[String] Switch scalar-aligned bit to a reserved bit.

stdlib/public/core/StringCharacterView.swift

@@ -64,10 +64,10 @@ extension String: BidirectionalCollection {
     let stride = _characterStride(startingAt: i)
     let nextOffset = i._encodedOffset &+ stride
     let nextStride = _characterStride(
-      startingAt: Index(_encodedOffset: nextOffset)._aligned)
+      startingAt: Index(_encodedOffset: nextOffset)._scalarAligned)
 
     return Index(
-      encodedOffset: nextOffset, characterStride: nextStride)._aligned
+      encodedOffset: nextOffset, characterStride: nextStride)._scalarAligned
   }
 
   /// Returns the position immediately before the given index.
@@ -82,7 +82,8 @@ extension String: BidirectionalCollection {
     let i = _guts.scalarAlign(i)
     let stride = _characterStride(endingAt: i)
     let priorOffset = i._encodedOffset &- stride
-    return Index(encodedOffset: priorOffset, characterStride: stride)._aligned
+    return Index(
+      encodedOffset: priorOffset, characterStride: stride)._scalarAligned
   }
   /// Returns an index that is the specified distance from the given index.
   ///
@@ -200,7 +201,7 @@ extension String: BidirectionalCollection {
 
   @inlinable @inline(__always)
   internal func _characterStride(startingAt i: Index) -> Int {
-    _internalInvariant(i._isAligned)
+    _internalInvariant(i._isScalarAligned)
 
     // Fast check if it's already been measured, otherwise check resiliently
     if let d = i.characterStride { return d }
@@ -212,7 +213,7 @@ extension String: BidirectionalCollection {
 
   @inlinable @inline(__always)
   internal func _characterStride(endingAt i: Index) -> Int {
-    _internalInvariant(i._isAligned)
+    _internalInvariant(i._isScalarAligned)
 
     if i == startIndex { return 0 }
 

stdlib/public/core/StringGuts.swift

@@ -268,11 +268,11 @@ extension _StringGuts {
 
   @inlinable @inline(__always)
   internal var startIndex: String.Index {
-   return Index(_encodedOffset: 0)._aligned
+   return Index(_encodedOffset: 0)._scalarAligned
   }
   @inlinable @inline(__always)
   internal var endIndex: String.Index {
-    return Index(_encodedOffset: self.count)._aligned
+    return Index(_encodedOffset: self.count)._scalarAligned
   }
 }
 

stdlib/public/core/StringIndex.swift

@@ -16,25 +16,36 @@ import SwiftShims
 
 String's Index has the following layout:
 
- ┌──────────┬───────────────────┬─────────╥────────────────┬──────────┐
- │ b63:b16  │      b15:b14      │   b13   ║     b12:b8     │  b6:b0   │
- ├──────────┼───────────────────┼─────────╫────────────────┼──────────┤
- │ position │ transcoded offset │ aligned ║ grapheme cache │ reserved │
- └──────────┴───────────────────┴─────────╨────────────────┴──────────┘
-
-Position, transcoded offset, and aligned are fully exposed in the ABI. Grapheme
-cache and reserved are partially resilient: the fact that there are 13 bits with
-a default value of `0` is ABI, but not the layout, construction, or
+ ┌──────────┬───────────────────╥────────────────┬──────────╥────────────────┐
+ │ b63:b16  │      b15:b14      ║     b13:b8     │  b7:b1   ║       b0       │
+ ├──────────┼───────────────────╫────────────────┼──────────╫────────────────┤
+ │ position │ transcoded offset ║ grapheme cache │ reserved ║ scalar aligned │
+ └──────────┴───────────────────╨────────────────┴──────────╨────────────────┘
+                                └──────── resilient ────────┘
+
+Position, transcoded offset, and scalar aligned are fully exposed in the ABI.
+Grapheme cache and reserved are partially resilient: the fact that there are 13
+bits with a default value of `0` is ABI, but not the layout, construction, or
 interpretation of those bits. All use of grapheme cache should be behind
-non-inlinable function calls.
+non-inlinable function calls. Inlinable code should not set a non-zero value to
+grapheme cache bits: doing so breaks back deployment as they will be interpreted
+as a set cache.
 
