Future-proof the ARM64 ABI by not reserving the entire top byte.

Targets that want to use armv8.5a memory tagging will need this.
Hopefully nobody comes up with a brilliant reason they need to use
anything else.

Author: rjmccall

stdlib/public/SwiftShims/System.h

@@ -136,9 +136,10 @@
 /// Darwin reserves the low 4GB of address space.
 #define SWIFT_ABI_DARWIN_ARM64_LEAST_VALID_POINTER 0x100000000ULL
 
-// TBI guarantees the top byte of pointers is unused.
+// TBI guarantees the top byte of pointers is unused, but ARMv8.5-A
+// claims the bottom four bits of that for memory tagging.
 // Heap objects are eight-byte aligned.
-#define SWIFT_ABI_ARM64_SWIFT_SPARE_BITS_MASK 0xFF00000000000007ULL
+#define SWIFT_ABI_ARM64_SWIFT_SPARE_BITS_MASK 0xF000000000000007ULL
 
 // Objective-C reserves just the high bit for tagged pointers.
 #define SWIFT_ABI_ARM64_OBJC_RESERVED_BITS_MASK 0x8000000000000000ULL

test/IRGen/bridge_object_arm64.sil

@@ -35,8 +35,8 @@ entry(%c : $C, %w : $Builtin.Word):
 // CHECK:         [[TAGGED_RESULT:%.*]] = bitcast [[BRIDGE]] %0 to [[C:%objc_object\*]]
 // CHECK:         br label %tagged-cont
 // CHECK:       not-tagged-pointer:
-// --                                                     0x00ff_ffff_ffff_fff8
-// CHECK:         [[MASKED_BITS:%.*]] = and i64 [[BOBITS]], 72057594037927928
+// --                                                     0x0fff_ffff_ffff_fff8
+// CHECK:         [[MASKED_BITS:%.*]] = and i64 [[BOBITS]], 1152921504606846968
 // CHECK:         [[MASKED_RESULT:%.*]] = inttoptr i64 [[MASKED_BITS]] to [[C]]
 // CHECK:         br label %tagged-cont
 // CHECK:      tagged-cont:

test/IRGen/enum_top_bits_reserved.sil

@@ -0,0 +1,58 @@
+// RUN: %swift -module-name test -emit-ir -target arm64-apple-ios10.0 %s | %FileCheck %s -check-prefix=CHECK -check-prefix=CHECK-arm64
+// RUN: %swift -module-name test -emit-ir -target x86_64-apple-macosx10.12 %s | %FileCheck %s -check-prefix=CHECK -check-prefix=CHECK-x86_64
+
+class NativeClass {}
+sil_vtable NativeClass {}
+
+// On 64-bit targets, there are 3 spare bits in the alignment bits and
+// either 4 (arm64) or 8 (everywhere else) spare bits in the high byte.
+// Consume those bits one by one.
+
+enum A {
+  case either(NativeClass)
+  case or(NativeClass)
+}
+
+enum B {
+  case either(A)
+  case or(A)
+}
+
+enum C {
+  case either(B)
+  case or(B)
+}
+
+enum D {
+  case either(C)
+  case or(C)
+}
+
+enum E {
+  case either(D)
+  case or(D)
+}
+
+enum F {
+  case either(E)
+  case or(E)
+}
+
+enum G {
+  case either(F)
+  case or(F)
+}
+
+// Okay, we've claimed 7 spare bits.  On ARM64, the eighth spare bit will
+// require an extra discriminator.
+
+// CHECK-LABEL: @"$s4test1HOWV" = internal constant %swift.enum_vwtable
+// CHECK-x86_64-SAME:     i64 8,
+// CHECK-x86_64-SAME:     i64 8,
+// CHECK-arm64-SAME:      i64 9,
+// CHECK-arm64-SAME:      i64 16,
+
+enum H {
+  case either(G)
+  case or(G)
+}