Merge pull request #38317 from mikeash/frame-pointer-mask-symbol-5.5

[5.5][Concurrency] Add an exported symbol on Darwin that contains the frame pointer flag bit for async frames.

Author: DougGregor

include/swift/Runtime/Concurrency.h

@@ -648,6 +648,18 @@ void swift_task_reportUnexpectedExecutor(
 SWIFT_EXPORT_FROM(swift_Concurrency) SWIFT_CC(swift)
 JobPriority swift_task_getCurrentThreadPriority(void);
 
+#ifdef __APPLE__
+/// A magic symbol whose address is the mask to apply to a frame pointer to
+/// signal that it is an async frame. Do not try to read the actual value of
+/// this global, it will crash.
+///
+/// On ARM64_32, the address is only 32 bits, and therefore this value covers
+/// the top 32 bits of the in-memory frame pointer. On other 32-bit platforms,
+/// the bit is not used and the address is always 0.
+SWIFT_EXPORT_FROM(swift_Concurrency)
+struct { char c; } swift_async_extendedFramePointerFlags;
+#endif
+
 }
 
 #pragma clang diagnostic pop

stdlib/public/Concurrency/Task.cpp

@@ -33,6 +33,19 @@
 #include <dlfcn.h>
 #endif
 
+#ifdef __APPLE__
+#if __POINTER_WIDTH__ == 64
+asm("\n .globl _swift_async_extendedFramePointerFlags" \
+    "\n _swift_async_extendedFramePointerFlags = 0x1000000000000000");
+#elif __ARM64_ARCH_8_32__
+asm("\n .globl _swift_async_extendedFramePointerFlags" \
+    "\n _swift_async_extendedFramePointerFlags = 0x10000000");
+#else
+asm("\n .globl _swift_async_extendedFramePointerFlags" \
+    "\n _swift_async_extendedFramePointerFlags = 0x0");
+#endif
+#endif // __APPLE__
+
 using namespace swift;
 using FutureFragment = AsyncTask::FutureFragment;
 using TaskGroup = swift::TaskGroup;