Rework the watchOS concurrency back-deployment async frame pointer.

Introduce a few changes to the logic for watchOS concurrency
back-deployment with respect to the async frame pointer.

* Only apply the change to watchOS device targets, not simulator targets
* Only introduce the override when no specific
  `-swift-async-frame-pointer=<value>` option is provided on the
command line
* Only override the default when deploying to watchOS < 8, and
* Use "never" for the default rather than "always".

This represents a different but safer trade-off than before. Setting
the async bit in the frame pointer can cause older APIs (such as
backtrace APIs in the OS) to crash when they encounter such frame
pointers. So, with this change we never set the bit when back-deploying
for watchOS device, to avoid said crashes.

The trade-off here is that a back-deployed watchOS app will never have
the async frame pointer bit set, so async backtraces will be
unavailable even when running on watchOS 8 or newer.

Author: DougGregor

lib/Frontend/CompilerInvocation.cpp

@@ -1982,16 +1982,17 @@ static bool ParseIRGenArgs(IRGenOptions &Opts, ArgList &Args,
       Diags.diagnose(SourceLoc(), diag::error_invalid_arg_value,
                      A->getAsString(Args), A->getValue());
     }
+  } else if (Triple.isWatchOS() && !Triple.isSimulatorEnvironment()) {
+    // watchOS does not support auto async frame pointers due to bitcode, so
+    // silently override "auto" to "never" when back-deploying. This approach
+    // sacrifies async backtraces when back-deploying but prevents crashes in
+    // older tools that cannot handle the async frame bit in the frame pointer.
+    unsigned major, minor, micro;
+    Triple.getWatchOSVersion(major, minor, micro);
+    if (major < 8)
+      Opts.SwiftAsyncFramePointer = SwiftAsyncFramePointerKind::Never;
   }
 
-  // watchOS does not support auto async frame pointers due to bitcode, so
-  // silently override "auto" to "always". This approach sacrifies async
-  // backtraces in older watchOS versions to gain better backtraces in newer
-  // versions.
-  if (Triple.isWatchOS() &&
-      Opts.SwiftAsyncFramePointer == SwiftAsyncFramePointerKind::Auto)
-    Opts.SwiftAsyncFramePointer = SwiftAsyncFramePointerKind::Always;
-
   return false;
 }
 

test/IRGen/swift_async_extended_frame_info_watchos.swift

@@ -1,12 +1,20 @@
-// RUN: %target-swift-frontend -disable-availability-checking -target arm64_32-apple-watchos7 %s -S | %FileCheck  -check-prefix=ALWAYS %s
+// RUN: %target-swift-frontend -disable-availability-checking -target arm64_32-apple-watchos7 %s -S | %FileCheck  -check-prefix=NEVER %s
 // RUN: %target-swift-frontend -disable-availability-checking -target arm64_32-apple-watchos8 %s -S | %FileCheck  -check-prefix=ALWAYS %s
-// RUN: %target-swift-frontend -disable-availability-checking -target arm64_32-apple-watchos7  -swift-async-frame-pointer=auto %s -S | %FileCheck  -check-prefix=ALWAYS %s
+// RUN: %target-swift-frontend -disable-availability-checking -target arm64_32-apple-watchos7  -swift-async-frame-pointer=always %s -S | %FileCheck  -check-prefix=ALWAYS %s
 // RUN: %target-swift-frontend -disable-availability-checking -target arm64_32-apple-watchos7  -swift-async-frame-pointer=never %s -S | %FileCheck  -check-prefix=NEVER %s
+// RUN: %target-swift-frontend -disable-availability-checking -target arm64_32-apple-watchos7  -swift-async-frame-pointer=auto %s -S | %FileCheck  -check-prefix=AUTO %s
 
 // REQUIRES: OS=watchos
+// REQUIRES: CPU=armv7k || CPU=arm64_32
 
 public func someAsyncFunction() async {
 }
 
+// AUTO: swift_async_extendedFramePointerFlags
+
+// ALWAYS-NOT: swift_async_extendedFramePointerFlags
+// ALWAYS: 0x1000000000000000
 // ALWAYS-NOT: swift_async_extendedFramePointerFlags
+
 // NEVER-NOT: swift_async_extendedFramePointerFlags
+// NEVER-NOT: 0x1000000000000000