[GitHub] [doris] mrhhsg commented on a diff in pull request #10386: [improvement]Use std::iota to set values of _block_rowids in SegmentIterator::_read_columns_by_index

[GitBox]

mrhhsg commented on code in PR #10386:
URL: https://github.com/apache/doris/pull/10386#discussion_r905877588


##########
be/src/olap/rowset/segment_v2/segment_iterator.cpp:
##########
@@ -872,9 +872,11 @@ Status SegmentIterator::_read_columns_by_index(uint32_t 
nrows_read_limit, uint32
                 _read_columns(_first_read_column_ids, _current_return_columns, 
rows_to_read));
         _cur_rowid += rows_to_read;
         if (set_block_rowid) {
-            for (uint32_t rid = range_from; rid < range_to; rid++) {
-                _block_rowids[nrows_read++] = rid;
-            }
+            // Here use std::iota is better performance than for-loop, maybe 
for-loop is not vectorized

Review Comment:
   I tested with the code below(compile command: ` g++ test.cc -O3 
-fopt-info-vec-all`:
   1. `nrows_read` as reference:
   ```c++
   #include <string>
   #include <vector>
   
   std::vector<uint32_t> _block_rowids;
   
   void func(uint32_t range_from, uint32_t range_to, uint32_t& nrows_read) {
       uint32_t* data = _block_rowids.data();
       for (uint32_t rid = range_from; rid < range_to; rid++) {
           data[nrows_read++] = rid;
       }
   }
   
   int main(int argc, char* argv[]) {
       uint32_t nrows_read = 0;
       _block_rowids.resize(4096);
       func(100, 2000, nrows_read);
       return 0;
   }
   ```
   got:
   <img width="1223" alt="image" 
src="https://user-images.githubusercontent.com/1179834/175503549-988a66e8-1ebc-42bc-a4bc-2e241ad165ee.png";>
   
   2. `nrows_read` as value and signed.
   ```c++
   #include <string>
   #include <vector>
   
   std::vector<uint32_t> _block_rowids;
   
   void func(uint32_t range_from, uint32_t range_to, int32_t nrows_read) {
       uint32_t* data = _block_rowids.data();
       for (uint32_t rid = range_from; rid < range_to; rid++) {
           data[nrows_read++] = rid;
       }
   }
   
   int main(int argc, char* argv[]) {
       uint32_t nrows_read = 0;
       _block_rowids.resize(4096);
       func(100, 2000, nrows_read);
       return 0;
   }
   ```
   <img width="1220" alt="image" 
src="https://user-images.githubusercontent.com/1179834/175503955-171ece10-0d2c-4d66-b7c6-5ab0e0fb012d.png";>
   



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