                          llvm::cl::desc("Enable the DestroyHoisting pass."));
+
 //===----------------------------------------------------------------------===//
 //                          Diagnostic Pass Pipeline
 //===----------------------------------------------------------------------===//
@@ -145,7 +149,7 @@ static void addMandatoryDiagnosticOptPipeline(SILPassPipelinePlan &P) {
     P.addSILSkippingChecker();
 #endif
 
-  if (Options.shouldOptimize()) {
+  if (Options.shouldOptimize() && EnableDestroyHoisting) {
     P.addDestroyHoisting();
   }
   P.addMandatoryInlining();

test/SILOptimizer/enumpayload.swift

@@ -0,0 +1,63 @@
+// RUN: %empty-directory(%t) 
+// RUN: %target-build-swift -O -Xllvm -enable-destroy-hoisting=false -module-name=test %s -o %t/a.out
+// RUN: %target-run %t/a.out | %FileCheck %s
+
+// REQUIRES: executable_test,optimized_stdlib
+
+final class Storage {
+  var v : Int
+  init(_ v : Int) {
+    self.v = v
+  }
+}
+
+struct IntBox {
+  var s : Storage
+
+  init(_ x : Int) {
+    s = Storage(x)
+  }
+
+  var value: Int { return s.v }
+
+  mutating func increment(_ delta: Int) {
+    if (!isKnownUniquelyReferenced(&s)) {
+      // We should never see this message
+      print("## copy on write")
+      s = Storage(s.v)
+    }
+    s.v += delta
+  }
+}
+
+enum E {
+  case value(IntBox)
+  case none
+
+  @inline(never)
+  mutating func simpleIncrement() {
+    switch self {
+    case .value(var i):
+      i.increment(1)
+      self = .value(i)
+    case .none:
+      break
+    }
+  }
+}
+
+func test_enum() {
+  // CHECK:      test_enum() simpleIncrement start
+  print(#function, "simpleIncrement start")
+  var e1 = E.value(IntBox(27))
+  e1.simpleIncrement()
+  // CHECK-NEXT: test_enum() simpleIncrement end: 28
+  switch e1 {
+  case .value(let i):
+    print(#function, "simpleIncrement end:", i.s.v.description)
+  case .none:
+    print("none")
+  }
+}
+
+test_enum()