TableCache.FindTable() to avoid the mem copy of file number

Summary: I'm not sure what's the purpose of encoding file number to a new buffer for looking up the table cache. It seems to be unnecessary to me. With this patch, we point the lookup key to the address of the int64 of the file number.

Test Plan: make all check

Reviewers: dhruba, haobo, igor, kailiu

Reviewed By: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D14811

db/table_cache.cc

@@ -29,6 +29,11 @@ static void UnrefEntry(void* arg1, void* arg2) {
   cache->Release(h);
 }
 
+static Slice GetSliceForFileNumber(uint64_t file_number) {
+  return Slice(reinterpret_cast<const char*>(&file_number),
+               sizeof(file_number));
+}
+
 TableCache::TableCache(const std::string& dbname,
                        const Options* options,
                        const EnvOptions& storage_options,
@@ -50,9 +55,7 @@ Status TableCache::FindTable(const EnvOptions& toptions,
                              Cache::Handle** handle, bool* table_io,
                              const bool no_io) {
   Status s;
-  char buf[sizeof(file_number)];
-  EncodeFixed64(buf, file_number);
-  Slice key(buf, sizeof(buf));
+  Slice key = GetSliceForFileNumber(file_number);
   *handle = cache_->Lookup(key);
   if (*handle == nullptr) {
     if (no_io) { // Dont do IO and return a not-found status
@@ -165,9 +168,7 @@ bool TableCache::PrefixMayMatch(const ReadOptions& options,
 }
 
 void TableCache::Evict(uint64_t file_number) {
-  char buf[sizeof(file_number)];
-  EncodeFixed64(buf, file_number);
-  cache_->Erase(Slice(buf, sizeof(buf)));
+  cache_->Erase(GetSliceForFileNumber(file_number));
 }
 
 }  // namespace rocksdb