[all-commits] [llvm/llvm-project] 81b813: [libc++] Optimize bitset::to_string (#128832)

[Peng Liu via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 81b81354f8c117fab07823fef24b97b3a1f47834
      https://github.com/llvm/llvm-project/commit/81b81354f8c117fab07823fef24b97b3a1f47834
  Author: Peng Liu <winner245 at hotmail.com>
  Date:   2025-05-21 (Wed, 21 May 2025)

  Changed paths:
    M libcxx/docs/ReleaseNotes/21.rst
    M libcxx/include/bitset
    A libcxx/test/benchmarks/bitset.bench.cpp

  Log Message:
  -----------
  [libc++] Optimize bitset::to_string (#128832)

This patch optimizes `bitset::to_string` by replacing the existing bit-by-bit processing with a more efficient
bit traversal strategy. Instead of checking each bit sequentially, we leverage `std::__countr_zero` to efficiently
locate the next set bit, skipping over consecutive zero bits. This greatly accelerates the conversion process,
especially for sparse `bitset`s where zero bits dominate. To ensure similar improvements for dense `bitset`s, we
exploit symmetry by inverting the bit pattern, allowing us to apply the same optimized traversal technique. Even
for uniformly distributed `bitset`s, the proposed approach offers measurable performance gains over the existing
implementation.

Benchmarks demonstrate substantial improvements, achieving up to 13.5x speedup for sparse `bitset`s with
`Pr(true bit) = 0.1`, 16.1x for dense `bitset`s with `Pr(true bit) = 0.9`, and 8.3x for uniformly distributed
`bitset`s with `Pr(true bit) = 0.5)`.



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