adonis0147 commented on code in PR #10386:
URL: https://github.com/apache/doris/pull/10386#discussion_r906043333
##########
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:
> There are two reasons why this loop can not be vectorized:
>
> 1. `nrows_read` is reference
> 2. `nrows_read` is unsigned
These reasons are not the root causes.
```cpp
#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;
}
```
In above snippet, if you change the type of `nrows_read` from `uint32_t` to
`uint64_t`, you will find compiler vectorizes the loop.
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