 - position aka `encodedOffset`: A 48-bit offset into the string's code units
+
 - transcoded offset: a 2-bit sub-scalar offset, derived from transcoding
-- aligned, whether this index is known to be scalar-aligned (see below)
+
+<resilience barrier>
+
+- grapheme cache: A 6-bit value remembering the distance to the next grapheme
+boundary.
+
+- reserved: 7-bit for future use.
+
 <resilience barrier>
-- grapheme cache: A 5-bit value remembering the distance to the next grapheme
-boundary
-- reserved: 8-bit for future use.
+
+- scalar aligned, whether this index is known to be scalar-aligned (see below)
+
 
 */
 extension String {
@@ -86,7 +97,7 @@ extension String.Index {
 
   @usableFromInline
   internal var characterStride: Int? {
-    let value = (_rawBits & 0x1F00) &>> 8
+    let value = (_rawBits & 0x3F00) &>> 8
     return value > 0 ? Int(truncatingIfNeeded: value) : nil
   }
 
@@ -136,7 +147,7 @@ extension String.Index {
     encodedOffset: Int, transcodedOffset: Int, characterStride: Int
   ) {
     self.init(encodedOffset: encodedOffset, transcodedOffset: transcodedOffset)
-    if _slowPath(characterStride > 0x1F) { return }
+    if _slowPath(characterStride > 0x3F) { return }
     self._rawBits |= UInt64(truncatingIfNeeded: characterStride &<< 8)
     self._invariantCheck()
   }
@@ -152,7 +163,7 @@ extension String.Index {
   @usableFromInline @inline(never) @_effects(releasenone)
   internal func _invariantCheck() {
     _internalInvariant(_encodedOffset >= 0)
-    if self._isAligned {
+    if self._isScalarAligned {
       _internalInvariant(transcodedOffset == 0)
     }
   }
@@ -209,35 +220,35 @@ extension String.Index {
 }
 
 /*
-  Index Alignment
+  Index Scalar Alignment
 
   SE-0180 unifies the Index type of String and all its views and allows
   non-scalar-aligned indices to be used across views. In order to guarantee
   behavior, we often have to check and perform scalar alignment. To speed up
-  these checks, we allocate a bit denoting known-to-be-aligned, so that the
-  alignment check can skip the load. The below shows what views need to check
-  for alignment before they can operate, and whether the indices they produce
-  are aligned.
-
-  ┌───────────────╥────────────────────┬──────────────────────────┐
-  │ View          ║ Requires Alignment │ Produces Aligned Indices │
-  ╞═══════════════╬════════════════════╪══════════════════════════╡
-  │ Native UTF8   ║ no                 │ no                       │
-  ├───────────────╫────────────────────┼──────────────────────────┤
-  │ Native UTF16  ║ yes                │ no                       │
-  ╞═══════════════╬════════════════════╪══════════════════════════╡
-  │ Foreign UTF8  ║ yes                │ no                       │
-  ├───────────────╫────────────────────┼──────────────────────────┤
-  │ Foreign UTF16 ║ no                 │ no                       │
-  ╞═══════════════╬════════════════════╪══════════════════════════╡
-  │ UnicodeScalar ║ yes                │ yes                      │
-  ├───────────────╫────────────────────┼──────────────────────────┤
-  │ Character     ║ yes                │ yes                      │
-  └───────────────╨────────────────────┴──────────────────────────┘
-
-  The "requires alignment" applies to any operation taking a String.Index that's
-  not defined entirely in terms of other operations taking a String.Index. These
-  include:
+  these checks, we allocate a bit denoting known-to-be-scalar-aligned, so that
+  the alignment check can skip the load. The below shows what views need to
+  check for alignment before they can operate, and whether the indices they
+  produce are aligned.
+
+  ┌───────────────╥───────────────────────────┬─────────────────────────┐
+  │ View          ║ Requires Scalar Alignment │ Produces Scalar Aligned │
+  ╞═══════════════╬═══════════════════════════╪═════════════════════════╡
+  │ Native UTF8   ║ no                        │ no                      │
+  ├───────────────╫───────────────────────────┼─────────────────────────┤
+  │ Native UTF16  ║ yes                       │ no                      │
+  ╞═══════════════╬═══════════════════════════╪═════════════════════════╡
+  │ Foreign UTF8  ║ yes                       │ no                      │
+  ├───────────────╫───────────────────────────┼─────────────────────────┤
+  │ Foreign UTF16 ║ no                        │ no                      │
+  ╞═══════════════╬═══════════════════════════╪═════════════════════════╡
+  │ UnicodeScalar ║ yes                       │ yes                     │
+  ├───────────────╫───────────────────────────┼─────────────────────────┤
+  │ Character     ║ yes                       │ yes                     │
+  └───────────────╨───────────────────────────┴─────────────────────────┘
+
+  The "requires scalar alignment" applies to any operation taking a String.Index
+  that's not defined entirely in terms of other operations taking a
+  String.Index. These include:
 
