[5.1] Avoid the overhead of looking up the current CFAllocator in String bridging

stdlib/public/SwiftShims/CoreFoundationShims.h

@@ -80,7 +80,7 @@ _swift_shims_CFIndex _swift_stdlib_CFStringGetLength(
 SWIFT_RUNTIME_STDLIB_API
 __attribute__((ns_returns_retained))
 _swift_shims_CFStringRef _Nonnull _swift_stdlib_CFStringCreateWithSubstring(
-    _swift_shims_CFAllocatorRef _Nullable alloc,
+    const void * _Nullable unused,
     _swift_shims_CFStringRef _Nonnull str, _swift_shims_CFRange range);
 
 SWIFT_RUNTIME_STDLIB_API
@@ -90,13 +90,13 @@ _swift_shims_UniChar _swift_stdlib_CFStringGetCharacterAtIndex(
 SWIFT_RUNTIME_STDLIB_API
 __attribute__((ns_returns_retained))
 _swift_shims_CFStringRef _Nonnull _swift_stdlib_CFStringCreateCopy(
-    _swift_shims_CFAllocatorRef _Nullable alloc,
+    const void * _Nullable unused,
     _swift_shims_CFStringRef _Nonnull theString);
 
 SWIFT_RUNTIME_STDLIB_API
 __attribute__((ns_returns_retained))
 _swift_shims_CFStringRef _Nonnull _swift_stdlib_CFStringCreateWithBytes(
-    _swift_shims_CFAllocatorRef _Nullable alloc,
+    const void * _Nullable unused,
     const __swift_uint8_t *_Nonnull bytes, _swift_shims_CFIndex numBytes,
     _swift_shims_CFStringEncoding encoding,
     _swift_shims_Boolean isExternalRepresentation);

stdlib/public/core/StringBridge.swift

@@ -266,18 +266,32 @@ extension String {
   }
 }
 
+@_effects(releasenone)
+private func _createCFString(
+  _ ptr: UnsafePointer<UInt8>,
+  _ count: Int,
+  _ encoding: UInt32
+) -> AnyObject {
+  return _swift_stdlib_CFStringCreateWithBytes(
+    nil, //ignored in the shim for perf reasons
+    ptr,
+    count,
+    kCFStringEncodingUTF8,
+    0
+  ) as AnyObject
+}
+
 extension String {
   @_effects(releasenone)
   public // SPI(Foundation)
   func _bridgeToObjectiveCImpl() -> AnyObject {
     if _guts.isSmall {
       return _guts.asSmall.withUTF8 { bufPtr in
-        // TODO(String bridging): worth isASCII check for different encoding?
-        return _swift_stdlib_CFStringCreateWithBytes(
-            nil, bufPtr.baseAddress._unsafelyUnwrappedUnchecked,
+        return _createCFString(
+            bufPtr.baseAddress._unsafelyUnwrappedUnchecked,
             bufPtr.count,
-            kCFStringEncodingUTF8, 0)
-        as AnyObject
+            kCFStringEncodingUTF8
+        )
       }
     }
     if _guts._object.isImmortal {

stdlib/public/stubs/FoundationHelpers.mm

@@ -77,10 +77,13 @@ static CFRange cast(_swift_shims_CFRange value) {
 
 _swift_shims_CFStringRef
 swift::_swift_stdlib_CFStringCreateWithSubstring(
-                                         _swift_shims_CFAllocatorRef alloc,
+                                         const void *unused,
                                          _swift_shims_CFStringRef str,
                                          _swift_shims_CFRange range) {
-  return cast(CFStringCreateWithSubstring(cast(alloc), cast(str), cast(range)));
+  assert(unused == NULL);
+  return cast(CFStringCreateWithSubstring(kCFAllocatorSystemDefault,
+                                          cast(str),
+                                          cast(range)));
 }
 
 _swift_shims_CFComparisonResult
@@ -108,17 +111,19 @@ static CFRange cast(_swift_shims_CFRange value) {
 }
 
 _swift_shims_CFStringRef
-swift::_swift_stdlib_CFStringCreateCopy(_swift_shims_CFAllocatorRef alloc,
+swift::_swift_stdlib_CFStringCreateCopy(const void *unused,
                                         _swift_shims_CFStringRef theString) {
-  return cast(CFStringCreateCopy(cast(alloc), cast(theString)));
+  assert(unused == NULL);
+  return cast(CFStringCreateCopy(kCFAllocatorSystemDefault, cast(theString)));
 }
 
 _swift_shims_CFStringRef
 swift::_swift_stdlib_CFStringCreateWithBytes(
-    _swift_shims_CFAllocatorRef _Nullable alloc, const uint8_t *bytes,
+    const void *unused, const uint8_t *bytes,
     _swift_shims_CFIndex numBytes, _swift_shims_CFStringEncoding encoding,
     _swift_shims_Boolean isExternalRepresentation) {
-  return cast(CFStringCreateWithBytes(cast(alloc), bytes, numBytes,
+  assert(unused == NULL);
+  return cast(CFStringCreateWithBytes(kCFAllocatorSystemDefault, bytes, numBytes,
                                       cast(encoding),
                                       isExternalRepresentation));
 }