[all-commits] [llvm/llvm-project] fd08af: [libc++] Optimize {std, ranges}::distance for segme...
Peng Liu via All-commits
all-commits at lists.llvm.org
Wed Oct 15 16:44:56 PDT 2025
Branch: refs/heads/main
Home: https://github.com/llvm/llvm-project
Commit: fd08af0a969a13d505c47a1f64e8f8def65a6aca
https://github.com/llvm/llvm-project/commit/fd08af0a969a13d505c47a1f64e8f8def65a6aca
Author: Peng Liu <winner245 at hotmail.com>
Date: 2025-10-16 (Thu, 16 Oct 2025)
Changed paths:
M libcxx/docs/ReleaseNotes/22.rst
M libcxx/include/__iterator/distance.h
A libcxx/test/benchmarks/iterators/distance.bench.cpp
M libcxx/test/std/iterators/iterator.primitives/iterator.operations/distance.pass.cpp
M libcxx/test/std/iterators/iterator.primitives/range.iter.ops/range.iter.ops.distance/iterator_sentinel.pass.cpp
Log Message:
-----------
[libc++] Optimize {std,ranges}::distance for segmented iterators (#133612)
This patch enhances the performance of `std::distance` and
`std::ranges::distance` for non-random-access segmented iterators, e.g.,
`std::join_view` iterators. The original implementation operates in
linear time, `O(n)`, where `n` is the total number of elements. The
optimized version reduces this to approximately `O(n / segment_size)` by
leveraging segmented structure, where `segment_size` is the average size
of each segment.
The table below summarizes the peak performance improvements observed
across different segment sizes, with the total element count `n` ranging
up to `1 << 20` (1,048,576 elements), based on benchmark results.
```
----------------------------------------------------------------------------------------
Container/n/segment_size std::distance std::ranges::distance
----------------------------------------------------------------------------------------
join_view(vector<vector<int>>)/1048576/256 401.6x 422.9x
join_view(deque<deque<int>>)/1048576/256 112.1x 132.6x
join_view(vector<vector<int>>)/1048576/1024 1669.2x 1559.1x
join_view(deque<deque<int>>)/1048576/1024 487.7x 497.4x
```
## Benchmarks
#### Segment size = 1024
```
-----------------------------------------------------------------------------------------
Benchmark Before After Speedup
-----------------------------------------------------------------------------------------
std::distance(join_view(vector<vector<int>>))/50 38.8 ns 1.01 ns 38.4x
std::distance(join_view(vector<vector<int>>))/1024 660 ns 1.02 ns 647.1x
std::distance(join_view(vector<vector<int>>))/4096 2934 ns 1.98 ns 1481.8x
std::distance(join_view(vector<vector<int>>))/8192 5751 ns 3.92 ns 1466.8x
std::distance(join_view(vector<vector<int>>))/16384 11520 ns 7.06 ns 1631.7x
std::distance(join_view(vector<vector<int>>))/65536 46367 ns 32.2 ns 1440.6x
std::distance(join_view(vector<vector<int>>))/262144 182611 ns 114 ns 1601.9x
std::distance(join_view(vector<vector<int>>))/1048576 737785 ns 442 ns 1669.2x
std::distance(join_view(deque<deque<int>>))/50 53.1 ns 6.13 ns 8.7x
std::distance(join_view(deque<deque<int>>))/1024 854 ns 7.53 ns 113.4x
std::distance(join_view(deque<deque<int>>))/4096 3507 ns 14.7 ns 238.6x
std::distance(join_view(deque<deque<int>>))/8192 7114 ns 17.6 ns 404.2x
std::distance(join_view(deque<deque<int>>))/16384 13997 ns 30.7 ns 455.9x
std::distance(join_view(deque<deque<int>>))/65536 55598 ns 114 ns 487.7x
std::distance(join_view(deque<deque<int>>))/262144 214293 ns 480 ns 446.4x
std::distance(join_view(deque<deque<int>>))/1048576 833000 ns 2183 ns 381.6x
rng::distance(join_view(vector<vector<int>>))/50 39.1 ns 1.10 ns 35.5x
rng::distance(join_view(vector<vector<int>>))/1024 689 ns 1.14 ns 604.4x
rng::distance(join_view(vector<vector<int>>))/4096 2753 ns 2.15 ns 1280.5x
rng::distance(join_view(vector<vector<int>>))/8192 5530 ns 4.61 ns 1199.6x
rng::distance(join_view(vector<vector<int>>))/16384 10968 ns 7.97 ns 1376.2x
