[String] Switch ABI to only use 4 discriminator bits.

In anticipation of potential future HW features, e.g. armv8.5 memory
tagging, only use the high 4 bytes as discriminator bits in
_BridgeObject rather than the top 8 bits. Utilize two perf flags to
cover this instead. This requires shifting around a fair amount of
internal complexity.

Author: milseman

stdlib/public/core/SmallString.swift

@@ -105,13 +105,10 @@ extension _SmallString {
 
   // Give raw, nul-terminated code units. This is only for limited internal
   // usage: it always clears the discriminator and count (in case it's full)
-  @inlinable
+  @inlinable @inline(__always)
   internal var zeroTerminatedRawCodeUnits: RawBitPattern {
-    @inline(__always) get {
-      return (
-        self._storage.0,
-        self._storage.1 & _StringObject.Nibbles.largeAddressMask)
-    }
+    let smallStringCodeUnitMask: UInt64 = 0x00FF_FFFF_FFFF_FFFF
+    return (self._storage.0, self._storage.1 & smallStringCodeUnitMask)
   }
 
   internal func computeIsASCII() -> Bool {
@@ -239,6 +236,7 @@ extension _SmallString {
     _internalInvariant(trailing & countAndDiscriminator == 0)
 
     self.init(raw: (leading, trailing | countAndDiscriminator))
+    _internalInvariant(self.count == count)
   }
 
   // Direct from UTF-8

stdlib/public/core/StringBridge.swift

@@ -281,9 +281,13 @@ extension String {
       }
     }
     if _guts._object.isImmortal {
+      // TODO: We'd rather emit a valid ObjC object statically than create a
+      // shared string class instance.
+      let gutsCountAndFlags = _guts._object._countAndFlags
       return _SharedStringStorage(
         immortal: _guts._object.fastUTF8.baseAddress!,
-        countAndFlags: _guts._object._countAndFlags)
+        countAndFlags: _StringObject.CountAndFlags(
+          sharedCount: _guts.count, isASCII: gutsCountAndFlags.isASCII))
     }
 
     _internalInvariant(_guts._object.hasObjCBridgeableObject,

stdlib/public/core/StringGuts.swift

@@ -61,8 +61,7 @@ extension _StringGuts {
   }
 
   internal init(_ storage: _SharedStringStorage) {
-    // TODO(cleanup): We should probably pass whole perf flags struct around
-    self.init(_StringObject(storage, isASCII: false))
+    self.init(_StringObject(storage))
   }
 
   internal init(
@@ -109,18 +108,15 @@ extension _StringGuts {
     @inline(__always) get { return isFastUTF8 && _object.isASCII }
   }
 
-  @inlinable
-  internal var isNFC: Bool  {
-    @inline(__always) get { return _object.isNFC }
-  }
+  @inline(__always)
+  internal var isNFC: Bool { return _object.isNFC }
 
-  @inlinable
-  internal var isNFCFastUTF8: Bool  {
+  @inline(__always)
+  internal var isNFCFastUTF8: Bool {
     // TODO(String micro-performance): Consider a dedicated bit for this
-    @inline(__always) get { return _object.isNFC && isFastUTF8 }
+    return _object.isNFC && isFastUTF8
   }
 
-  @inlinable
   internal var hasNativeStorage: Bool { return _object.hasNativeStorage }
 
   internal var hasSharedStorage: Bool { return _object.hasSharedStorage }