Fix simplifySwitchEnumUnreachableBlocks for default cases in ossa

Unlike non-ossa, ossa's switch_enum accepts an argument for the default case
When all other cases are unreachable, replace the default block's phi with
the switch_enum's operand and transform the switch_enum to a branch.

Fixes rdar://139441002

Author: meg-gupta

lib/SILOptimizer/Transforms/SimplifyCFG.cpp

@@ -1790,9 +1790,7 @@ bool SimplifyCFG::simplifySwitchEnumUnreachableBlocks(SwitchEnumInst *SEI) {
     return true;
   }
 
-  if (!Element || !Element->hasAssociatedValues() || Dest->args_empty()) {
-    assert(Dest->args_empty() && "Unexpected argument at destination!");
-
+  if (Dest->args_empty()) {
     SILBuilderWithScope(SEI).createBranch(SEI->getLoc(), Dest);
 
     addToWorklist(SEI->getParent());
@@ -1802,16 +1800,25 @@ bool SimplifyCFG::simplifySwitchEnumUnreachableBlocks(SwitchEnumInst *SEI) {
     return true;
   }
 
-  auto &Mod = SEI->getModule();
-  auto OpndTy = SEI->getOperand()->getType();
-  auto Ty = OpndTy.getEnumElementType(
-      Element, Mod, TypeExpansionContext(*SEI->getFunction()));
-  auto *UED = SILBuilderWithScope(SEI)
-    .createUncheckedEnumData(SEI->getLoc(), SEI->getOperand(), Element, Ty);
-
+  // If the Dest has an argument, it's case should have an associated value or
+  // it is the default case in ossa.
+  assert((Element && Element->hasAssociatedValues()) ||
+         (SEI->getFunction()->hasOwnership() && SEI->hasDefault() &&
+          Dest == SEI->getDefaultBB()));
+  SILValue newValue;
+  if (SEI->getFunction()->hasOwnership() && SEI->hasDefault() &&
+      Dest == SEI->getDefaultBB()) {
+    newValue = SEI->getOperand();
+  } else {
+    auto OpndTy = SEI->getOperand()->getType();
+    auto Ty = OpndTy.getEnumElementType(
+        Element, SEI->getModule(), TypeExpansionContext(*SEI->getFunction()));
+    newValue = SILBuilderWithScope(SEI).createUncheckedEnumData(
+        SEI->getLoc(), SEI->getOperand(), Element, Ty);
+  }
   assert(Dest->args_size() == 1 && "Expected only one argument!");
   auto *DestArg = Dest->getArgument(0);
-  DestArg->replaceAllUsesWith(UED);
+  DestArg->replaceAllUsesWith(newValue);
   Dest->eraseArgument(0);
 
   SILBuilderWithScope(SEI).createBranch(SEI->getLoc(), Dest);

test/SILOptimizer/simplify_cfg.sil

@@ -3259,3 +3259,24 @@ bb21:
   %75 = tuple ()
   return %75 : $()
 }
+
+// CHECK-LABEL: sil @simplify_switch_enum_unreachable :
+// CHECK-NOT:     switch_enum
+// CHECK:       } // end sil function 'simplify_switch_enum_unreachable'
+sil @simplify_switch_enum_unreachable : $@convention(thin) (@owned Optional<String>) -> () {
+bb0(%0 : $Optional<String>):
+  switch_enum %0 : $Optional<String>, case #Optional.some!enumelt: bb5, case #Optional.none!enumelt: bb4
+
+bb4:
+  unreachable
+
+bb5:
+  %1 = unchecked_enum_data %0 : $Optional<String>, #Optional.some!enumelt
+  release_value %1 : $String
+  br bb6
+
+bb6:
+  %17 = tuple ()
+  return %17 : $()
+}
+

test/SILOptimizer/simplify_cfg_ossa.sil

@@ -1919,3 +1919,23 @@ bb6:
   %17 = tuple ()
   return %17 : $()
 }
+
+// CHECK-LABEL: sil [ossa] @simplify_switch_enum_unreachable :
+// CHECK-NOT:     switch_enum
+// CHECK:       } // end sil function 'simplify_switch_enum_unreachable'
+sil [ossa] @simplify_switch_enum_unreachable : $@convention(thin) (A) -> () {
+bb0(%0 : $A):
+  switch_enum %0 : $A, case #A.B!enumelt: bb1, default bb2
+
+bb1:
+  unreachable
+
+bb2(%defaultArg : $A):
+  apply undef(%defaultArg) : $@convention(thin) (A) -> ()
+  br bb3
+
+bb3:
+  %t = tuple ()
+  return %t : $()
+}
+