SILGen: Emit a fallback variant of a back deployable function and cal…

…l it when the original function is not available at run time.

include/swift/AST/Decl.h

@@ -6268,7 +6268,7 @@ class AbstractFunctionDecl : public GenericContext, public ValueDecl {
   /// Returns 'true' if the function is distributed.
   bool isDistributed() const;
 
-  /// Returns 'true' if the function can be back deployed.
+  /// Returns 'true' if the function has the @c @_backDeploy attribute.
   bool isBackDeployed() const;
 
   PolymorphicEffectKind getPolymorphicEffectKind(EffectKind kind) const;

include/swift/SIL/SILDeclRef.h

@@ -162,6 +162,21 @@ struct SILDeclRef {
     AsyncEntryPoint,
   };
 
+  /// Represents the variants of a back deployable function.
+  enum class BackDeploymentKind : unsigned {
+    /// Default value. If a SILDecRef references a function that has been back
+    /// deployed and has this back deployment kind, then it references the
+    /// original ABI stable function.
+    None,
+    /// The thunk variant of a function that calls either the original function
+    /// or the fallback variant if the original is unavailable. This thunk will
+    /// be emitted with PublicNonABI linkage.
+    Thunk,
+    /// The fallback variant of the function. This function will be emitted with
+    /// PublicNonABI linkage.
+    Fallback,
+  };
+
   /// The AST node represented by this SILDeclRef.
   Loc loc;
   /// The Kind of this SILDeclRef.
@@ -170,8 +185,8 @@ struct SILDeclRef {
   unsigned isForeign : 1;
   /// True if this references a distributed function.
   unsigned isDistributed : 1;
-  /// True if this references a back deployed entry point for the referenced decl.
-  unsigned isBackDeployed : 1;
+  /// The BackDeploymentKind of this SILDeclRef.
+  BackDeploymentKind backDeploymentKind : 2;
   /// The default argument index for a default argument getter.
   unsigned defaultArgIndex : 10;
 
@@ -207,14 +222,14 @@ struct SILDeclRef {
   /// Produces a null SILDeclRef.
   SILDeclRef()
       : loc(), kind(Kind::Func), isForeign(0), isDistributed(0),
-        isBackDeployed(0), defaultArgIndex(0) {}
+        backDeploymentKind(BackDeploymentKind::None), defaultArgIndex(0) {}
 
   /// Produces a SILDeclRef of the given kind for the given decl.
   explicit SILDeclRef(
       ValueDecl *decl, Kind kind,
       bool isForeign = false,
       bool isDistributed = false,
-      bool isBackDeployed = false,
+      BackDeploymentKind backDeploymentKind = BackDeploymentKind::None,
       AutoDiffDerivativeFunctionIdentifier *derivativeId = nullptr);
 
   /// Produces a SILDeclRef for the given ValueDecl or
@@ -231,8 +246,7 @@ struct SILDeclRef {
   explicit SILDeclRef(
       Loc loc,
       bool isForeign = false,
-      bool isDistributed = false,
-      bool isBackDeployed = false);
+      bool isDistributed = false);
 
