Add descriptors of weak reference bits for lldb.

Author: Greg Parker

include/swift/ABI/System.h

@@ -56,16 +56,36 @@
 /// ``pointer & SWIFT_ABI_XXX_OBJC_RESERVED_BITS_MASK == 0 && 
 /// pointer & SWIFT_ABI_XXX_SWIFT_SPARE_BITS_MASK != 0``.
 
+// Weak references use a marker to tell when they are controlled by
+// the ObjC runtime and when they are controlled by the Swift runtime.
+// Non-ObjC platforms don't use this marker.
+#define SWIFT_ABI_DEFAULT_OBJC_WEAK_REFERENCE_MARKER_MASK  0
+#define SWIFT_ABI_DEFAULT_OBJC_WEAK_REFERENCE_MARKER_VALUE 0
+
 /*********************************** i386 *************************************/
 
 // Heap objects are pointer-aligned, so the low two bits are unused.
 #define SWIFT_ABI_I386_SWIFT_SPARE_BITS_MASK 0x00000003U
 
+// ObjC weak reference discriminator is the LSB.
+#define SWIFT_ABI_I386_OBJC_WEAK_REFERENCE_MARKER_MASK  \
+  (SWIFT_ABI_DEFAULT_OBJC_RESERVED_BITS_MASK |          \
+   1<<SWIFT_ABI_DEFAULT_OBJC_NUM_RESERVED_LOW_BITS)
+#define SWIFT_ABI_I386_OBJC_WEAK_REFERENCE_MARKER_VALUE \
+  (1<<SWIFT_ABI_DEFAULT_OBJC_NUM_RESERVED_LOW_BITS)
+
 /*********************************** arm **************************************/
 
 // Heap objects are pointer-aligned, so the low two bits are unused.
 #define SWIFT_ABI_ARM_SWIFT_SPARE_BITS_MASK 0x00000003U
 
+// ObjC weak reference discriminator is the LSB.
+#define SWIFT_ABI_ARM_OBJC_WEAK_REFERENCE_MARKER_MASK  \
+  (SWIFT_ABI_DEFAULT_OBJC_RESERVED_BITS_MASK |          \
+   1<<SWIFT_ABI_DEFAULT_OBJC_NUM_RESERVED_LOW_BITS)
+#define SWIFT_ABI_ARM_OBJC_WEAK_REFERENCE_MARKER_VALUE \
+  (1<<SWIFT_ABI_DEFAULT_OBJC_NUM_RESERVED_LOW_BITS)
+
 /*********************************** x86-64 ***********************************/
 
 /// Darwin reserves the low 4GB of address space.
@@ -79,6 +99,14 @@
 #define SWIFT_ABI_X86_64_OBJC_RESERVED_BITS_MASK 0x8000000000000001ULL
 #define SWIFT_ABI_X86_64_OBJC_NUM_RESERVED_LOW_BITS 1
 
+// ObjC weak reference discriminator is the two bits
+// reserved for ObjC tagged pointers plus one more low bit.
+#define SWIFT_ABI_X86_64_OBJC_WEAK_REFERENCE_MARKER_MASK  \
+  (SWIFT_ABI_X86_64_OBJC_RESERVED_BITS_MASK |          \
+   1<<SWIFT_ABI_X86_64_OBJC_NUM_RESERVED_LOW_BITS)
+#define SWIFT_ABI_X86_64_OBJC_WEAK_REFERENCE_MARKER_VALUE \
+  (1<<SWIFT_ABI_X86_64_OBJC_NUM_RESERVED_LOW_BITS)
+
 /*********************************** arm64 ************************************/
 
 /// Darwin reserves the low 4GB of address space.
@@ -92,6 +120,14 @@
 #define SWIFT_ABI_ARM64_OBJC_RESERVED_BITS_MASK 0x8000000000000000ULL
 #define SWIFT_ABI_ARM64_OBJC_NUM_RESERVED_LOW_BITS 0
 
