Merge pull request #24616 from mikeash/hardcode-swift-isamask

[Runtime] Use a #define for the value of swift_isaMask.

Author: mikeash

stdlib/public/runtime/Private.h

@@ -22,6 +22,10 @@
 #include "swift/Runtime/Metadata.h"
 #include "llvm/Support/Compiler.h"
 
+#if defined(__APPLE__) && defined(__MACH__)
+#include <TargetConditionals.h>
+#endif
+
 // Opaque ISAs need to use object_getClass which is in runtime.h
 #if SWIFT_HAS_OPAQUE_ISAS
 #include <objc/runtime.h>
@@ -84,6 +88,24 @@ class TypeInfo {
 #if SWIFT_HAS_ISA_MASKING
   SWIFT_RUNTIME_EXPORT
   uintptr_t swift_isaMask;
+
+// Hardcode the mask. We have our own copy of the value, as it's hard to work
+// out the proper includes from libobjc. The values MUST match the ones from
+// libobjc. Debug builds check these values against objc_debug_isa_class_mask
+// from libobjc.
+#  if TARGET_OS_SIMULATOR
+// Simulators don't currently use isa masking, but we still want to emit
+// swift_isaMask and the corresponding code in case that changes. libobjc's
+// mask has the bottom bits clear to include pointer alignment, match that
+// value here.
+#    define SWIFT_ISA_MASK 0xfffffffffffffff8ULL
+#  elif __arm64__
+#    define SWIFT_ISA_MASK 0x0000000ffffffff8ULL
+#  elif __x86_64__
+#    define SWIFT_ISA_MASK 0x00007ffffffffff8ULL
+#  else
+#    error Unknown architecture for masked isa.
+#  endif
 #endif
 
 #if SWIFT_OBJC_INTEROP
@@ -134,7 +156,7 @@ class TypeInfo {
 
 #if SWIFT_HAS_ISA_MASKING
     // Apply the mask.
-    bits &= swift_isaMask;
+    bits &= SWIFT_ISA_MASK;
 #endif
 
     // The result is a class pointer.

stdlib/public/runtime/SwiftObject.mm

@@ -58,17 +58,7 @@
 OBJC_EXPORT __attribute__((__weak_import__))
 const uintptr_t objc_debug_isa_class_mask;
 
-static uintptr_t computeISAMask() {
-  // The versions of the Objective-C runtime which use non-pointer
-  // ISAs also export this symbol.
-  if (auto runtimeSymbol = &objc_debug_isa_class_mask)
-    return *runtimeSymbol;
-  return ~uintptr_t(0);
-}
-
-SWIFT_ALLOWED_RUNTIME_GLOBAL_CTOR_BEGIN
-uintptr_t swift::swift_isaMask = computeISAMask();
-SWIFT_ALLOWED_RUNTIME_GLOBAL_CTOR_END
+uintptr_t swift::swift_isaMask = SWIFT_ISA_MASK;
 #endif
 
 const ClassMetadata *swift::_swift_getClass(const void *object) {
@@ -204,7 +194,12 @@ Class _swift_classOfObjCHeapObject(OpaqueValue *value) {
 
 
 @implementation SwiftObject
-+ (void)initialize {}
++ (void)initialize {
+#if SWIFT_HAS_ISA_MASKING
+  assert(&objc_debug_isa_class_mask);
+  assert(objc_debug_isa_class_mask == SWIFT_ISA_MASK);
+#endif
+}
 
 + (instancetype)allocWithZone:(struct _NSZone *)zone {
   assert(zone == nullptr);