[libcxx-commits] [libcxx] [libc++] Optimize bitset::to_string (PR #128832)

[Nikolas Klauser via libcxx-commits]

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@@ -0,0 +1,115 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03
+
+#include "benchmark/benchmark.h"
+#include <bitset>
+#include <cmath>
+#include <cstddef>
+
+template <std::size_t N>
+struct GenerateBitset {
+  // Construct a bitset with p*N true bits
+  static std::bitset<N> generate(double p) {
+    std::bitset<N> b;
+    if (p <= 0.0)
+      return b;
+    if (p >= 1.0)
+      return ~b;
+
+    std::size_t num_ones = std::round(N * p);
+    if (num_ones == 0)
+      return b;
+
+    double step  = static_cast<double>(N) / num_ones;
+    double error = 0.0;
+
+    std::size_t pos = 0;
+    for (std::size_t i = 0; i < num_ones; ++i) {
+      if (pos >= N)
+        break;
+      b.set(pos);
+      error += step;
+      pos += std::floor(error);
+      error -= std::floor(error);
+    }
+    return b;
+  }
+
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philnik777 wrote:

I think having random data here is really important to check your current implementation, since it likely heavily depends on the branch predictor.

https://github.com/llvm/llvm-project/pull/128832