Not generating "__attribute__((__unused__))" for padding fields if it is not CLANG

Summary: Adding "__attribute__((__unused__))" after padding fields will pass CLANG build but will fail gcc 4.8.1. Fix it by not generating it under GCC 4.8.1.

Test Plan: Build under four combinations of USE_CLANG=0,1 and ROCKSDB_FBCODE_BUILD_WITH_481=0.1.

Reviewers: yhchiang, rven, ngbronson, anthony, IslamAbdelRahman

Reviewed By: anthony

Subscribers: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D52371

util/concurrent_arena.h

@@ -17,6 +17,14 @@
 #include "util/mutexlock.h"
 #include "util/thread_local.h"
 
+// Only generate field unused warning for padding array, or build under
+// GCC 4.8.1 will fail.
+#ifdef __clang__
+#define ROCKSDB_FIELD_UNUSED __attribute__((__unused__))
+#else
+#define ROCKSDB_FIELD_UNUSED
+#endif  // __clang__
+
 namespace rocksdb {
 
 class Logger;
@@ -78,7 +86,7 @@ class ConcurrentArena : public Allocator {
 
  private:
   struct Shard {
-    char padding[40] __attribute__((__unused__));
+    char padding[40] ROCKSDB_FIELD_UNUSED;
     mutable SpinMutex mutex;
     char* free_begin_;
     std::atomic<size_t> allocated_and_unused_;
@@ -92,7 +100,7 @@ class ConcurrentArena : public Allocator {
   enum ZeroFirstEnum : uint32_t { tls_cpuid = 0 };
 #endif
 
-  char padding0[56] __attribute__((__unused__));
+  char padding0[56] ROCKSDB_FIELD_UNUSED;
 
   size_t shard_block_size_;
 
@@ -106,7 +114,7 @@ class ConcurrentArena : public Allocator {
   std::atomic<size_t> memory_allocated_bytes_;
   std::atomic<size_t> irregular_block_num_;
 
-  char padding1[56] __attribute__((__unused__));
+  char padding1[56] ROCKSDB_FIELD_UNUSED;
 
   Shard* Repick();