perf: add repeat_slice_n_times to MutableBuffer
#8658
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this will be used in: - apache#8653
This was referenced Oct 20, 2025
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alamb
approved these changes
Oct 20, 2025
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| /// Adding to this mutable buffer `slice_to_repeat` repeated `repeat_count` times. |
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I am wondering how much the unsafe log copying here makes a difference, vs ensuring reserve is called correctly.
Did you measure with code that was like:
reserve(slice.len() * repeat_count);
for _ in 0..repeat_count {
buf.extend_from_slice(slice_to_repeat)
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| // Get to the start of the data before we started copying anything | ||
| let src = self.data.as_ptr().add(length_before) as *const u8; |
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rustc can probably figure it out, but src is the same for all loop iterations so could be pulled out of the loop I think
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yeah, I thought about it but decided not to do it so the code for src and dst is close
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Yes, I committed the benchmark I tested with
these are the results: Result |
👍 |
alamb
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that referenced
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Oct 20, 2025
# Which issue does this PR close? N/A # Rationale for this change doing `OffsetBuffer::from_lengths(std::iter::repeat_n(size, value.len()));` does not utilize SIMD (I explain further if you want) See [GodBolt Link](https://godbolt.org/z/PTsfvfjqx) Extracted from: - #8653 After this and the pr below is merged will improve the datafusion scalar to array to use this and make it really really fast: - #8658 # What changes are included in this PR? added new function # Are these changes tested? yes # Are there any user-facing changes? yes
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@alamb can you please merge |
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Done |
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Which issue does this PR close?
N/A
Rationale for this change
I want to repeat the same value multiple times in a very fast way
which will be used in:
After this and the pr below is merged will improve the datafusion scalar to array to use this and make it really really fast:
What changes are included in this PR?
Created a function in
MutableBufferto repeat a slice a number of times in a logarithmic way to reduce memcopy callsAre these changes tested?
Yes
Are there any user-facing changes?
Yes, and added docs
Extracted from:
Benchmark results on local machine
these are the results:
Result