[CoroutineAccessors] Use async bit in descriptors.

To facilitate back deployment, make use of the fact that the async bit
has up to now never been set for read and modify accessors and claim
that set bit to indicate that it is a callee-allocated coroutine.  This
has the virtue of being completely back deployable because like async
function pointers coro function pointers must be auth'd and signed as
data.

Author: nate-chandler

include/swift/ABI/MetadataValues.h

@@ -374,8 +374,6 @@ class MethodDescriptorFlags {
     Setter,
     ModifyCoroutine,
     ReadCoroutine,
-    Read2Coroutine,
-    Modify2Coroutine,
   };
 
 private:
@@ -438,23 +436,27 @@ class MethodDescriptorFlags {
   /// Note that 'init' is not considered an instance member.
   bool isInstance() const { return Value & IsInstanceMask; }
 
-  bool isAsync() const { return Value & IsAsyncMask; }
+  bool _hasAsyncBitSet() const { return Value & IsAsyncMask; }
 
-  bool isCalleeAllocatedCoroutine() const {
+  bool isAsync() const { return !isCoroutine() && _hasAsyncBitSet(); }
+
+  bool isCoroutine() const {
     switch (getKind()) {
     case Kind::Method:
     case Kind::Init:
     case Kind::Getter:
     case Kind::Setter:
+      return false;
     case Kind::ModifyCoroutine:
     case Kind::ReadCoroutine:
-      return false;
-    case Kind::Read2Coroutine:
-    case Kind::Modify2Coroutine:
       return true;
     }
   }
 
+  bool isCalleeAllocatedCoroutine() const {
+    return isCoroutine() && _hasAsyncBitSet();
+  }
+
   bool isData() const { return isAsync() || isCalleeAllocatedCoroutine(); }
 
   uint16_t getExtraDiscriminator() const {
@@ -615,8 +617,6 @@ class ProtocolRequirementFlags {
     ModifyCoroutine,
     AssociatedTypeAccessFunction,
     AssociatedConformanceAccessFunction,
-    Read2Coroutine,
-    Modify2Coroutine,
   };
 
 private:
@@ -666,26 +666,30 @@ class ProtocolRequirementFlags {
   /// Note that 'init' is not considered an instance member.
   bool isInstance() const { return Value & IsInstanceMask; }
 
-  bool isAsync() const { return Value & IsAsyncMask; }
+  bool _hasAsyncBitSet() const { return Value & IsAsyncMask; }
 
-  bool isCalleeAllocatedCoroutine() const {
+  bool isAsync() const { return !isCoroutine() && _hasAsyncBitSet(); }
+
+  bool isCoroutine() const {
     switch (getKind()) {
     case Kind::BaseProtocol:
     case Kind::Method:
     case Kind::Init:
     case Kind::Getter:
     case Kind::Setter:
-    case Kind::ReadCoroutine:
-    case Kind::ModifyCoroutine:
     case Kind::AssociatedTypeAccessFunction:
     case Kind::AssociatedConformanceAccessFunction:
       return false;
-    case Kind::Read2Coroutine:
-    case Kind::Modify2Coroutine:
+    case Kind::ReadCoroutine:
+    case Kind::ModifyCoroutine:
       return true;
     }
   }
 
+  bool isCalleeAllocatedCoroutine() const {
+    return isCoroutine() && _hasAsyncBitSet();
+  }
+
   bool isData() const { return isAsync() || isCalleeAllocatedCoroutine(); }
 
   bool isSignedWithAddress() const {

lib/IRGen/GenMeta.cpp

@@ -255,22 +255,24 @@ static Flags getMethodDescriptorFlags(ValueDecl *fn) {
     return flags;
   }
 
