SILGen: Function availability for linkage should use @_backDeploy OS version if present.

When computing the availability of a `SILFunction` for linkage the OS version specified in the `@_backDeploy` attribute should be preferred over the version in the `@available` attribute. This ensures that the corresponding symbol is weakly linked when deploying to older OSes than the back deploy "before" version.

Resolves rdar://99962885

Author: tshortli

include/swift/AST/Attr.h

@@ -2221,6 +2221,9 @@ class BackDeployAttr: public DeclAttribute {
   /// The earliest platform version that may use the back deployed implementation.
   const llvm::VersionTuple Version;
 
+  /// Returns true if this attribute is active given the current platform.
+  bool isActivePlatform(const ASTContext &ctx) const;
+
   static bool classof(const DeclAttribute *DA) {
     return DA->getKind() == DAK_BackDeploy;
   }

include/swift/AST/Decl.h

@@ -837,7 +837,7 @@ class alignas(1 << DeclAlignInBits) Decl : public ASTAllocated<Decl> {
   /// Returns the OS version in which the decl became ABI as specified by the
   /// @_backDeploy attribute.
   Optional<llvm::VersionTuple>
-  getBackDeployBeforeOSVersion(PlatformKind Kind) const;
+  getBackDeployBeforeOSVersion(ASTContext &Ctx) const;
 
   /// Returns the starting location of the entire declaration.
   SourceLoc getStartLoc() const { return getSourceRange().Start; }

include/swift/SIL/SILDeclRef.h

@@ -19,11 +19,12 @@
 #ifndef SWIFT_SIL_SILDeclRef_H
 #define SWIFT_SIL_SILDeclRef_H
 
+#include "swift/AST/Availability.h"
 #include "swift/AST/ClangNode.h"
 #include "swift/AST/GenericSignature.h"
 #include "swift/AST/TypeAlignments.h"
-#include "llvm/ADT/Hashing.h"
 #include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/Hashing.h"
 #include "llvm/ADT/PointerUnion.h"
 #include "llvm/Support/PrettyStackTrace.h"
 
@@ -530,6 +531,9 @@ struct SILDeclRef {
   static AbstractFunctionDecl *getOverriddenWitnessTableEntry(
                                                     AbstractFunctionDecl *func);
 
+  /// Returns the availability of the decl for computing linkage.
+  Optional<AvailabilityContext> getAvailabilityForLinkage() const;
+
   /// True if the referenced entity is some kind of thunk.
   bool isThunk() const;
 

lib/AST/Attr.cpp

@@ -1695,6 +1695,10 @@ bool AvailableAttr::isActivePlatform(const ASTContext &ctx) const {
   return isPlatformActive(Platform, ctx.LangOpts);
 }
 
+bool BackDeployAttr::isActivePlatform(const ASTContext &ctx) const {
+  return isPlatformActive(Platform, ctx.LangOpts);
+}
+
 AvailableAttr *AvailableAttr::clone(ASTContext &C, bool implicit) const {
   return new (C) AvailableAttr(implicit ? SourceLoc() : AtLoc,
                                implicit ? SourceRange() : getRange(),

lib/AST/Decl.cpp

@@ -382,18 +382,19 @@ Decl::getIntroducedOSVersion(PlatformKind Kind) const {
 }
 
 Optional<llvm::VersionTuple>
-Decl::getBackDeployBeforeOSVersion(PlatformKind Kind) const {
+Decl::getBackDeployBeforeOSVersion(ASTContext &Ctx) const {
   for (auto *attr : getAttrs()) {
     if (auto *backDeployAttr = dyn_cast<BackDeployAttr>(attr)) {
-      if (backDeployAttr->Platform == Kind && backDeployAttr->Version) {
+      if (backDeployAttr->isActivePlatform(Ctx) && backDeployAttr->Version) {
         return backDeployAttr->Version;
       }
     }
   }
 
   // Accessors may inherit `@_backDeploy`.
   if (getKind() == DeclKind::Accessor) {
-    return cast<AccessorDecl>(this)->getStorage()->getBackDeployBeforeOSVersion(Kind);
+    return cast<AccessorDecl>(this)->getStorage()->getBackDeployBeforeOSVersion(
+        Ctx);
   }
 
   return None;
@@ -937,14 +938,20 @@ bool Decl::isStdlibDecl() const {
 }
 
 AvailabilityContext Decl::getAvailabilityForLinkage() const {
+  ASTContext &ctx = getASTContext();
+
+  // When computing availability for linkage, use the "before" version from
+  // the @_backDeploy attribute, if present.
+  if (auto backDeployVersion = getBackDeployBeforeOSVersion(ctx))
+    return AvailabilityContext{VersionRange::allGTE(*backDeployVersion)};
+
   auto containingContext =
       AvailabilityInference::annotatedAvailableRange(this, getASTContext());
   if (containingContext.hasValue()) {
-    // If this entity comes from the concurrency module, adjust it's
+    // If this entity comes from the concurrency module, adjust its
     // availability for linkage purposes up to Swift 5.5, so that we use
     // weak references any time we reference those symbols when back-deploying
     // concurrency.
-    ASTContext &ctx = getASTContext();
     if (getModuleContext()->getName() == ctx.Id_Concurrency) {
       containingContext->intersectWith(ctx.getConcurrencyAvailability());
     }

lib/SIL/IR/SILDeclRef.cpp

@@ -207,6 +207,16 @@ ASTContext &SILDeclRef::getASTContext() const {
   llvm_unreachable("Unhandled case in switch");
 }
 