   * index(after:)
   * index(before:)
@@ -249,13 +260,13 @@ extension String.Index {
 extension String.Index {
   @_alwaysEmitIntoClient // Swift 5.1
   @inline(__always)
-  internal var _isAligned: Bool { return 0 != _rawBits & 0x2000 }
+  internal var _isScalarAligned: Bool { return 0 != _rawBits & 0x1 }
 
   @_alwaysEmitIntoClient // Swift 5.1
   @inline(__always)
-  internal var _aligned: String.Index {
+  internal var _scalarAligned: String.Index {
     var idx = self
-    idx._rawBits |= 0x2000
+    idx._rawBits |= 0x1
     idx._invariantCheck()
     return idx
   }

stdlib/public/core/StringUTF16View.swift

@@ -154,7 +154,7 @@ extension String.UTF16View: BidirectionalCollection {
     if len == 4 && idx.transcodedOffset == 0 {
       return idx.nextTranscoded
     }
-    return idx.strippingTranscoding.encoded(offsetBy: len)._aligned
+    return idx.strippingTranscoding.encoded(offsetBy: len)._scalarAligned
   }
 
   @inlinable @inline(__always)
@@ -178,7 +178,7 @@ extension String.UTF16View: BidirectionalCollection {
 
     // Single UTF-16 code unit
     _internalInvariant((1...3) ~= len)
-    return idx.encoded(offsetBy: -len)._aligned
+    return idx.encoded(offsetBy: -len)._scalarAligned
   }
 
   public func index(_ i: Index, offsetBy n: Int) -> Index {
@@ -587,7 +587,7 @@ extension String.UTF16View {
             _internalInvariant(utf16Len == 2)
             return Index(encodedOffset: readIdx, transcodedOffset: 1)
           }
-          return Index(_encodedOffset: readIdx &+ len)._aligned
+          return Index(_encodedOffset: readIdx &+ len)._scalarAligned
         }
 
         readIdx &+= len

stdlib/public/core/StringUnicodeScalarView.swift

@@ -112,7 +112,7 @@ extension String.UnicodeScalarView: BidirectionalCollection {
 
     if _fastPath(_guts.isFastUTF8) {
       let len = _guts.fastUTF8ScalarLength(startingAt: i._encodedOffset)
-      return i.encoded(offsetBy: len)._aligned
+      return i.encoded(offsetBy: len)._scalarAligned
     }
 
     return _foreignIndex(after: i)
@@ -137,7 +137,7 @@ extension String.UnicodeScalarView: BidirectionalCollection {
         return _utf8ScalarLength(utf8, endingAt: i._encodedOffset)
       }
       _internalInvariant(len <= 4, "invalid UTF8")
-      return i.encoded(offsetBy: -len)._aligned
+      return i.encoded(offsetBy: -len)._scalarAligned
     }
 
     return _foreignIndex(before: i)
@@ -419,7 +419,7 @@ extension String.UnicodeScalarView {
     let cu = _guts.foreignErrorCorrectedUTF16CodeUnit(at: i)
     let len = UTF16.isLeadSurrogate(cu) ? 2 : 1
 
