[llvm] [ADT] Introduce EytzingerTableSpan (PR #208885)

[David Li via llvm-commits]

================
@@ -0,0 +1,68 @@
+//===- Eytzinger.h - Eytzinger Search Tree Span -----------------*- C++ -*-===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+///
+/// \file
+/// This file defines the EytzingerTableSpan class, a non-owning view of a
+/// buffer formatted as a complete binary search tree in Eytzinger
+/// (breadth-first) order.
+///
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_ADT_EYTZINGER_H
+#define LLVM_ADT_EYTZINGER_H
+
+#include "llvm/ADT/bit.h"
+#include <cassert>
+#include <cstddef>
+#include <optional>
+
+namespace llvm {
+
+/// Non-owning view of a buffer formatted as a complete binary search tree in
+/// Eytzinger (breadth-first) order.
+template <typename T> class EytzingerTableSpan {
+public:
+  EytzingerTableSpan() = default;
+  EytzingerTableSpan(const T *Data, size_t NumEntries)
+      : Data(Data), NumEntries(NumEntries) {}
+
+  [[nodiscard]] const T *data() const { return Data; }
+  [[nodiscard]] bool empty() const { return !Data || NumEntries == 0; }
+  [[nodiscard]] size_t size() const { return NumEntries; }
+  [[nodiscard]] const T &operator[](size_t Idx) const {
+    assert(Idx < NumEntries && "Index out of bounds");
+    return Data[Idx];
+  }
+
+  /// Search this Eytzinger table for Target using branchless binary search.
+  /// Returns the 0-based array index if found.
+  ///
+  /// KeyT enables heterogeneous lookups, allowing callers to search tables of
+  /// endian-specific wrappers (e.g., support::ulittle64_t) using native integer
+  /// keys without explicit conversions at the call site.
+  template <typename KeyT = T>
+  [[nodiscard]] std::optional<size_t> findIndex(const KeyT &Target) const {
+    if (empty())
+      return std::nullopt;
+    size_t K = 1;
+    while (K <= NumEntries)
+      K = 2 * K + (Data[K - 1] < Target);
+    K >>= llvm::countr_one(K) + 1;
----------------
david-xl wrote:

The bit manipulation code is hard to read. Does this version work?
size_t i = 0;
while (i < n) {
    if (Data[i] == target)
        return i;

    i = 2 * i + 1 + (Data[i] < target);
}

https://github.com/llvm/llvm-project/pull/208885