+// ObjC weak reference discriminator is the high bit
+// reserved for ObjC tagged pointers plus the LSB.
+#define SWIFT_ABI_ARM64_OBJC_WEAK_REFERENCE_MARKER_MASK  \
+  (SWIFT_ABI_ARM64_OBJC_RESERVED_BITS_MASK |          \
+   1<<SWIFT_ABI_ARM64_OBJC_NUM_RESERVED_LOW_BITS)
+#define SWIFT_ABI_ARM64_OBJC_WEAK_REFERENCE_MARKER_VALUE \
+  (1<<SWIFT_ABI_ARM64_OBJC_NUM_RESERVED_LOW_BITS)
+
 /*********************************** powerpc64 ********************************/
 
 // Heap objects are pointer-aligned, so the low three bits are unused.

stdlib/public/runtime/WeakReference.h

@@ -66,23 +66,46 @@ namespace swift {
 //  * concurrent writes other than zeroing
 
 class WeakReferenceBits {
-#if SWIFT_OBJC_INTEROP
-  // NativeMarker: ObjC low bits all zero, next bit 1
+  // On ObjC platforms, a weak variable may be controlled by the ObjC
+  // runtime or by the Swift runtime. NativeMarkerMask and NativeMarkerValue
+  // are used to distinguish them.
+  //   if ((ptr & NativeMarkerMask) == NativeMarkerValue) it's Swift
+  //   else it's ObjC
+  // NativeMarkerMask incorporates the ObjC tagged pointer bits
+  // plus one more bit that is set in Swift-controlled weak pointer values.
+  // Non-ObjC platforms don't use any markers.
   enum : uintptr_t {
-    NativeMarkerValue =
-      uintptr_t(1) << swift::heap_object_abi::ObjCReservedLowBits,
-    NativeMarkerMask =
-      NativeMarkerValue | (NativeMarkerValue - 1)
-  };
+#if !SWIFT_OBJC_INTEROP
+    NativeMarkerMask  = 0,
+    NativeMarkerValue = 0
+#elif __x86_64__
+    NativeMarkerMask  = SWIFT_ABI_X86_64_OBJC_WEAK_REFERENCE_MARKER_MASK,
+    NativeMarkerValue = SWIFT_ABI_X86_64_OBJC_WEAK_REFERENCE_MARKER_VALUE
+#elif __i386__
+    NativeMarkerMask  = SWIFT_ABI_I386_OBJC_WEAK_REFERENCE_MARKER_MASK,
+    NativeMarkerValue = SWIFT_ABI_I386_OBJC_WEAK_REFERENCE_MARKER_VALUE
+#elif __arm__
+    NativeMarkerMask  = SWIFT_ABI_ARM_OBJC_WEAK_REFERENCE_MARKER_MASK,
+    NativeMarkerValue = SWIFT_ABI_ARM_OBJC_WEAK_REFERENCE_MARKER_VALUE
+#elif __arm64__
+    NativeMarkerMask  = SWIFT_ABI_ARM64_OBJC_WEAK_REFERENCE_MARKER_MASK,
+    NativeMarkerValue = SWIFT_ABI_ARM64_OBJC_WEAK_REFERENCE_MARKER_VALUE
 #else
-  // No NativeMarkerValue needed.
-  enum : uintptr_t {
-    NativeMarkerValue = 0, NativeMarkerMask = 0
-  };
+    #error unknown architecture
 #endif
+  };
 
-  static_assert(NativeMarkerMask < alignof(void*),
-                "native marker bit must not interfere with pointer bits");
+  static_assert((NativeMarkerMask & NativeMarkerValue) == NativeMarkerValue,
+                "native marker value must fall within native marker mask");
+  static_assert((NativeMarkerMask & heap_object_abi::SwiftSpareBitsMask)
+                == NativeMarkerMask,
+                "native marker mask must fall within Swift spare bits");
+  static_assert((NativeMarkerMask & heap_object_abi::ObjCReservedBitsMask)
+                == heap_object_abi::ObjCReservedBitsMask,
+                "native marker mask must contain all ObjC tagged pointer bits");
+  static_assert((NativeMarkerValue & heap_object_abi::ObjCReservedBitsMask)
+                == 0,
+                "native marker value must not interfere with ObjC bits");
 
   uintptr_t bits;