   /// See above put produces a prespecialization according to the signature.
   explicit SILDeclRef(Loc loc, GenericSignature prespecializationSig);
@@ -368,7 +382,7 @@ struct SILDeclRef {
     return loc.getOpaqueValue() == rhs.loc.getOpaqueValue() &&
            kind == rhs.kind && isForeign == rhs.isForeign &&
            isDistributed == rhs.isDistributed &&
-           isBackDeployed == rhs.isBackDeployed &&
+           backDeploymentKind == rhs.backDeploymentKind &&
            defaultArgIndex == rhs.defaultArgIndex &&
            pointer == rhs.pointer;
   }
@@ -387,7 +401,7 @@ struct SILDeclRef {
     return SILDeclRef(loc.getOpaqueValue(), kind,
                       /*foreign=*/foreign,
                       /*distributed=*/false,
-                      /*backDeployed=*/false,
+                      backDeploymentKind,
                       defaultArgIndex,
                       pointer.get<AutoDiffDerivativeFunctionIdentifier *>());
   }
@@ -397,17 +411,16 @@ struct SILDeclRef {
     return SILDeclRef(loc.getOpaqueValue(), kind,
                       /*foreign=*/false,
                       /*distributed=*/distributed,
-                      /*backDeployed=*/false,
+                      backDeploymentKind,
                       defaultArgIndex,
                       pointer.get<AutoDiffDerivativeFunctionIdentifier *>());
   }
-  /// Returns the back deployment entry point corresponding to the same
-  /// decl.
-  SILDeclRef asBackDeployed(bool backDeployed = true) const {
+  /// Returns a copy of the decl with the given back deployment kind.
+  SILDeclRef asBackDeploymentKind(BackDeploymentKind backDeploymentKind) const {
     return SILDeclRef(loc.getOpaqueValue(), kind,
-                      /*foreign=*/false,
-                      /*distributed=*/false,
-                      /*backDeployed=*/backDeployed,
+                      isForeign,
+                      isDistributed,
+                      backDeploymentKind,
                       defaultArgIndex,
                       pointer.get<AutoDiffDerivativeFunctionIdentifier *>());
   }
@@ -452,9 +465,13 @@ struct SILDeclRef {
   /// True if the decl ref references a thunk handling potentially distributed actor functions
   bool isDistributedThunk() const;
 
-  /// True if the decl ref references a thunk handling a call to a back deployed
-  /// function.
-  bool isBackDeployedThunk() const;
+  /// True if the decl ref references a thunk handling a call to a function that
+  /// supports back deployment.
+  bool isBackDeploymentThunk() const;
+
+  /// True if the decl ref references a function that is the back deployment
+  /// fallback for an original function which may be unavailable at runtime.
+  bool isBackDeploymentFallback() const;
 
   /// True if the decl ref references a method which introduces a new vtable
   /// entry.
@@ -533,12 +550,13 @@ struct SILDeclRef {
   explicit SILDeclRef(void *opaqueLoc, Kind kind,
                       bool isForeign,
                       bool isDistributed,
-                      bool isBackDeployed,
+                      BackDeploymentKind backDeploymentKind,
                       unsigned defaultArgIndex,
                       AutoDiffDerivativeFunctionIdentifier *derivativeId)
       : loc(Loc::getFromOpaqueValue(opaqueLoc)), kind(kind),
         isForeign(isForeign), isDistributed(isDistributed),
-        isBackDeployed(isBackDeployed), defaultArgIndex(defaultArgIndex),
+        backDeploymentKind(backDeploymentKind),
+        defaultArgIndex(defaultArgIndex),
         pointer(derivativeId) {}
 };
 
@@ -555,17 +573,18 @@ namespace llvm {
 template<> struct DenseMapInfo<swift::SILDeclRef> {
   using SILDeclRef = swift::SILDeclRef;
   using Kind = SILDeclRef::Kind;
+  using BackDeploymentKind = SILDeclRef::BackDeploymentKind;
   using Loc = SILDeclRef::Loc;
   using PointerInfo = DenseMapInfo<void*>;
   using UnsignedInfo = DenseMapInfo<unsigned>;
 
