runtime: add and switch to SWIFT_FALLTHROUGH (NFC)

This duplicates and switches the uses of `LLVM_FALLTHROUGH` to a local
macro for the same annotation.

Author: compnerd

stdlib/public/Reflection/TypeLowering.cpp

@@ -1118,7 +1118,7 @@ class ExistentialTypeInfoBuilder {
       switch (FD->Kind) {
       case FieldDescriptorKind::Class:
         Refcounting = ReferenceCounting::Native;
-        LLVM_FALLTHROUGH;
+        SWIFT_FALLTHROUGH;
 
       case FieldDescriptorKind::ObjCClass:
         addAnyObject();

stdlib/public/Reflection/TypeRef.cpp

@@ -537,7 +537,7 @@ class DemanglingForTypeRef
       }
 
       // Otherwise it requires a tuple wrapper.
-      LLVM_FALLTHROUGH;
+      SWIFT_FALLTHROUGH;
     }
 
     // This covers both none and multiple parameters.

stdlib/public/SwiftShims/Visibility.h

@@ -30,6 +30,10 @@
 #define __has_builtin(builtin) 0
 #endif
 
+#if !defined(__has_cpp_attribute)
+#define __has_cpp_attribute(attribute) 0
+#endif
+
 #if __has_feature(nullability)
 // Provide macros to temporarily suppress warning about the use of
 // _Nullable and _Nonnull.
@@ -127,6 +131,18 @@
 #define SWIFT_RUNTIME_EXPORT SWIFT_EXPORT_ATTRIBUTE
 #endif
 
+#if __cplusplus > 201402l && __has_cpp_attribute(fallthrough)
+#define SWIFT_FALLTHROUGH [[fallthrough]]
+#elif __has_cpp_attribute(gnu::fallthrough)
+#define SWIFT_FALLTHROUGH [[gnu::fallthrough]]
+#elif __has_cpp_attribute(clang::fallthrough)
+#define SWIFT_FALLTHROUGH [[clang::fallthrough]]
+#elif __has_attribute(fallthrough)
+#define SWIFT_FALLTHROUGH __attribute__((__fallthrough__))
+#else
+#define SWIFT_FALLTHROUGH
+#endif
+
 
 /// Attributes for runtime-stdlib interfaces.
 /// Use these for C implementations that are imported into Swift via SwiftShims

stdlib/public/runtime/Casting.cpp

@@ -896,7 +896,7 @@ static bool _dynamicCastToExistential(OpaqueValue *dest,
         maybeDeallocateSource(result);
         return result;
       }
-      LLVM_FALLTHROUGH;
+      SWIFT_FALLTHROUGH;
 
     case MetadataKind::Enum:
     case MetadataKind::Optional:
@@ -912,7 +912,7 @@ static bool _dynamicCastToExistential(OpaqueValue *dest,
         maybeDeallocateSource(success);
         return success;
       }
-      LLVM_FALLTHROUGH;
+      SWIFT_FALLTHROUGH;
 
     default:
       return fallbackForNonClass();
@@ -1129,15 +1129,15 @@ swift_dynamicCastMetatypeImpl(const Metadata *sourceType,
     // Get the actual class object.
     targetType = static_cast<const ObjCClassWrapperMetadata*>(targetType)
       ->Class;
-    LLVM_FALLTHROUGH;
+    SWIFT_FALLTHROUGH;
   case MetadataKind::Class:
     // The source value must also be a class; otherwise the cast fails.
     switch (sourceType->getKind()) {
     case MetadataKind::ObjCClassWrapper:
       // Get the actual class object.
       sourceType = static_cast<const ObjCClassWrapperMetadata*>(sourceType)
         ->Class;
-      LLVM_FALLTHROUGH;
+      SWIFT_FALLTHROUGH;
     case MetadataKind::Class: {
       // Check if the source is a subclass of the target.
 #if SWIFT_OBJC_INTEROP
@@ -1173,7 +1173,7 @@ swift_dynamicCastMetatypeImpl(const Metadata *sourceType,
       // Get the actual class object.
       sourceType = static_cast<const ObjCClassWrapperMetadata*>(sourceType)
         ->Class;
-      LLVM_FALLTHROUGH;
+      SWIFT_FALLTHROUGH;
     case MetadataKind::Class:
     case MetadataKind::ForeignClass:
       // Check if the source is a subclass of the target.
@@ -1216,15 +1216,15 @@ swift_dynamicCastMetatypeUnconditionalImpl(const Metadata *sourceType,
     // Get the actual class object.
     targetType = static_cast<const ObjCClassWrapperMetadata*>(targetType)
       ->Class;
-    LLVM_FALLTHROUGH;
+    SWIFT_FALLTHROUGH;
   case MetadataKind::Class:
     // The source value must also be a class; otherwise the cast fails.
     switch (sourceType->getKind()) {
     case MetadataKind::ObjCClassWrapper:
       // Get the actual class object.
       sourceType = static_cast<const ObjCClassWrapperMetadata*>(sourceType)
         ->Class;
-      LLVM_FALLTHROUGH;
+      SWIFT_FALLTHROUGH;
     case MetadataKind::Class: {
       // Check if the source is a subclass of the target.
 #if SWIFT_OBJC_INTEROP
@@ -1263,7 +1263,7 @@ swift_dynamicCastMetatypeUnconditionalImpl(const Metadata *sourceType,
       // Get the actual class object.
       sourceType = static_cast<const ObjCClassWrapperMetadata*>(sourceType)
         ->Class;
-      LLVM_FALLTHROUGH;
+      SWIFT_FALLTHROUGH;
     case MetadataKind::Class:
     case MetadataKind::ForeignClass:
       // Check if the source is a subclass of the target.
@@ -2412,7 +2412,7 @@ static bool swift_dynamicCastImpl(OpaqueValue *dest, OpaqueValue *src,
         return _dynamicCastFromAnyHashable(dest, src, srcType,
                                            targetType, flags);
       }
-      LLVM_FALLTHROUGH;
+      SWIFT_FALLTHROUGH;
 
     case MetadataKind::Enum:
     case MetadataKind::Optional: {
@@ -2527,7 +2527,7 @@ static bool swift_dynamicCastImpl(OpaqueValue *dest, OpaqueValue *src,
       break;
     }
 
-    LLVM_FALLTHROUGH;
+    SWIFT_FALLTHROUGH;
 
   // The non-polymorphic types.
   default:

stdlib/public/runtime/Demangle.cpp

@@ -517,7 +517,7 @@ swift::_swift_buildDemanglingForMetadata(const Metadata *type,
       }
 
       // Otherwise it requires a tuple wrapper.
-      LLVM_FALLTHROUGH;
+      SWIFT_FALLTHROUGH;
     }
 
     // This covers both none and multiple parameters.

stdlib/public/runtime/MetadataCache.h

@@ -1076,7 +1076,7 @@ class MetadataCacheEntryBase
     case LSK::CompletionQueue:
       // Move the existing completion queue to the cache entry.
       queueEntry->CompletionQueue = LockedStorage.CompletionQueue;
-      LLVM_FALLTHROUGH;
+      SWIFT_FALLTHROUGH;
 
     case LSK::AllocatingThread:
       LockedStorageKind = LSK::QueueEntry;

stdlib/public/runtime/ReflectionMirror.mm

@@ -755,7 +755,7 @@ auto call(OpaqueValue *passedValue, const Metadata *T, const Metadata *passedTyp
           return callClass();
         }
       }
-      LLVM_FALLTHROUGH;
+      SWIFT_FALLTHROUGH;
     }
 
     /// TODO: Implement specialized mirror witnesses for all kinds.