1 //===----------------------------------------------------------------------===// 2 // 3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. 4 // See https://llvm.org/LICENSE.txt for license information. 5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception 6 // 7 //===----------------------------------------------------------------------===// 8 9 #include <__algorithm/min.h> 10 #include <__algorithm/pstl_backends/cpu_backends/libdispatch.h> 11 #include <__config> 12 #include <dispatch/dispatch.h> 13 #include <thread> 14 15 _LIBCPP_BEGIN_NAMESPACE_STD 16 17 namespace __par_backend::inline __libdispatch { 18 19 20 void __dispatch_apply(size_t chunk_count, void* context, void (*func)(void* context, size_t chunk)) noexcept { 21 ::dispatch_apply_f(chunk_count, DISPATCH_APPLY_AUTO, context, func); 22 } 23 24 __chunk_partitions __partition_chunks(ptrdiff_t element_count) { 25 if (element_count == 0) { 26 return __chunk_partitions{1, 0, 0}; 27 } else if (element_count == 1) { 28 return __chunk_partitions{1, 0, 1}; 29 } 30 31 __chunk_partitions partitions; 32 partitions.__chunk_count_ = [&] { 33 ptrdiff_t cores = std::max(1u, thread::hardware_concurrency()); 34 35 auto medium = [&](ptrdiff_t n) { return cores + ((n - cores) / cores); }; 36 37 // This is an approximation of `log(1.01, sqrt(n))` which seemes to be reasonable for `n` larger than 500 and tops 38 // at 800 tasks for n ~ 8 million 39 auto large = [](ptrdiff_t n) { return static_cast<ptrdiff_t>(100.499 * std::log(std::sqrt(n))); }; 40 41 if (element_count < cores) 42 return element_count; 43 else if (element_count < 500) 44 return medium(element_count); 45 else 46 return std::min(medium(element_count), large(element_count)); // provide a "smooth" transition 47 }(); 48 partitions.__chunk_size_ = element_count / partitions.__chunk_count_; 49 partitions.__first_chunk_size_ = partitions.__chunk_size_; 50 51 const ptrdiff_t leftover_item_count = element_count - (partitions.__chunk_count_ * partitions.__chunk_size_); 52 53 if (leftover_item_count == 0) 54 return partitions; 55 56 if (leftover_item_count == partitions.__chunk_size_) { 57 partitions.__chunk_count_ += 1; 58 return partitions; 59 } 60 61 const ptrdiff_t n_extra_items_per_chunk = leftover_item_count / partitions.__chunk_count_; 62 const ptrdiff_t n_final_leftover_items = leftover_item_count - (n_extra_items_per_chunk * partitions.__chunk_count_); 63 64 partitions.__chunk_size_ += n_extra_items_per_chunk; 65 partitions.__first_chunk_size_ = partitions.__chunk_size_ + n_final_leftover_items; 66 return partitions; 67 } 68 69 // NOLINTNEXTLINE(llvm-namespace-comment) // This is https://llvm.org/PR56804 70 } // namespace __par_backend::inline __libdispatch 71 72 _LIBCPP_END_NAMESPACE_STD 73