Migrate away from deprecated OSAtomic APIs. (protocolbuffers#4184)

* Migrate away from deprecated OSAtomic APIs.

Author: dierksen

objectivec/GPBCodedInputStream_PackagePrivate.h

@@ -34,8 +34,6 @@
 
 #import "GPBCodedInputStream.h"
 
-#import <libkern/OSAtomic.h>
-
 @class GPBUnknownFieldSet;
 @class GPBFieldDescriptor;
 

objectivec/GPBMessage.m

@@ -32,6 +32,7 @@
 
 #import <objc/runtime.h>
 #import <objc/message.h>
+#import <stdatomic.h>
 
 #import "GPBArray_PackagePrivate.h"
 #import "GPBCodedInputStream_PackagePrivate.h"
@@ -742,14 +743,14 @@ void GPBClearMessageAutocreator(GPBMessage *self) {
 void GPBPrepareReadOnlySemaphore(GPBMessage *self) {
 #pragma clang diagnostic push
 #pragma clang diagnostic ignored "-Wdirect-ivar-access"
-#pragma clang diagnostic ignored "-Wdeprecated-declarations"
 
   // Create the semaphore on demand (rather than init) as developers might not cause them
   // to be needed, and the heap usage can add up.  The atomic swap is used to avoid needing
   // another lock around creating it.
   if (self->readOnlySemaphore_ == nil) {
     dispatch_semaphore_t worker = dispatch_semaphore_create(1);
-    if (!OSAtomicCompareAndSwapPtrBarrier(NULL, worker, (void * volatile *)&(self->readOnlySemaphore_))) {
+    dispatch_semaphore_t expected = nil;
+    if (!atomic_compare_exchange_strong(&self->readOnlySemaphore_, &expected, worker)) {
       dispatch_release(worker);
     }
   }

objectivec/GPBMessage_PackagePrivate.h

@@ -34,7 +34,7 @@
 
 #import "GPBMessage.h"
 
-#import <libkern/OSAtomic.h>
+#import <stdatomic.h>
 
 #import "GPBBootstrap.h"
 
@@ -70,7 +70,7 @@ typedef struct GPBMessage_Storage *GPBMessage_StoragePtr;
   // Use of readOnlySemaphore_ must be prefaced by a call to
   // GPBPrepareReadOnlySemaphore to ensure it has been created. This allows
   // readOnlySemaphore_ to be only created when actually needed.
-  dispatch_semaphore_t readOnlySemaphore_;
+  _Atomic(dispatch_semaphore_t) readOnlySemaphore_;
 }
 
 // Gets an extension value without autocreating the result if not found. (i.e.

objectivec/google/protobuf/Struct.pbobjc.m

@@ -149,7 +149,7 @@ void EnumGenerator::GenerateSource(io::Printer* printer) {
 
   printer->Print(
       "GPBEnumDescriptor *$name$_EnumDescriptor(void) {\n"
-      "  static GPBEnumDescriptor *descriptor = NULL;\n"
+      "  static _Atomic(GPBEnumDescriptor*) descriptor = NULL;\n"
       "  if (!descriptor) {\n",
       "name", name_);
 
@@ -192,7 +192,8 @@ void EnumGenerator::GenerateSource(io::Printer* printer) {
         "extraTextFormatInfo", CEscape(text_format_decode_data.Data()));
     }
     printer->Print(
-      "    if (!OSAtomicCompareAndSwapPtrBarrier(nil, worker, (void * volatile *)&descriptor)) {\n"
+      "    GPBEnumDescriptor *expected = nil;\n"
+      "    if (!atomic_compare_exchange_strong(&descriptor, &expected, worker)) {\n"
       "      [worker release];\n"
       "    }\n"
       "  }\n"