rng::distance(join_view(vector<vector<int>>))/65536 46009 ns 35.3 ns 1303.4x
rng::distance(join_view(vector<vector<int>>))/262144 190569 ns 124 ns 1536.9x
rng::distance(join_view(vector<vector<int>>))/1048576 746724 ns 479 ns 1559.1x
rng::distance(join_view(deque<deque<int>>))/50 51.6 ns 6.57 ns 7.9x
rng::distance(join_view(deque<deque<int>>))/1024 826 ns 6.50 ns 127.1x
rng::distance(join_view(deque<deque<int>>))/4096 3323 ns 12.5 ns 265.8x
rng::distance(join_view(deque<deque<int>>))/8192 6619 ns 19.1 ns 346.5x
rng::distance(join_view(deque<deque<int>>))/16384 13495 ns 33.2 ns 406.5x
rng::distance(join_view(deque<deque<int>>))/65536 53668 ns 114 ns 470.8x
rng::distance(join_view(deque<deque<int>>))/262144 236277 ns 475 ns 497.4x
rng::distance(join_view(deque<deque<int>>))/1048576 914177 ns 2157 ns 423.8x
-----------------------------------------------------------------------------------------
```
#### Segment size = 256
```
-----------------------------------------------------------------------------------------
Benchmark Before After Speedup
-----------------------------------------------------------------------------------------
std::distance(join_view(vector<vector<int>>))/50 38.1 ns 1.02 ns 37.4x
std::distance(join_view(vector<vector<int>>))/1024 689 ns 2.06 ns 334.5x
std::distance(join_view(vector<vector<int>>))/4096 2815 ns 7.01 ns 401.6x
std::distance(join_view(vector<vector<int>>))/8192 5507 ns 14.3 ns 385.1x
std::distance(join_view(vector<vector<int>>))/16384 11050 ns 33.7 ns 327.9x
std::distance(join_view(vector<vector<int>>))/65536 44197 ns 118 ns 374.6x
std::distance(join_view(vector<vector<int>>))/262144 175793 ns 449 ns 391.5x
std::distance(join_view(vector<vector<int>>))/1048576 703242 ns 2140 ns 328.7x
std::distance(join_view(deque<deque<int>>))/50 50.2 ns 6.12 ns 8.2x
std::distance(join_view(deque<deque<int>>))/1024 835 ns 11.4 ns 73.2x
std::distance(join_view(deque<deque<int>>))/4096 3353 ns 32.9 ns 101.9x
std::distance(join_view(deque<deque<int>>))/8192 6711 ns 64.2 ns 104.5x
std::distance(join_view(deque<deque<int>>))/16384 13231 ns 118 ns 112.1x
std::distance(join_view(deque<deque<int>>))/65536 53523 ns 556 ns 96.3x
std::distance(join_view(deque<deque<int>>))/262144 219101 ns 2166 ns 101.2x
std::distance(join_view(deque<deque<int>>))/1048576 880277 ns 15852 ns 55.5x
rng::distance(join_view(vector<vector<int>>))/50 37.7 ns 1.13 ns 33.4x
rng::distance(join_view(vector<vector<int>>))/1024 697 ns 2.14 ns 325.7x
rng::distance(join_view(vector<vector<int>>))/4096 2804 ns 7.52 ns 373.0x
rng::distance(join_view(vector<vector<int>>))/8192 5749 ns 15.2 ns 378.2x
rng::distance(join_view(vector<vector<int>>))/16384 11742 ns 34.8 ns 337.4x
rng::distance(join_view(vector<vector<int>>))/65536 47274 ns 116 ns 407.7x
rng::distance(join_view(vector<vector<int>>))/262144 187774 ns 444 ns 422.9x
rng::distance(join_view(vector<vector<int>>))/1048576 749724 ns 2109 ns 355.5x
rng::distance(join_view(deque<deque<int>>))/50 53.0 ns 6.09 ns 8.7x
rng::distance(join_view(deque<deque<int>>))/1024 895 ns 11.0 ns 81.4x
rng::distance(join_view(deque<deque<int>>))/4096 3825 ns 30.6 ns 125.0x
rng::distance(join_view(deque<deque<int>>))/8192 7550 ns 60.5 ns 124.8x
rng::distance(join_view(deque<deque<int>>))/16384 14847 ns 112 ns 132.6x
rng::distance(join_view(deque<deque<int>>))/65536 56888 ns 453 ns 125.6x
rng::distance(join_view(deque<deque<int>>))/262144 231395 ns 2034 ns 113.8x
rng::distance(join_view(deque<deque<int>>))/1048576 933093 ns 15012 ns 62.2x
-----------------------------------------------------------------------------------------
```
Addresses a subtask of #102817.
---------
Co-authored-by: Louis Dionne <ldionne.2 at gmail.com>
Co-authored-by: A. Jiang <de34 at live.cn>
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