[IRGen] Properly handle empty payloads in getEnumTagMultipayload (#60590)

* [IRGen] Properly handle empty payloads in getEnumTagMultipayload

rdar://97914498

The generated code assumed that payloads would always be at least
1 byte long, ignoring the possibility of empty payloads, causing
runtime crashes when using empty payloads in multi payload enums.

* Fix test

* Remove unnecessary basic block

Author: drexin

lib/IRGen/TypeLayout.cpp

@@ -2321,7 +2321,7 @@ EnumTypeLayoutEntry::getEnumTagMultipayload(IRGenFunction &IGF,
   auto usePayloadBB = IGF.createBasicBlock("use-payload-for-tag");
   auto numPayloads = IGM.getInt32(cases.size());
   auto usePayloadValue = Builder.CreateICmpUGE(loadedTag, numPayloads);
-  auto tagValue = llvm::PHINode::Create(IGM.Int32Ty, 3);
+  auto tagValue = llvm::PHINode::Create(IGM.Int32Ty, 4);
   tagValue->addIncoming(loadedTag, Builder.GetInsertBlock());
   Builder.CreateCondBr(usePayloadValue, usePayloadBB, resultBB);
 
@@ -2330,6 +2330,13 @@ EnumTypeLayoutEntry::getEnumTagMultipayload(IRGenFunction &IGF,
   auto truncSize = Builder.CreateZExtOrTrunc(maxPayloadSize(IGF), IGM.Int32Ty);
   auto sizeGTE4 = Builder.CreateICmpUGE(truncSize, four);
   auto sizeClampedTo4 = Builder.CreateSelect(sizeGTE4, four, truncSize);
+  auto sizeGreaterZeroBB = IGF.createBasicBlock("");
+  auto zero = IGM.getInt32(0);
+  auto sizeGreaterZero = Builder.CreateICmpUGT(sizeClampedTo4, zero);
+  tagValue->addIncoming(loadedTag, Builder.GetInsertBlock());
+  Builder.CreateCondBr(sizeGreaterZero, sizeGreaterZeroBB, resultBB);
+
+  Builder.emitBlock(sizeGreaterZeroBB);
   auto payloadValue = emitLoad1to4Bytes(IGF, enumAddr, sizeClampedTo4);
   auto payloadGTE4BB = IGF.createBasicBlock("");
   auto payloadLT4BB = IGF.createBasicBlock("");

test/Interpreter/rdar97914498.swift

@@ -0,0 +1,32 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swiftc_driver -Xfrontend -disable-availability-checking -O -Xllvm -sil-opt-pass-count=0 -Xfrontend -disable-llvm-optzns %s -o %t/out
+// RUN: %target-codesign %t/out
+// RUN: %target-run %t/out
+
+// REQUIRES: executable_test
+// REQUIRES: foundation
+
+// This is a regression test that ensures that empty payloads in multi
+// payload enums are properly handled.
+
+import Foundation
+
+@inline(never)
+func crash() {
+    let testURL = URL(string: "https://www.google.com")!
+    let r = Resource<()>(url: testURL, method: .get)
+    print(r.url)
+}
+
+enum HTTPMethod<Payload> {
+    case get
+    case post(Payload)
+    case patch(Payload)
+}
+
+struct Resource<Payload> {
+    let url: URL
+    let method: HTTPMethod<Payload>
+}
+
+crash()