+Optional<AvailabilityContext> SILDeclRef::getAvailabilityForLinkage() const {
+  // Back deployment thunks and fallbacks don't have availability since they
+  // are non-ABI.
+  // FIXME: Generalize this check to all kinds of non-ABI functions.
+  if (backDeploymentKind != SILDeclRef::BackDeploymentKind::None)
+    return None;
+
+  return getDecl()->getAvailabilityForLinkage();
+}
+
 bool SILDeclRef::isThunk() const {
   return isForeignToNativeThunk() || isNativeToForeignThunk() ||
          isDistributedThunk() || isBackDeploymentThunk();

lib/SIL/IR/SILFunctionBuilder.cpp

@@ -311,7 +311,9 @@ SILFunction *SILFunctionBuilder::getOrCreateFunction(
     if (constant.isForeign && decl->hasClangNode())
       F->setClangNodeOwner(decl);
 
-    F->setAvailabilityForLinkage(decl->getAvailabilityForLinkage());
+    if (auto availability = constant.getAvailabilityForLinkage())
+      F->setAvailabilityForLinkage(*availability);
+
     F->setIsAlwaysWeakImported(decl->isAlwaysWeakImported());
 
     if (auto *accessor = dyn_cast<AccessorDecl>(decl)) {

lib/SILGen/SILGenBackDeploy.cpp

@@ -52,8 +52,7 @@ static void emitBackDeployIfAvailableCondition(SILGenFunction &SGF,
                                                SILLocation loc,
                                                SILBasicBlock *availableBB,
                                                SILBasicBlock *unavailableBB) {
-  PlatformKind platform = targetPlatform(SGF.SGM.getASTContext().LangOpts);
-  auto version = AFD->getBackDeployBeforeOSVersion(platform);
+  auto version = AFD->getBackDeployBeforeOSVersion(SGF.SGM.getASTContext());
   VersionRange OSVersion = VersionRange::empty();
   if (version.hasValue()) {
     OSVersion = VersionRange::allGTE(*version);

test/IRGen/weak_import_back_deploy_attr.swift

@@ -0,0 +1,22 @@
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+// RUN: %target-swift-frontend -emit-module -emit-module-path %t/Library.swiftmodule -parse-as-library %t/Library.swift -enable-library-evolution
+
+// RUN: %target-swift-frontend -primary-file %t/Client.swift -I %t -emit-ir -target %target-cpu-apple-macosx10.50 | %FileCheck %t/Client.swift --check-prefix=CHECK-OLD
+// RUN: %target-swift-frontend -primary-file %t/Client.swift -I %t -emit-ir -target %target-cpu-apple-macosx10.60 | %FileCheck %t/Client.swift --check-prefix=CHECK-NEW
+
+// REQUIRES: OS=macosx
+
+//--- Library.swift
+
+@available(macOS 10.50, *)
+@_backDeploy(before: macOS 10.60)
+public func backDeployedFunc() {}
+
+//--- Client.swift
+
+import Library
+
+// CHECK-OLD: declare extern_weak {{.*}} void @"$s7Library16backDeployedFuncyyF"()
+// CHECK-NEW: declare {{.*}} void @"$s7Library16backDeployedFuncyyF"()
+backDeployedFunc()

test/SILGen/back_deploy_attribute_accessor.swift

@@ -8,12 +8,12 @@
 @available(macOS 10.50, *)
 public struct TopLevelStruct {
   // -- Fallback definition for TopLevelStruct.property.getter
-  // CHECK-LABEL: sil non_abi [serialized] [available 10.51] [ossa] @$s11back_deploy14TopLevelStructV8propertyACvgTwB : $@convention(method) (TopLevelStruct) -> TopLevelStruct
+  // CHECK-LABEL: sil non_abi [serialized] [ossa] @$s11back_deploy14TopLevelStructV8propertyACvgTwB : $@convention(method) (TopLevelStruct) -> TopLevelStruct
   // CHECK: bb0([[SELF:%.*]] : $TopLevelStruct):
   // CHECK:   return [[SELF]] : $TopLevelStruct
 
   // -- Back deployment thunk for TopLevelStruct.property.getter
-  // CHECK-LABEL: sil non_abi [serialized] [thunk] [available 10.51] [ossa] @$s11back_deploy14TopLevelStructV8propertyACvgTwb : $@convention(method) (TopLevelStruct) -> TopLevelStruct
+  // CHECK-LABEL: sil non_abi [serialized] [thunk] [ossa] @$s11back_deploy14TopLevelStructV8propertyACvgTwb : $@convention(method) (TopLevelStruct) -> TopLevelStruct
   // CHECK: bb0([[BB0_ARG:%.*]] : $TopLevelStruct):
   // CHECK:   [[MAJOR:%.*]] = integer_literal $Builtin.Word, 10
   // CHECK:   [[MINOR:%.*]] = integer_literal $Builtin.Word, 52
@@ -36,7 +36,7 @@ public struct TopLevelStruct {
   // CHECK:   return [[RETURN_BB_ARG]] : $TopLevelStruct
 
   // -- Original definition of TopLevelStruct.property.getter
-  // CHECK-LABEL: sil [available 10.51] [ossa] @$s11back_deploy14TopLevelStructV8propertyACvg : $@convention(method) (TopLevelStruct) -> TopLevelStruct
+  // CHECK-LABEL: sil [available 10.52] [ossa] @$s11back_deploy14TopLevelStructV8propertyACvg : $@convention(method) (TopLevelStruct) -> TopLevelStruct
   @available(macOS 10.51, *)
   @_backDeploy(before: macOS 10.52)
   public var property: TopLevelStruct { self }