Merge pull request swiftlang#30244 from milseman/string_shrink

[string] Shrink storage class sizes

Author: milseman

stdlib/public/core/LegacyABI.swift

@@ -79,3 +79,11 @@ extension String {
     range: Range<String.Index>
   ) { fatalError() }
 }
+
+extension String.UTF16View {
+  // Swift 5.x: This was accidentally shipped as inlinable, but was never used
+  // from an inlinable context. The definition is kept around for techincal ABI
+  // compatibility (even though it shouldn't matter), but is unused.
+  @inlinable @inline(__always)
+  internal var _shortHeuristic: Int { return 32 }
+}

stdlib/public/core/StringBreadcrumbs.swift

@@ -13,16 +13,16 @@
 
 // @opaque
 internal final class _StringBreadcrumbs {
-  static var breadcrumbStride: Int { return 32 }
+  internal static var breadcrumbStride: Int { 64 }
 
-  var utf16Length: Int
+  internal var utf16Length: Int
 
   // TODO: does this need to be a pair?.... Can we be smaller than Int?
-  var crumbs: [String.Index]
+  internal var crumbs: [String.Index]
 
   // TODO: Does this need to be inout, unique, or how will we be enforcing
   // atomicity?
-  init(_ str: String) {
+  internal init(_ str: String) {
     let stride = _StringBreadcrumbs.breadcrumbStride
 
     self.crumbs = []
@@ -62,7 +62,7 @@ internal final class _StringBreadcrumbs {
 }
 
 extension _StringBreadcrumbs {
-  var stride: Int {
+  internal var stride: Int {
     @inline(__always) get { return _StringBreadcrumbs.breadcrumbStride }
   }
 
@@ -108,37 +108,3 @@ extension _StringBreadcrumbs {
   #endif // INTERNAL_CHECKS_ENABLED
 }
 
-extension _StringGuts {
-  @_effects(releasenone)
-  internal func getBreadcrumbsPtr() -> UnsafePointer<_StringBreadcrumbs> {
-    _internalInvariant(hasBreadcrumbs)
-
-    let mutPtr: UnsafeMutablePointer<_StringBreadcrumbs?>
-    if hasNativeStorage {
-      mutPtr = _object.nativeStorage._breadcrumbsAddress
-    } else {
-      mutPtr = UnsafeMutablePointer(
-        Builtin.addressof(&_object.sharedStorage._breadcrumbs))
-    }
-
-    if _slowPath(mutPtr.pointee == nil) {
-      populateBreadcrumbs(mutPtr)
-    }
-
-    _internalInvariant(mutPtr.pointee != nil)
-    // assuming optional class reference and class reference can alias
-    return UnsafeRawPointer(mutPtr).assumingMemoryBound(to: _StringBreadcrumbs.self)
-  }
-
-  @inline(never) // slow-path
-  @_effects(releasenone)
-  internal func populateBreadcrumbs(
-    _ mutPtr: UnsafeMutablePointer<_StringBreadcrumbs?>
-  ) {
-    // Thread-safe compare-and-swap
-    let crumbs = _StringBreadcrumbs(String(self))
-    _stdlib_atomicInitializeARCRef(
-      object: UnsafeMutableRawPointer(mutPtr).assumingMemoryBound(to: Optional<AnyObject>.self), 
-      desired: crumbs)
-  }
-}

stdlib/public/core/StringBridge.swift

@@ -462,6 +462,7 @@ extension String {
         // and bridge that instead. Also avoids CF deleting any BOM that may be
         // present
         var copy = self
+        // TODO: small capacity minimum is lifted, just need to make native
         copy._guts.grow(_SmallString.capacity + 1)
         _internalInvariant(!copy._guts.isSmall)
         return copy._bridgeToObjectiveCImpl()

stdlib/public/core/StringCreate.swift

@@ -109,7 +109,7 @@ extension String {
     ) throws -> Int
   ) rethrows -> String {
     let result = try __StringStorage.create(
-      uninitializedCapacity: capacity,
+      uninitializedCodeUnitCapacity: capacity,
       initializingUncheckedUTF8With: initializer)
 
     switch validateUTF8(result.codeUnits) {

stdlib/public/core/StringGuts.swift

@@ -120,8 +120,10 @@ extension _StringGuts {
 
   internal var hasSharedStorage: Bool { return _object.hasSharedStorage }
 
+  // Whether this string has breadcrumbs
   internal var hasBreadcrumbs: Bool {
-    return hasNativeStorage || hasSharedStorage
+    return hasSharedStorage
+      || (hasNativeStorage && _object.nativeStorage.hasBreadcrumbs)
   }
 }
 

stdlib/public/core/StringGutsRangeReplaceable.swift

@@ -58,7 +58,7 @@ extension _StringGuts {
   internal init(_initialCapacity capacity: Int) {
     self.init()
     if _slowPath(capacity > _SmallString.capacity) {
-      self.grow(capacity)
+      self.grow(capacity) // TODO: no factor should be applied
     }
   }
 