   static SILDeclRef getEmptyKey() {
     return SILDeclRef(PointerInfo::getEmptyKey(), Kind::Func, false, false,
-                      false, 0, nullptr);
+                      BackDeploymentKind::None, 0, nullptr);
   }
   static SILDeclRef getTombstoneKey() {
     return SILDeclRef(PointerInfo::getTombstoneKey(), Kind::Func, false, false,
-                      false, 0, nullptr);
+                      BackDeploymentKind::None, 0, nullptr);
   }
   static unsigned getHashValue(swift::SILDeclRef Val) {
     unsigned h1 = PointerInfo::getHashValue(Val.loc.getOpaqueValue());
@@ -576,7 +595,7 @@ template<> struct DenseMapInfo<swift::SILDeclRef> {
     unsigned h4 = UnsignedInfo::getHashValue(Val.isForeign);
     unsigned h5 = PointerInfo::getHashValue(Val.pointer.getOpaqueValue());
     unsigned h6 = UnsignedInfo::getHashValue(Val.isDistributed);
-    unsigned h7 = UnsignedInfo::getHashValue(Val.isBackDeployed);
+    unsigned h7 = UnsignedInfo::getHashValue(unsigned(Val.backDeploymentKind));
     return h1 ^ (h2 << 4) ^ (h3 << 9) ^ (h4 << 7) ^ (h5 << 11) ^ (h6 << 8) ^
            (h7 << 10);
   }

lib/SIL/IR/SILDeclRef.cpp

@@ -121,15 +121,18 @@ bool swift::requiresForeignEntryPoint(ValueDecl *vd) {
   return false;
 }
 
-SILDeclRef::SILDeclRef(ValueDecl *vd, SILDeclRef::Kind kind,
-                       bool isForeign, bool isDistributed, bool isBackDeployed,
+SILDeclRef::SILDeclRef(ValueDecl *vd, SILDeclRef::Kind kind, bool isForeign,
+                       bool isDistributed,
+                       SILDeclRef::BackDeploymentKind backDeploymentKind,
                        AutoDiffDerivativeFunctionIdentifier *derivativeId)
     : loc(vd), kind(kind), isForeign(isForeign), isDistributed(isDistributed),
-      isBackDeployed(isBackDeployed), defaultArgIndex(0), pointer(derivativeId) {}
+      backDeploymentKind(backDeploymentKind), defaultArgIndex(0),
+      pointer(derivativeId) {}
 
 SILDeclRef::SILDeclRef(SILDeclRef::Loc baseLoc, bool asForeign,
-                       bool asDistributed, bool asBackDeployed)
-    : defaultArgIndex(0),
+                       bool asDistributed)
+    : backDeploymentKind(SILDeclRef::BackDeploymentKind::None),
+      defaultArgIndex(0),
       pointer((AutoDiffDerivativeFunctionIdentifier *)nullptr) {
   if (auto *vd = baseLoc.dyn_cast<ValueDecl*>()) {
     if (auto *fd = dyn_cast<FuncDecl>(vd)) {
@@ -168,7 +171,6 @@ SILDeclRef::SILDeclRef(SILDeclRef::Loc baseLoc, bool asForeign,
 
   isForeign = asForeign;
   isDistributed = asDistributed;
-  isBackDeployed = asBackDeployed;
 }
 
 SILDeclRef::SILDeclRef(SILDeclRef::Loc baseLoc,
@@ -205,7 +207,7 @@ ASTContext &SILDeclRef::getASTContext() const {
 
 bool SILDeclRef::isThunk() const {
   return isForeignToNativeThunk() || isNativeToForeignThunk() ||
-         isDistributedThunk() || isBackDeployedThunk();
+         isDistributedThunk() || isBackDeploymentThunk();
 }
 
 bool SILDeclRef::isClangImported() const {
@@ -317,9 +319,9 @@ SILLinkage SILDeclRef::getLinkage(ForDefinition_t forDefinition) const {
       return maybeAddExternal(SILLinkage::PublicNonABI);
   }
 
-  // Back deployment thunks are emitted into the client and therefore have
-  // PublicNonABI linkage.
-  if (isBackDeployedThunk())
+  // Back deployment thunks and fallbacks are emitted into the client and
+  // therefore have PublicNonABI linkage.
+  if (backDeploymentKind != SILDeclRef::BackDeploymentKind::None)
     return maybeAddExternal(SILLinkage::PublicNonABI);
 
   enum class Limit {
@@ -736,10 +738,8 @@ bool SILDeclRef::isAlwaysInline() const {
 }
 
 bool SILDeclRef::isAnyThunk() const {
-  return isForeignToNativeThunk() ||
-    isNativeToForeignThunk() ||
-    isDistributedThunk() ||
-    isBackDeployedThunk();
+  return isForeignToNativeThunk() || isNativeToForeignThunk() ||
+         isDistributedThunk() || isBackDeploymentThunk();
 }
 
 bool SILDeclRef::isForeignToNativeThunk() const {
@@ -793,8 +793,14 @@ bool SILDeclRef::isDistributedThunk() const {
   return kind == Kind::Func;
 }
 
