[IRGen] Don't store payload in destructiveInjectEnumTag when payload is empty type

rdar://142485229

In generic multi payload enums, if the payload was empty, we missed a check for that when storing the payload and always cleared out the extra tag byte.

Author: drexin

lib/IRGen/TypeLayout.cpp

@@ -3262,13 +3262,29 @@ void EnumTypeLayoutEntry::storeMultiPayloadValue(IRGenFunction &IGF,
                                                  Address enumAddr) const {
   auto &IGM = IGF.IGM;
   auto &Builder = IGF.Builder;
+
+  auto emptyBB = IGF.createBasicBlock("empty-payload");
+  auto nonEmptyBB = IGF.createBasicBlock("non-empty-payload");
+  auto contBB = IGF.createBasicBlock("");
+
   auto truncSize = Builder.CreateZExtOrTrunc(maxPayloadSize(IGF), IGM.Int32Ty);
+  auto empty =
+      Builder.CreateICmpEQ(truncSize, llvm::ConstantInt::get(IGM.Int32Ty, 0));
+
+  Builder.CreateCondBr(empty, emptyBB, nonEmptyBB);
+
+  Builder.emitBlock(nonEmptyBB);
   auto four = IGM.getInt32(4);
   auto sizeGTE4 = Builder.CreateICmpUGE(truncSize, four);
   auto sizeClampedTo4 = Builder.CreateSelect(sizeGTE4, four, truncSize);
   Builder.CreateMemSet(enumAddr, llvm::ConstantInt::get(IGF.IGM.Int8Ty, 0),
                        truncSize);
   emitStore1to4Bytes(IGF, enumAddr, value, sizeClampedTo4);
+  Builder.CreateBr(contBB);
+
+  Builder.emitBlock(emptyBB);
+  Builder.CreateBr(contBB);
+  Builder.emitBlock(contBB);
 }
 
 void EnumTypeLayoutEntry::storeEnumTagSinglePayloadForMultiPayloadEnum(

test/IRGen/typelayout_based_value_witness.swift

@@ -127,6 +127,11 @@ public enum ForwardEnum<T> {
 // OPT:   ret void
 // CHECK: }
 
+// OPT: define internal void @"$s30typelayout_based_value_witness2E3Owui"(ptr noalias nocapture writeonly %value, i32 %tag, ptr nocapture readonly %"E3<T>")
+// OPT:  [[IS_EMPTY:%.*]] = icmp eq i32 {{%.*}}, 0
+// OPT:  br i1 [[IS_EMPTY]], label %empty-payload, label %non-empty-payload
+// OPT: }
+
 // Let's not crash on the following example.
 public protocol P {}
 
@@ -145,3 +150,9 @@ public enum E2<T: P> {
     case first(Ref<T>)
     case second(String)
 }
+
+public enum E3<T> {
+  case empty
+  case first(T)
+  case second(T)
+}