@@ -68,7 +68,7 @@ extension _StringGuts {
     if let currentCap = self.uniqueNativeCapacity, currentCap >= n { return }
 
     // Grow
-    self.grow(n)
+    self.grow(n) // TODO: no factor should be applied
   }
 
   // Grow to accomodate at least `n` code units
@@ -79,13 +79,16 @@ extension _StringGuts {
     _internalInvariant(
       self.uniqueNativeCapacity == nil || self.uniqueNativeCapacity! < n)
 
+    // TODO: Dont' do this! Growth should only happen for append...
     let growthTarget = Swift.max(n, (self.uniqueNativeCapacity ?? 0) * 2)
 
     if _fastPath(isFastUTF8) {
       let isASCII = self.isASCII
       let storage = self.withFastUTF8 {
         __StringStorage.create(
-          initializingFrom: $0, capacity: growthTarget, isASCII: isASCII)
+          initializingFrom: $0,
+          codeUnitCapacity: growthTarget,
+          isASCII: isASCII)
       }
 
       self = _StringGuts(storage)
@@ -128,11 +131,11 @@ extension _StringGuts {
     } else {
       sufficientCapacity = false
     }
-        
+
     if self.isUniqueNative && sufficientCapacity {
       return
     }
-    
+
     // If we have to resize anyway, and we fit in smol, we should have made one
     _internalInvariant(totalCount > _SmallString.capacity)
 
@@ -145,7 +148,7 @@ extension _StringGuts {
       growthTarget = Swift.max(
         totalCount, _growArrayCapacity(nativeCapacity ?? 0))
     }
-    self.grow(growthTarget)
+    self.grow(growthTarget) // NOTE: this already has exponential growth...
   }
 
   internal mutating func append(_ other: _StringGuts) {
@@ -157,7 +160,7 @@ extension _StringGuts {
     }
     append(_StringGutsSlice(other))
   }
-  
+
   @inline(never)
   @_effects(readonly)
   private func _foreignConvertedToSmall() -> _SmallString {
@@ -170,7 +173,7 @@ extension _StringGuts {
     _internalInvariant(smol._guts.isSmall)
     return smol._guts.asSmall
   }
-  
+
   private func _convertedToSmall() -> _SmallString {
     _internalInvariant(utf8Count <= _SmallString.capacity)
     if _fastPath(isSmall) {
@@ -186,25 +189,25 @@ extension _StringGuts {
     defer { self._invariantCheck() }
 
     let otherCount = slicedOther.utf8Count
-    
+
     let totalCount = utf8Count + otherCount
-    
+
     /*
      Goal: determine if we need to allocate new native capacity
      Possible scenarios in which we need to allocate:
      • Not uniquely owned and native: we can't use the capacity to grow into,
         have to become unique + native by allocating
      • Not enough capacity: have to allocate to grow
-     
+
      Special case: a non-smol String that can fit in a smol String but doesn't
         meet the above criteria shouldn't throw away its buffer just to be smol.
         The reasoning here is that it may be bridged or have reserveCapacity'd
         in preparation for appending more later, in which case we would end up
         have to allocate anyway to convert back from smol.
-     
+
         If we would have to re-allocate anyway then that's not a problem and we
         should just be smol.
-     
+
         e.g. consider
         var str = "" // smol
         str.reserveCapacity(100) // large native unique
@@ -215,15 +218,15 @@ extension _StringGuts {
       nativeUnusedCapacity! >= otherCount
     let shouldBeSmol = totalCount <= _SmallString.capacity &&
       (isSmall || !hasEnoughUsableSpace)
-    
+
     if shouldBeSmol {
       let smolSelf = _convertedToSmall()
       let smolOther = String(Substring(slicedOther))._guts._convertedToSmall()
       // TODO: In-register slicing
       self = _StringGuts(_SmallString(smolSelf, appending: smolOther)!)
       return
     }
-    
+
     prepareForAppendInPlace(totalCount: totalCount, otherUTF8Count: otherCount)
 
     if slicedOther.isFastUTF8 {