-bool SILDeclRef::isBackDeployedThunk() const {
-  if (!isBackDeployed)
+bool SILDeclRef::isBackDeploymentFallback() const {
+  if (backDeploymentKind != BackDeploymentKind::Fallback)
+    return false;
+  return kind == Kind::Func;
+}
+
+bool SILDeclRef::isBackDeploymentThunk() const {
+  if (backDeploymentKind != BackDeploymentKind::Thunk)
     return false;
   return kind == Kind::Func;
 }
@@ -879,8 +885,10 @@ std::string SILDeclRef::mangle(ManglingKind MKind) const {
         SKind = ASTMangler::SymbolKind::ObjCAsSwiftThunk;
       } else if (isDistributedThunk()) {
         SKind = ASTMangler::SymbolKind::DistributedThunk;
-      } else if (isBackDeployedThunk()) {
-        SKind = ASTMangler::SymbolKind::BackDeployedThunk;
+      } else if (isBackDeploymentThunk()) {
+        SKind = ASTMangler::SymbolKind::BackDeploymentThunk;
+      } else if (isBackDeploymentFallback()) {
+        SKind = ASTMangler::SymbolKind::BackDeploymentFallback;
       }
       break;
     case SILDeclRef::ManglingKind::DynamicThunk:
@@ -1012,7 +1020,7 @@ bool SILDeclRef::requiresNewVTableEntry() const {
     return false;
   if (isDistributedThunk())
     return false;
-  if (isBackDeployedThunk())
+  if (isBackDeploymentThunk())
     return false;
   auto fnDecl = dyn_cast<AbstractFunctionDecl>(getDecl());
   if (!fnDecl)
@@ -1041,6 +1049,9 @@ SILDeclRef SILDeclRef::getOverridden() const {
 
 SILDeclRef SILDeclRef::getNextOverriddenVTableEntry() const {
   if (auto overridden = getOverridden()) {
+    // Back deployed methods should not be overridden.
+    assert(backDeploymentKind == SILDeclRef::BackDeploymentKind::None);
+
     // If we overrode a foreign decl or dynamic method, if this is an
     // accessor for a property that overrides an ObjC decl, or if it is an
     // @NSManaged property, then it won't be in the vtable.
@@ -1051,10 +1062,6 @@ SILDeclRef SILDeclRef::getNextOverriddenVTableEntry() const {
     if (isDistributedThunk())
       return SILDeclRef();
 
-    // Back deployed thunks are not in the vtable.
-    if (isBackDeployedThunk())
-      return SILDeclRef();
-
     // An @objc convenience initializer can be "overridden" in the sense that
     // its selector is reclaimed by a subclass's convenience init with the
     // same name. The AST models this as an override for the purposes of
@@ -1336,7 +1343,7 @@ bool SILDeclRef::canBeDynamicReplacement() const {
     return false;
   if (isDistributedThunk())
     return false;
-  if (isBackDeployedThunk())
+  if (backDeploymentKind != SILDeclRef::BackDeploymentKind::None)
     return false;
   if (kind == SILDeclRef::Kind::Destroyer ||
       kind == SILDeclRef::Kind::DefaultArgGenerator)
@@ -1357,7 +1364,7 @@ bool SILDeclRef::isDynamicallyReplaceable() const {
   if (isDistributedThunk())
     return false;
 