-  auto kind = [&] {
+  auto kindAndIsCalleeAllocatedCoroutine =
+      [&]() -> std::pair<typename Flags::Kind, bool> {
     auto accessor = dyn_cast<AccessorDecl>(fn);
-    if (!accessor) return Flags::Kind::Method;
+    if (!accessor)
+      return {Flags::Kind::Method, false};
     switch (accessor->getAccessorKind()) {
     case AccessorKind::Get:
-      return Flags::Kind::Getter;
+      return {Flags::Kind::Getter, false};
     case AccessorKind::Set:
-      return Flags::Kind::Setter;
+      return {Flags::Kind::Setter, false};
     case AccessorKind::Read:
-      return Flags::Kind::ReadCoroutine;
+      return {Flags::Kind::ReadCoroutine, false};
     case AccessorKind::Read2:
-      return Flags::Kind::Read2Coroutine;
+      return {Flags::Kind::ReadCoroutine, true};
     case AccessorKind::Modify:
-      return Flags::Kind::ModifyCoroutine;
+      return {Flags::Kind::ModifyCoroutine, false};
     case AccessorKind::Modify2:
-      return Flags::Kind::Modify2Coroutine;
+      return {Flags::Kind::ModifyCoroutine, true};
 #define OPAQUE_ACCESSOR(ID, KEYWORD)
 #define ACCESSOR(ID) \
     case AccessorKind::ID:
@@ -280,9 +282,20 @@ static Flags getMethodDescriptorFlags(ValueDecl *fn) {
     }
     llvm_unreachable("bad kind");
   }();
-  bool hasAsync = false;
+  auto kind = kindAndIsCalleeAllocatedCoroutine.first;
+
+  // Because no async old-ABI accessor coroutines exist or presumably ever will
+  // (if async coroutines accessors are added, they will presumably be new-ABI),
+  // the pairs {Flags::Kind::ReadCoroutine, isAsync} and
+  // {Flags::Kind::ModifyCoroutine, isAsync} can't mean "async old-ABI
+  // accessor coroutine".  As such, we repurpose that pair to mean "new-ABI
+  // accessor coroutine".  This has the important virtue of resulting in ptrauth
+  // authing/signing coro function pointers as data on old OSes where the bit
+  // means "async" and where adding new accessor kinds requires a back
+  // deployment library.
+  bool hasAsync = kindAndIsCalleeAllocatedCoroutine.second;
   if (auto *afd = dyn_cast<AbstractFunctionDecl>(fn))
-    hasAsync = afd->hasAsync();
+    hasAsync |= afd->hasAsync();
   return Flags(kind).withIsInstance(!fn->isStatic()).withIsAsync(hasAsync);
 }
 

stdlib/public/runtime/Metadata.cpp

@@ -6162,9 +6162,7 @@ static void initProtocolWitness(void **slot, void *witness,
   case ProtocolRequirementFlags::Kind::Getter:
   case ProtocolRequirementFlags::Kind::Setter:
   case ProtocolRequirementFlags::Kind::ReadCoroutine:
-  case ProtocolRequirementFlags::Kind::Read2Coroutine:
   case ProtocolRequirementFlags::Kind::ModifyCoroutine:
-  case ProtocolRequirementFlags::Kind::Modify2Coroutine:
     swift_ptrauth_init_code_or_data(slot, witness,
                                     reqt.Flags.getExtraDiscriminator(),
                                     !reqt.Flags.isData());
@@ -6205,9 +6203,7 @@ static void copyProtocolWitness(void **dest, void * const *src,
   case ProtocolRequirementFlags::Kind::Getter:
   case ProtocolRequirementFlags::Kind::Setter:
   case ProtocolRequirementFlags::Kind::ReadCoroutine:
-  case ProtocolRequirementFlags::Kind::Read2Coroutine:
   case ProtocolRequirementFlags::Kind::ModifyCoroutine:
-  case ProtocolRequirementFlags::Kind::Modify2Coroutine:
     swift_ptrauth_copy_code_or_data(
         dest, src, reqt.Flags.getExtraDiscriminator(), !reqt.Flags.isData(),
         /*allowNull*/ true); // NULL allowed for VFE (methods in the vtable