Merge pull request swiftlang#40044 from etcwilde/ewilde/5.5/fix-weak-extendedFramePointerFlags

5.5 fix weak extended frame pointer flags

Author: DougGregor

lib/IRGen/IRGenModule.cpp

@@ -1778,6 +1778,19 @@ void IRGenModule::emitSwiftAsyncExtendedFrameInfoWeakRef() {
   extendedFramePointerFlagsWeakRef = new llvm::GlobalVariable(Module, Int8PtrTy, false,
                                          llvm::GlobalValue::ExternalWeakLinkage, nullptr,
                                          symbolName);
+
+  // The weak imported extendedFramePointerFlagsWeakRef gets optimized out
+  // before being added back as a strong import.
+  // Declarations can't be added to the used list, so we create a little
+  // global that can't be used from the program, but can be in the used list to
+  // avoid optimizations.
+  llvm::GlobalVariable *usage = new llvm::GlobalVariable(
+      Module, extendedFramePointerFlagsWeakRef->getType(), false,
+      llvm::GlobalValue::LinkOnceODRLinkage,
+      static_cast<llvm::GlobalVariable *>(extendedFramePointerFlagsWeakRef),
+      "_swift_async_extendedFramePointerFlagsUser");
+  usage->setVisibility(llvm::GlobalValue::HiddenVisibility);
+  addUsedGlobal(usage);
 }
 
 bool IRGenModule::isConcurrencyAvailable() {

test/Concurrency/Backdeploy/weak_linking.swift

@@ -3,6 +3,9 @@
 // RUN: %FileCheck %s --check-prefix=NEW < %t/new.ir
 // RUN: %target-swift-frontend %s -target x86_64-apple-macosx10.15 -module-name main -emit-ir -o %t/old.ir -disable-availability-checking
 // RUN: %FileCheck %s --check-prefix=OLD < %t/old.ir
+// RUN: %target-swift-frontend %s -target x86_64-apple-macosx10.15 -O -module-name main -emit-ir -o %t/optimized.ir -disable-availability-checking
+// RUN: %FileCheck %s --check-prefix=OPTIMIZED < %t/optimized.ir
+
 
 // REQUIRES: OS=macosx
 
@@ -12,6 +15,11 @@
 // OLD: declare extern_weak swiftcc %swift.metadata_response @"$sScPMa"
 // OLD: declare extern_weak swiftcc i8 @"$sScP8rawValues5UInt8Vvg"
 
+// OPTIMIZED: @swift_async_extendedFramePointerFlags = extern_weak global i8*
+// OPTIMIZED: @_swift_async_extendedFramePointerFlagsUser = linkonce_odr hidden global i8** @swift_async_extendedFramePointerFlags
+// OPTIMIZED: @llvm.used =
+// OPTIMIZED-SAME: (i8*** @_swift_async_extendedFramePointerFlagsUser to i8*)
+
 @available(macOS 12.0, *)
 public func g() async -> String { "hello" }