[libcxx-commits] [libcxx] [libc++] Optimize std::min_element (PR #100616)
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Thu Jul 25 10:56:08 PDT 2024
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-libcxx
Author: vaibhav (mrdaybird)
<details>
<summary>Changes</summary>
This PR provides an alternate implementation of std::min_element which is faster in most cases and can leverage clang's auto-vectorization capabilities.
The algorithm is inspired by [Argmin in SIMD](https://en.algorithmica.org/hpc/algorithms/argmin/). You can refer to it for reason of choosing the algorithm.
The algorithm performs min-reduction and std::find in blocks, to get the global minimum element. Since, clang can auto-vectorize min-reduction and soon may also auto-vectorize std::find(#<!-- -->88385), the new algorithm can partially be auto-vectorized, and in the future may get completely auto-vectorized.
You can see the benchmark results in [this repo](https://github.com/mrdaybird/argminlibcxx)
Gist of benchmark-
- If vectorization is enabled, up-to 22x speedup
- If vectorization is not enabled, up-to 3x speedup, though regression in case of *(unsigned) long long*.
I would encourage the reviewers to perform the benchmark on other systems as well.
---
Full diff: https://github.com/llvm/llvm-project/pull/100616.diff
3 Files Affected:
- (modified) libcxx/benchmarks/CMakeLists.txt (+1)
- (added) libcxx/benchmarks/min_element.bench.cpp (+48)
- (modified) libcxx/include/__algorithm/min_element.h (+50-5)
``````````diff
diff --git a/libcxx/benchmarks/CMakeLists.txt b/libcxx/benchmarks/CMakeLists.txt
index d96ccc1e49f66..952777d5346f7 100644
--- a/libcxx/benchmarks/CMakeLists.txt
+++ b/libcxx/benchmarks/CMakeLists.txt
@@ -121,6 +121,7 @@ set(BENCHMARK_TESTS
algorithms/make_heap_then_sort_heap.bench.cpp
algorithms/min.bench.cpp
algorithms/minmax.bench.cpp
+ algorithms/min_element.bench.cpp
algorithms/min_max_element.bench.cpp
algorithms/mismatch.bench.cpp
algorithms/pop_heap.bench.cpp
diff --git a/libcxx/benchmarks/min_element.bench.cpp b/libcxx/benchmarks/min_element.bench.cpp
new file mode 100644
index 0000000000000..ed769aeeea7d2
--- /dev/null
+++ b/libcxx/benchmarks/min_element.bench.cpp
@@ -0,0 +1,48 @@
+#include <vector>
+#include <algorithm>
+#include <limits>
+
+#include <benchmark/benchmark.h>
+#include <random>
+
+template<typename T>
+static void BM_stdmin_element_decreasing(benchmark::State &state){
+ std::vector<T> v(state.range(0));
+ T start = std::numeric_limits<T>::max();
+ T end = std::numeric_limits<T>::min();
+
+ for(size_t i = 0; i < v.size(); i++)
+ v[i] = ((start != end) ? start-- : end);
+
+ for(auto _ : state){
+ benchmark::DoNotOptimize(v);
+ benchmark::DoNotOptimize(std::min_element(v.begin(), v.end()));
+ }
+}
+
+BENCHMARK(BM_stdmin_element_decreasing<char>)
+ ->DenseRange(1, 8)->Range(32, 128)->Range(256, 4096)->DenseRange(5000, 10000, 1000)
+ ->Range(1<<14, 1<<16)->Arg(70000);
+BENCHMARK(BM_stdmin_element_decreasing<short>)
+ ->DenseRange(1, 8)->Range(32, 128)->Range(256, 4096)->DenseRange(5000, 10000, 1000)
+ ->Range(1<<14, 1<<16)->Arg(70000);
+BENCHMARK(BM_stdmin_element_decreasing<int>)
+ ->DenseRange(1, 8)->Range(32, 128)->Range(256, 4096)->DenseRange(5000, 10000, 1000)
+ ->Range(1<<14, 1<<16)->Arg(70000);
