class Block to store num_restarts_

Summary:
Right now, every Block::NewIterator() reads num_restarts_ from the block, which is already read in Block::Block(). This sometimes cause a CPU cache miss. Although fetching this cacheline can usually benefit follow-up block restart offset reading, as they are close to each other, it's almost free to get ride of this read by storing it in the Block class.
Closes https://github.com/facebook/rocksdb/pull/3869

Differential Revision: D8052493

Pulled By: siying

fbshipit-source-id: 9c72360f0c2d7329f3c198ce4eaedd2bc14b87c1

table/block.cc

@@ -402,12 +402,14 @@ Block::Block(BlockContents&& contents, SequenceNumber _global_seqno,
       data_(contents_.data.data()),
       size_(contents_.data.size()),
       restart_offset_(0),
+      num_restarts_(0),
       global_seqno_(_global_seqno) {
   if (size_ < sizeof(uint32_t)) {
     size_ = 0;  // Error marker
   } else {
+    num_restarts_ = NumRestarts();
     restart_offset_ =
-        static_cast<uint32_t>(size_) - (1 + NumRestarts()) * sizeof(uint32_t);
+        static_cast<uint32_t>(size_) - (1 + num_restarts_) * sizeof(uint32_t);
     if (restart_offset_ > size_ - sizeof(uint32_t)) {
       // The size is too small for NumRestarts() and therefore
       // restart_offset_ wrapped around.
@@ -432,15 +434,14 @@ BlockIter* Block::NewIterator(const Comparator* cmp, BlockIter* iter,
     ret_iter->Invalidate(Status::Corruption("bad block contents"));
     return ret_iter;
   }
-  const uint32_t num_restarts = NumRestarts();
+  if (num_restarts_ == 0) {
-  if (num_restarts == 0) {
     // Empty block.
     ret_iter->Invalidate(Status::OK());
     return ret_iter;
   } else {
     BlockPrefixIndex* prefix_index_ptr =
         total_order_seek ? nullptr : prefix_index_.get();
-    ret_iter->Initialize(cmp, data_, restart_offset_, num_restarts,
+    ret_iter->Initialize(cmp, data_, restart_offset_, num_restarts_,
                          prefix_index_ptr, global_seqno_,
                          read_amp_bitmap_.get());
 

table/block.h

@@ -184,6 +184,7 @@ class Block {
   const char* data_;            // contents_.data.data()
   size_t size_;                 // contents_.data.size()
   uint32_t restart_offset_;     // Offset in data_ of restart array
+  uint32_t num_restarts_;
   std::unique_ptr<BlockPrefixIndex> prefix_index_;
   std::unique_ptr<BlockReadAmpBitmap> read_amp_bitmap_;
   // All keys in the block will have seqno = global_seqno_, regardless of