Take <atomic> for granted in port/atomic_pointer.h.

C++11 requires <atomic>. This lets us remove the header detection
(LEVELDB_ATOMIC_PRESENT) and simplify port/atomic_pointer.h.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=189919098

CMakeLists.txt

@@ -25,9 +25,6 @@ test_big_endian(LEVELDB_IS_BIG_ENDIAN)
 include(CheckIncludeFile)
 check_include_file("unistd.h" HAVE_UNISTD_H)
 
-include(CheckIncludeFileCXX)
-check_include_file_cxx("atomic" LEVELDB_ATOMIC_PRESENT)
-
 include(CheckLibraryExists)
 check_library_exists(crc32c crc32c_value "" HAVE_CRC32C)
 check_library_exists(snappy snappy_compress "" HAVE_SNAPPY)

port/atomic_pointer.h

@@ -19,11 +19,9 @@
 #define PORT_ATOMIC_POINTER_H_
 
 #include <stdint.h>
-#ifdef LEVELDB_ATOMIC_PRESENT
+
 #include <atomic>
-#elif defined(__APPLE__)
-#include <libkern/OSAtomic.h>
-#endif
+
 #ifdef OS_WIN
 #include <windows.h>
 #endif
@@ -55,11 +53,7 @@ namespace port {
 // Mac OS
 #elif defined(__APPLE__)
 inline void MemoryBarrier() {
-#if defined(LEVELDB_ATOMIC_PRESENT)
   std::atomic_thread_fence(std::memory_order_seq_cst);
-#else
-  OSMemoryBarrier();
-#endif  // defined(LEVELDB_ATOMIC_PRESENT)
 }
 #define LEVELDB_HAVE_MEMORY_BARRIER
 
@@ -124,7 +118,7 @@ inline void MemoryBarrier() {
 
 #endif
 
-// AtomicPointer built using platform-specific MemoryBarrier()
+// AtomicPointer built using platform-specific MemoryBarrier().
 #if defined(LEVELDB_HAVE_MEMORY_BARRIER)
 class AtomicPointer {
  private:
@@ -145,8 +139,8 @@ class AtomicPointer {
   }
 };
 
-// AtomicPointer based on <cstdatomic>
-#elif defined(LEVELDB_ATOMIC_PRESENT)
+// AtomicPointer based on C++11 <atomic>.
+#else
 class AtomicPointer {
  private:
   std::atomic<void*> rep_;
@@ -167,70 +161,6 @@ class AtomicPointer {
   }
 };
 
-// Atomic pointer based on sparc memory barriers
-#elif defined(__sparcv9) && defined(__GNUC__)
-class AtomicPointer {
- private:
-  void* rep_;
- public:
-  AtomicPointer() { }
-  explicit AtomicPointer(void* v) : rep_(v) { }
-  inline void* Acquire_Load() const {
-    void* val;
-    __asm__ __volatile__ (
-        "ldx [%[rep_]], %[val] \n\t"
-         "membar #LoadLoad|#LoadStore \n\t"
-        : [val] "=r" (val)
-        : [rep_] "r" (&rep_)
-        : "memory");
-    return val;
-  }
-  inline void Release_Store(void* v) {
-    __asm__ __volatile__ (
-        "membar #LoadStore|#StoreStore \n\t"
-        "stx %[v], [%[rep_]] \n\t"
-        :
-        : [rep_] "r" (&rep_), [v] "r" (v)
-        : "memory");
-  }
-  inline void* NoBarrier_Load() const { return rep_; }
-  inline void NoBarrier_Store(void* v) { rep_ = v; }
-};
-
-// Atomic pointer based on ia64 acq/rel
-#elif defined(__ia64) && defined(__GNUC__)
-class AtomicPointer {
- private:
-  void* rep_;
- public:
-  AtomicPointer() { }
-  explicit AtomicPointer(void* v) : rep_(v) { }
-  inline void* Acquire_Load() const {
-    void* val    ;
-    __asm__ __volatile__ (
-        "ld8.acq %[val] = [%[rep_]] \n\t"
-        : [val] "=r" (val)
-        : [rep_] "r" (&rep_)
-        : "memory"
-        );
-    return val;
-  }
-  inline void Release_Store(void* v) {
-    __asm__ __volatile__ (
-        "st8.rel [%[rep_]] = %[v]  \n\t"
-        :
-        : [rep_] "r" (&rep_), [v] "r" (v)
-        : "memory"
-        );
-  }
-  inline void* NoBarrier_Load() const { return rep_; }
-  inline void NoBarrier_Store(void* v) { rep_ = v; }
-};
-
-// We have neither MemoryBarrier(), nor <atomic>
-#else
-#error Please implement AtomicPointer for this platform.
-
 #endif
 
 #undef LEVELDB_HAVE_MEMORY_BARRIER

port/port_config.h.in

@@ -26,11 +26,4 @@
 #cmakedefine01 LEVELDB_IS_BIG_ENDIAN
 #endif  // !defined(LEVELDB_IS_BIG_ENDIAN)
 
-// Define to 1 if you have the <atomic> header.
-// NOTE: <atomic> was standardized in C++11, which will be required to build
-// LevelDB soon.
-#if !defined(LEVELDB_ATOMIC_PRESENT)
-#cmakedefine01 LEVELDB_ATOMIC_PRESENT
-#endif  // !defined(LEVELDB_ATOMIC_PRESENT)
-
 #endif  // STORAGE_LEVELDB_PORT_PORT_CONFIG_H_