+BENCHMARK(BM_stdmin_element_decreasing<long long>)
+ ->DenseRange(1, 8)->Range(32, 128)->Range(256, 4096)->DenseRange(5000, 10000, 1000)
+ ->Range(1<<14, 1<<16)->Arg(70000);
+BENCHMARK(BM_stdmin_element_decreasing<unsigned char>)
+ ->DenseRange(1, 8)->Range(32, 128)->Range(256, 4096)->DenseRange(5000, 10000, 1000)
+ ->Range(1<<14, 1<<16)->Arg(70000);
+BENCHMARK(BM_stdmin_element_decreasing<unsigned short>)
+ ->DenseRange(1, 8)->Range(32, 128)->Range(256, 4096)->DenseRange(5000, 10000, 1000)
+ ->Range(1<<14, 1<<16)->Arg(70000);
+BENCHMARK(BM_stdmin_element_decreasing<unsigned int>)
+ ->DenseRange(1, 8)->Range(32, 128)->Range(256, 4096)->DenseRange(5000, 10000, 1000)
+ ->Range(1<<14, 1<<16)->Arg(70000);
+BENCHMARK(BM_stdmin_element_decreasing<unsigned long long>)
+ ->DenseRange(1, 8)->Range(32, 128)->Range(256, 4096)->DenseRange(5000, 10000, 1000)
+ ->Range(1<<14, 1<<16)->Arg(70000);
+
+BENCHMARK_MAIN();
diff --git a/libcxx/include/__algorithm/min_element.h b/libcxx/include/__algorithm/min_element.h
index 65f3594d630ce..891a4afae3ea1 100644
--- a/libcxx/include/__algorithm/min_element.h
+++ b/libcxx/include/__algorithm/min_element.h
@@ -11,6 +11,7 @@
#include <__algorithm/comp.h>
#include <__algorithm/comp_ref_type.h>
+#include <__algorithm/iterator_operations.h>
#include <__config>
#include <__functional/identity.h>
#include <__functional/invoke.h>
@@ -33,12 +34,56 @@ __min_element(_Iter __first, _Sent __last, _Comp __comp, _Proj& __proj) {
if (__first == __last)
return __first;
- _Iter __i = __first;
- while (++__i != __last)
- if (std::__invoke(__comp, std::__invoke(__proj, *__i), std::__invoke(__proj, *__first)))
- __first = __i;
+ const size_t __n = static_cast<size_t>(std::distance(__first, __last));
- return __first;
+ if (__n <= 64) {
+ _Iter __i = __first;
+ while (++__i != __last)
+ if (std::__invoke(__comp, std::__invoke(__proj, *__i), std::__invoke(__proj, *__first)))
+ __first = __i;
+ return __first;
+ }
+
+ size_t __block_size = 256;
+
+ size_t __n_blocked = __n - (__n % __block_size);
+ _Iter __block_start = __first, __block_end = __first;
+
+ typedef typename std::iterator_traits<_Iter>::value_type value_type;
+ value_type __min_val = std::invoke(__proj, *__first);
+
+ _Iter __curr = __first;
+ for (size_t __i = 0; __i < __n_blocked; __i += __block_size) {
+ _Iter __start = __curr;
+ value_type __block_min = __min_val;
+ for (size_t j = 0; j < __block_size; j++) {
+ if (std::__invoke(__comp, std::__invoke(__proj, *__curr), __block_min)) {
+ __block_min = *__curr;
+ }
+ __curr++;
+ }
+ if (std::invoke(__comp, __block_min, __min_val)) {
+ __min_val = __block_min;
+ __block_start = __start;
+ __block_end = __curr;
+ }
+ }
+
+ value_type __epilogue_min = __min_val;
+ _Iter __epilogue_start = __curr;
+ while (__curr != __last) {
+ if (std::__invoke(__comp, std::__invoke(__proj, *__curr), __epilogue_min)) {
+ __epilogue_min = *__curr;
+ }
+ __curr++;
+ }
+ if (std::__invoke(__comp, __epilogue_min, __min_val)) {
+ __min_val = __epilogue_min;
+ __block_start = __epilogue_start;
+ __block_end = __last;
+ }
+
+ return find(__block_start, __block_end, __min_val);
}
template <class _Comp, class _Iter, class _Sent>
``````````
</details>
https://github.com/llvm/llvm-project/pull/100616
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