-    return i.encoded(offsetBy: len)._aligned
+    return i.encoded(offsetBy: len)._scalarAligned
   }
 
   @usableFromInline @inline(never)
@@ -430,6 +430,6 @@ extension String.UnicodeScalarView {
     let cu = _guts.foreignErrorCorrectedUTF16CodeUnit(at: priorIdx)
     let len = UTF16.isTrailSurrogate(cu) ? 2 : 1
 
-    return i.encoded(offsetBy: -len)._aligned
+    return i.encoded(offsetBy: -len)._scalarAligned
   }
 }

stdlib/public/core/UnicodeHelpers.swift

@@ -163,7 +163,7 @@ extension _StringGuts {
   @inline(__always) // fast-path: fold common fastUTF8 check
   internal func scalarAlign(_ idx: Index) -> Index {
     let result: String.Index
-    if _fastPath(idx._isAligned) {
+    if _fastPath(idx._isScalarAligned) {
       result = idx
     } else {
       // TODO(String performance): isASCII check
@@ -172,28 +172,28 @@ extension _StringGuts {
 
     _internalInvariant(isOnUnicodeScalarBoundary(result),
       "Alignment bit is set for non-aligned index")
-    _internalInvariant(result._isAligned)
+    _internalInvariant(result._isScalarAligned)
     return result
   }
 
   @inline(never) // slow-path
   @_alwaysEmitIntoClient // Swift 5.1
   internal func scalarAlignSlow(_ idx: Index) -> Index {
-    _internalInvariant(!idx._isAligned)
+    _internalInvariant(!idx._isScalarAligned)
 
     if _slowPath(idx.transcodedOffset != 0 || idx._encodedOffset == 0) {
       // Transcoded index offsets are already scalar aligned
-      return String.Index(_encodedOffset: idx._encodedOffset)._aligned
+      return String.Index(_encodedOffset: idx._encodedOffset)._scalarAligned
     }
     if _slowPath(self.isForeign) {
       let foreignIdx = foreignScalarAlign(idx)
-      _internalInvariant(foreignIdx._isAligned)
+      _internalInvariant(foreignIdx._isScalarAligned)
       return foreignIdx
     }
 
     return String.Index(_encodedOffset:
       self.withFastUTF8 { _scalarAlign($0, idx._encodedOffset) }
-    )._aligned
+    )._scalarAligned
   }
 
   @inlinable
@@ -359,17 +359,17 @@ extension _StringGuts {
   @usableFromInline @inline(never) // slow-path
   @_effects(releasenone)
   internal func foreignScalarAlign(_ idx: Index) -> Index {
-    guard idx._encodedOffset != self.count else { return idx._aligned }
+    guard idx._encodedOffset != self.count else { return idx._scalarAligned }
 
     _internalInvariant(idx._encodedOffset < self.count)
 
     let ecCU = foreignErrorCorrectedUTF16CodeUnit(at: idx)
     if _fastPath(!UTF16.isTrailSurrogate(ecCU)) {
-      return idx._aligned
+      return idx._scalarAligned
     }
     _internalInvariant(idx._encodedOffset > 0,
       "Error-correction shouldn't give trailing surrogate at position zero")
-    return String.Index(_encodedOffset: idx._encodedOffset &- 1)._aligned
+    return String.Index(_encodedOffset: idx._encodedOffset &- 1)._scalarAligned
   }
 
   @usableFromInline @inline(never)

stdlib/public/core/UnicodeScalar.swift

@@ -428,7 +428,6 @@ extension Unicode.Scalar.UTF16View : RandomAccessCollection {
   ///   `endIndex` property.
   @inlinable
   public subscript(position: Int) -> UTF16.CodeUnit {
-    _internalInvariant((0..<self.count).contains(position))
     if position == 1 { return UTF16.trailSurrogate(value) }
     if endIndex == 1 { return UTF16.CodeUnit(value.value) }
     return UTF16.leadSurrogate(value)