Fix incompatibilities with 16 bit pointers

- fix code generation for enum types to zext or trunc 32 bit data as appropriate for the platform
- fix IR generation code to use a relative address type of 16 bit width on that platform

- correct order of statements and line up comments

Author: carlos4242

lib/IRGen/GenType.cpp

@@ -392,7 +392,7 @@ static llvm::Value *computeExtraTagBytes(IRGenFunction &IGF, IRBuilder &Builder,
   auto *entryBB = Builder.GetInsertBlock();
   llvm::Value *size = asSizeConstant(IGM, fixedSize);
   auto *returnBB = llvm::BasicBlock::Create(Ctx);
-  size = Builder.CreateTrunc(size, int32Ty); // We know size < 4.
+  size = Builder.CreateZExtOrTrunc(size, int32Ty); // We know size < 4.
 
   auto *two = llvm::ConstantInt::get(int32Ty, 2U);
   auto *four = llvm::ConstantInt::get(int32Ty, 4U);
@@ -487,7 +487,7 @@ llvm::Value *FixedTypeInfo::getEnumTagSinglePayload(IRGenFunction &IGF,
 
   Builder.emitBlock(extraTagBitsBB);
 
-  auto *truncSize = Builder.CreateTrunc(size, IGM.Int32Ty);
+  auto *truncSize = Builder.CreateZExtOrTrunc(size, IGM.Int32Ty);
   Address caseIndexFromValueSlot = IGF.createAlloca(IGM.Int32Ty, Alignment(4));
   Builder.CreateStore(zero, caseIndexFromValueSlot);
 
@@ -683,7 +683,7 @@ void FixedTypeInfo::storeEnumTagSinglePayload(IRGenFunction &IGF,
   auto *nonPayloadElementIndex = Builder.CreateSub(whichCase, one);
   auto *caseIndex =
       Builder.CreateSub(nonPayloadElementIndex, numExtraInhabitants);
-  auto *truncSize = Builder.CreateTrunc(size, IGM.Int32Ty);
+  auto *truncSize = Builder.CreateZExtOrTrunc(size, IGM.Int32Ty);
   auto *isFourBytesPayload = Builder.CreateICmpUGE(truncSize, four);
   auto *payloadGE4BB = Builder.GetInsertBlock();
   auto *payloadLT4BB = llvm::BasicBlock::Create(Ctx);

lib/IRGen/IRGenModule.cpp

@@ -169,6 +169,18 @@ IRGenModule::IRGenModule(IRGenerator &irgen,
   Int8PtrPtrTy = Int8PtrTy->getPointerTo(0);
   SizeTy = DataLayout.getIntPtrType(getLLVMContext(), /*addrspace*/ 0);
 
+  // For the relative address type, we want to use the int32 bit type
+  // on most architectures, e.g. x86_64, because it produces valid
+  // fixups/relocations. The exception is 16-bit architectures,
+  // so we shorten the relative address type there.
+  if (SizeTy->getBitWidth()<32) {
+    RelativeAddressTy = SizeTy;
+  } else {
+    RelativeAddressTy = Int32Ty;
+  }
+
+  RelativeAddressPtrTy = RelativeAddressTy->getPointerTo();
+
   FloatTy = llvm::Type::getFloatTy(getLLVMContext());
   DoubleTy = llvm::Type::getDoubleTy(getLLVMContext());
 

lib/IRGen/IRGenModule.h

@@ -511,14 +511,10 @@ class IRGenModule {
   llvm::IntegerType *Int1Ty;           /// i1
   llvm::IntegerType *Int8Ty;           /// i8
   llvm::IntegerType *Int16Ty;          /// i16
-  union {
-    llvm::IntegerType *Int32Ty;          /// i32
-    llvm::IntegerType *RelativeAddressTy;
-  };
-  union {
-    llvm::PointerType *Int32PtrTy;            /// i32 *
-    llvm::PointerType *RelativeAddressPtrTy;
-  };
+  llvm::IntegerType *Int32Ty;          /// i32
+  llvm::PointerType *Int32PtrTy;       /// i32 *
+  llvm::IntegerType *RelativeAddressTy;
+  llvm::PointerType *RelativeAddressPtrTy;
   llvm::IntegerType *Int64Ty;          /// i64
   union {
     llvm::IntegerType *SizeTy;         /// usually i32 or i64
@@ -541,7 +537,7 @@ class IRGenModule {
     llvm::PointerType *WitnessTablePtrTy;
   };
   llvm::StructType *RefCountedStructTy;/// %swift.refcounted = type { ... }
-  Size RefCountedStructSize;     /// sizeof(%swift.refcounted)
+  Size RefCountedStructSize;           /// sizeof(%swift.refcounted)
   llvm::PointerType *RefCountedPtrTy;  /// %swift.refcounted*
 #define CHECKED_REF_STORAGE(Name, ...) \
   llvm::PointerType *Name##ReferencePtrTy; /// %swift. #name _reference*
@@ -569,11 +565,11 @@ class IRGenModule {
   llvm::PointerType *ProtocolDescriptorPtrTy; /// %swift.protocol*
   llvm::StructType *ProtocolRequirementStructTy; /// %swift.protocol_requirement
   union {
-    llvm::PointerType *ObjCPtrTy;        /// %objc_object*
+    llvm::PointerType *ObjCPtrTy;      /// %objc_object*
     llvm::PointerType *UnknownRefCountedPtrTy;
   };
-  llvm::PointerType *BridgeObjectPtrTy; /// %swift.bridge*
-  llvm::StructType *OpaqueTy;           /// %swift.opaque
+  llvm::PointerType *BridgeObjectPtrTy;/// %swift.bridge*
+  llvm::StructType *OpaqueTy;          /// %swift.opaque
   llvm::PointerType *OpaquePtrTy;      /// %swift.opaque*
   llvm::StructType *ObjCClassStructTy; /// %objc_class
   llvm::PointerType *ObjCClassPtrTy;   /// %objc_class*