-  if (isBackDeployedThunk())
+  if (backDeploymentKind != SILDeclRef::BackDeploymentKind::None)
     return false;
 
   if (kind == SILDeclRef::Kind::DefaultArgGenerator)

lib/SIL/Parser/ParseSIL.cpp

@@ -1519,7 +1519,7 @@ bool SILParser::parseSILDeclRef(SILDeclRef &Result,
   if (!P.consumeIf(tok::sil_exclamation)) {
     // Construct SILDeclRef.
     Result = SILDeclRef(VD, Kind, IsObjC, /*distributed=*/false,
-                        /*backDeployed=*/false, DerivativeId);
+                        SILDeclRef::BackDeploymentKind::None, DerivativeId);
     return false;
   }
 
@@ -1640,7 +1640,7 @@ bool SILParser::parseSILDeclRef(SILDeclRef &Result,
 
   // Construct SILDeclRef.
   Result = SILDeclRef(VD, Kind, IsObjC, /*distributed=*/false,
-                      /*backDeployed=*/false, DerivativeId);
+                      SILDeclRef::BackDeploymentKind::None, DerivativeId);
   return false;
 }
 

lib/SILGen/SILGen.cpp

@@ -853,18 +853,18 @@ void SILGenModule::emitFunctionDefinition(SILDeclRef constant, SILFunction *f) {
     return;
   }
 
-  if (constant.isBackDeployedThunk()) {
+  if (constant.isBackDeploymentThunk()) {
     auto loc = constant.getAsRegularLocation();
     loc.markAutoGenerated();
     auto *dc = loc.getAsDeclContext();
     assert(dc);
 
     preEmitFunction(constant, f, loc);
-    PrettyStackTraceSILFunction X("silgen emitBackDeployedThunk", f);
+    PrettyStackTraceSILFunction X("silgen emitBackDeploymentThunk", f);
     f->setBare(IsBare);
     f->setThunk(IsThunk);
 
-    SILGenFunction(*this, *f, dc).emitBackDeployedThunk(constant);
+    SILGenFunction(*this, *f, dc).emitBackDeploymentThunk(constant);
 
     postEmitFunction(constant, f);
     return;
@@ -1402,9 +1402,17 @@ void SILGenModule::emitAbstractFuncDecl(AbstractFunctionDecl *AFD) {
   }
 
   if (AFD->isBackDeployed()) {
-    // FIXME(backDeploy): Emit client copy of back deployed function
-    auto thunk = SILDeclRef(AFD).asBackDeployed();
-    emitBackDeployedThunk(thunk);
+    // Emit the fallback function that will be used when the original function
+    // is unavailable at runtime.
+    auto fallback = SILDeclRef(AFD).asBackDeploymentKind(
+        SILDeclRef::BackDeploymentKind::Fallback);
+    emitFunctionDefinition(fallback, getFunction(fallback, ForDefinition));
+
+    // Emit the thunk that either invokes the original function or the fallback
+    // function depending on the availability of the original.
+    auto thunk = SILDeclRef(AFD).asBackDeploymentKind(
+        SILDeclRef::BackDeploymentKind::Thunk);
+    emitBackDeploymentThunk(thunk);
   }
 }
 

lib/SILGen/SILGen.h

@@ -337,9 +337,9 @@ class LLVM_LIBRARY_VISIBILITY SILGenModule : public ASTVisitor<SILGenModule> {
   void emitDistributedThunk(SILDeclRef thunk);
 
   /// Emits a thunk that calls either the original function if it is available
-  /// or otherwise calls a copy of the function that was emitted into the
-  /// calling module.
-  void emitBackDeployedThunk(SILDeclRef thunk);
+  /// or otherwise calls a fallback variant of the function that was emitted
+  /// into the client module.
+  void emitBackDeploymentThunk(SILDeclRef thunk);
 
   void preEmitFunction(SILDeclRef constant, SILFunction *F, SILLocation L);
   void postEmitFunction(SILDeclRef constant, SILFunction *F);