[IRGen] Use proper linkage for async function pointers to partial apply forwarders

rdar://163631865

Under certain circumstances the same symbol can be used for different implementations of the forwarders.
The forwarders themselves are already emitted with "internal" LLVM linkage, but some particularities in
the mapping from SILLinkage to LLVM linkage caused async function pointers to partial apply forwarders
to be emitted with linkonce_odr instead, which caused the wrong forwarder to be called at runtime, causing
unexpected behavior and crashes.

Author: drexin

include/swift/IRGen/Linking.h

@@ -1861,6 +1861,15 @@ class LinkEntity {
 
   bool isAlwaysSharedLinkage() const;
 
+  /// Partial apply forwarders always need real private linkage,
+  /// to ensure the correct implementation is used in case of
+  /// colliding symbols.
+  bool privateMeansPrivate() const {
+    return getKind() == Kind::PartialApplyForwarder ||
+           getKind() == Kind::PartialApplyForwarderAsyncFunctionPointer ||
+           getKind() == Kind::PartialApplyForwarderCoroFunctionPointer;
+  }
+
   /// Whether the link entity's definitions must be considered non-unique.
   ///
   /// This applies only in the Embedded Swift linkage model, and is used for

lib/IRGen/GenDecl.cpp

@@ -2324,11 +2324,11 @@ void IRGenerator::emitEntryPointInfo() {
   IGM.addUsedGlobal(var);
 }
 
-static IRLinkage
-getIRLinkage(StringRef name, const UniversalLinkageInfo &info,
-             SILLinkage linkage, ForDefinition_t isDefinition,
-             bool isWeakImported, bool isKnownLocal,
-             bool hasNonUniqueDefinition) {
+static IRLinkage getIRLinkage(StringRef name, const UniversalLinkageInfo &info,
+                              SILLinkage linkage, ForDefinition_t isDefinition,
+                              bool isWeakImported, bool isKnownLocal,
+                              bool hasNonUniqueDefinition,
+                              bool privateMeansPrivate) {
 #define RESULT(LINKAGE, VISIBILITY, DLL_STORAGE)                               \
   IRLinkage{llvm::GlobalValue::LINKAGE##Linkage,                               \
             llvm::GlobalValue::VISIBILITY##Visibility,                         \
@@ -2390,12 +2390,15 @@ getIRLinkage(StringRef name, const UniversalLinkageInfo &info,
   case SILLinkage::Private: {
     if (info.forcePublicDecls() && !isDefinition)
       return getIRLinkage(name, info, SILLinkage::PublicExternal, isDefinition,
-                          isWeakImported, isKnownLocal, hasNonUniqueDefinition);
-
-    auto linkage = info.needLinkerToMergeDuplicateSymbols()
-                       ? llvm::GlobalValue::LinkOnceODRLinkage
-                       : llvm::GlobalValue::InternalLinkage;
-    auto visibility = info.shouldAllPrivateDeclsBeVisibleFromOtherFiles()
+                          isWeakImported, isKnownLocal, hasNonUniqueDefinition,
+                          privateMeansPrivate);
+
+    auto linkage =
+        info.needLinkerToMergeDuplicateSymbols() && !privateMeansPrivate
+            ? llvm::GlobalValue::LinkOnceODRLinkage
+            : llvm::GlobalValue::InternalLinkage;
+    auto visibility = info.shouldAllPrivateDeclsBeVisibleFromOtherFiles() &&
+                              !privateMeansPrivate
                           ? llvm::GlobalValue::HiddenVisibility
                           : llvm::GlobalValue::DefaultVisibility;
     return {linkage, visibility, llvm::GlobalValue::DefaultStorageClass};
@@ -2446,8 +2449,8 @@ void irgen::updateLinkageForDefinition(IRGenModule &IGM,
   auto IRL =
       getIRLinkage(global->hasName() ? global->getName() : StringRef(),
                    linkInfo, entity.getLinkage(ForDefinition), ForDefinition,
-                   weakImported, isKnownLocal,
-                   entity.hasNonUniqueDefinition());
+                   weakImported, isKnownLocal, entity.hasNonUniqueDefinition(),
+                   entity.privateMeansPrivate());
   ApplyIRLinkage(IRL).to(global);
 
   LinkInfo link = LinkInfo::get(IGM, entity, ForDefinition);
@@ -2498,10 +2501,10 @@ LinkInfo LinkInfo::get(const UniversalLinkageInfo &linkInfo,
   }
 
   bool weakImported = entity.isWeakImported(swiftModule);
-  result.IRL = getIRLinkage(result.Name, linkInfo,
-                            entity.getLinkage(isDefinition), isDefinition,
-                            weakImported, isKnownLocal,
-                            entity.hasNonUniqueDefinition());
+  result.IRL = getIRLinkage(
+      result.Name, linkInfo, entity.getLinkage(isDefinition), isDefinition,
+      weakImported, isKnownLocal, entity.hasNonUniqueDefinition(),
+      entity.privateMeansPrivate());
   result.ForDefinition = isDefinition;
   return result;
 }
@@ -2513,7 +2516,8 @@ LinkInfo LinkInfo::get(const UniversalLinkageInfo &linkInfo, StringRef name,
   result.Name += name;
   result.IRL = getIRLinkage(name, linkInfo, linkage, isDefinition,
                             isWeakImported, linkInfo.Internalize,
-                            /*hasNonUniqueDefinition=*/false);
+                            /*hasNonUniqueDefinition=*/false,
+                            /*privateMeansPrivate=*/false);
   result.ForDefinition = isDefinition;
   return result;
 }

test/IRGen/async/partial_apply_forwarder.sil

@@ -89,6 +89,11 @@ entry(%e : $EmptyType, %b : $WeakBox<τ_0_0>):
   unreachable
 }
 
+// CHECK-DAG: @"$s7takingQTATu" = internal
+// CHECK-DAG: @"$s11takingQAndSTATu" = internal
+// CHECK-DAG: @"$s13inner_closureTATu" = internal
+// CHECK-DAG: @"$s15returns_closureTATu" = internal
+
 // CHECK-LABEL: define{{( dllexport)?}}{{( protected)?}} swift{{(tail)?}}cc void @bind_polymorphic_param_from_context(
 // CHECK-LABEL: define internal swift{{(tail)?}}cc void @"$s7takingQTA"(
 sil public @bind_polymorphic_param_from_context : $@async @convention(thin) <τ_0_1>(@in τ_0_1) -> @owned @async @callee_guaranteed () -> () {