[String] Refactor 32-bit StringObject.

Remove Discriminator, Flags, etc., abstractions from
StringObject. These cause code divergence between 32-bit and 64-bit
ABI, complicate ABI changes, and otherwise contribute to bloat.

Author: milseman

stdlib/public/core/SmallString.swift

@@ -72,18 +72,9 @@ extension _SmallString {
     }
   }
 
-  @inlinable
-  internal var discriminator: _StringObject.Discriminator {
-    @inline(__always) get {
-      let value = _storage.1 &>> _StringObject.Nibbles.discriminatorShift
-      return _StringObject.Discriminator(UInt8(truncatingIfNeeded: value))
-    }
-    @inline(__always) set {
-      _storage.1 &= _StringObject.Nibbles.largeAddressMask
-      _storage.1 |= (
-        UInt64(truncatingIfNeeded: newValue._value)
-          &<< _StringObject.Nibbles.discriminatorShift)
-    }
+  @inlinable @inline(__always)
+  internal var rawDiscriminatedObject: UInt64 {
+    return _storage.1
   }
 
   @inlinable
@@ -96,7 +87,7 @@ extension _SmallString {
   @inlinable
   internal var count: Int {
     @inline(__always) get {
-      return discriminator.smallCount
+      return _StringObject.getSmallCount(fromRaw: rawDiscriminatedObject)
     }
   }
 
@@ -108,7 +99,7 @@ extension _SmallString {
   @inlinable
   internal var isASCII: Bool {
     @inline(__always) get {
-      return discriminator.smallIsASCII
+      return _StringObject.getSmallIsASCII(fromRaw: rawDiscriminatedObject)
     }
   }
 
@@ -123,12 +114,10 @@ extension _SmallString {
     }
   }
 
-  @inlinable
   internal func computeIsASCII() -> Bool {
-    // TODO(String micro-performance): Evaluate other expressions, e.g. | first
     let asciiMask: UInt64 = 0x8080_8080_8080_8080
     let raw = zeroTerminatedRawCodeUnits
-    return (raw.0 & asciiMask == 0) && (raw.1 & asciiMask == 0)
+    return (raw.0 | raw.1) & asciiMask == 0
   }
 }
 
@@ -220,7 +209,7 @@ extension _SmallString {
 
   // Overwrite stored code units, including uninitialized. `f` should return the
   // new count.
-  @inlinable @inline(__always)
+  @inline(__always)
   internal mutating func withMutableCapacity(
     _ f: (UnsafeMutableBufferPointer<UInt8>) throws -> Int
   ) rethrows {
@@ -231,14 +220,27 @@ extension _SmallString {
       return try f(UnsafeMutableBufferPointer(
         start: ptr, count: _SmallString.capacity))
     }
-
     _internalInvariant(len <= _SmallString.capacity)
-    discriminator = .small(withCount: len, isASCII: self.computeIsASCII())
+
+    let (leading, trailing) = self.zeroTerminatedRawCodeUnits
+    self = _SmallString(leading: leading, trailing: trailing, count: len)
   }
 }
 
 // Creation
 extension _SmallString {
+  @inlinable @inline(__always)
+  internal init(leading: UInt64, trailing: UInt64, count: Int) {
+    _internalInvariant(count <= _SmallString.capacity)
+
+    let isASCII = (leading | trailing) & 0x8080_8080_8080_8080 == 0
+    let countAndDiscriminator = UInt64(truncatingIfNeeded: count) &<< 56
+                              | _StringObject.Nibbles.small(isASCII: isASCII)
+    _internalInvariant(trailing & countAndDiscriminator == 0)
+
+    self.init(raw: (leading, trailing | countAndDiscriminator))
+  }
+
   // Direct from UTF-8
   @inlinable @inline(__always)
   internal init?(_ input: UnsafeBufferPointer<UInt8>) {
@@ -251,11 +253,7 @@ extension _SmallString {
     let leading = _bytesToUInt64(ptr, Swift.min(input.count, 8))
     let trailing = count > 8 ? _bytesToUInt64(ptr + 8, count &- 8) : 0
 
-    let isASCII = (leading | trailing) & 0x8080_8080_8080_8080 == 0
-    let discriminator = _StringObject.Discriminator.small(
-      withCount: count,
-      isASCII: isASCII)
-    self.init(raw: (leading, trailing | discriminator.rawBits))
+    self.init(leading: leading, trailing: trailing, count: count)
   }
 
   @usableFromInline // @testable
@@ -273,13 +271,8 @@ extension _SmallString {
     }
     _internalInvariant(writeIdx == totalCount)
 
-    let isASCII = base.isASCII && other.isASCII
-    let discriminator = _StringObject.Discriminator.small(
-      withCount: totalCount,
-      isASCII: isASCII)
-
     let (leading, trailing) = result.zeroTerminatedRawCodeUnits
-    self.init(raw: (leading, trailing | discriminator.rawBits))
+    self.init(leading: leading, trailing: trailing, count: totalCount)
   }
 }