[ADT] Add TrieRawHashMap (llvm#69528)

Implement TrieRawHashMap can be used to store object with its associated
hash. User needs to supply a strong hashing function to guarantee the
uniqueness of the hash of the objects to be inserted. A hash collision
is not supported and will lead to error or failed to insert.

TrieRawHashMap is thread-safe and lock-free and can be used as
foundation data structure to implement a content addressible storage.
TrieRawHashMap owns the data stored in it and is designed to be:
* Fast to lookup.
* Fast to "insert" if the data has already been inserted.
* Can be used without lock and doesn't require any knowledge of the
participating threads or extra coordination between threads.

It is not currently designed to be used to insert unique new data with
high contention, due to the limitation on the memory allocator.

llvm/include/llvm/ADT/TrieHashIndexGenerator.h

@@ -0,0 +1,122 @@
+//===- TrieHashIndexGenerator.h ---------------------------------*- 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_ADT_TRIEHASHINDEXGENERATOR_H
+#define LLVM_ADT_TRIEHASHINDEXGENERATOR_H
+
+#include "llvm/ADT/ArrayRef.h"
+#include <optional>
+
+namespace llvm {
+
+/// The utility class that helps computing the index of the object inside trie
+/// from its hash. The generator can be configured with the number of bits
+/// used for each level of trie structure with \c NumRootsBits and \c
+/// NumSubtrieBits.
+/// For example, try computing indexes for a 16-bit hash 0x1234 with 8-bit root
+/// and 4-bit sub-trie:
+///
+///   IndexGenerator IndexGen{8, 4, Hash};
+///   size_t index1 = IndexGen.next(); // index 18 in root node.
+///   size_t index2 = IndexGen.next(); // index 3 in sub-trie level 1.
+///   size_t index3 = IndexGen.next(); // index 4 in sub-tire level 2.
+///
+/// This is used by different trie implementation to figure out where to
+/// insert/find the object in the data structure.
+struct TrieHashIndexGenerator {
+  size_t NumRootBits;
+  size_t NumSubtrieBits;
+  ArrayRef<uint8_t> Bytes;
+  std::optional<size_t> StartBit = std::nullopt;
+
+  // Get the number of bits used to generate current index.
+  size_t getNumBits() const {
+    assert(StartBit);
+    size_t TotalNumBits = Bytes.size() * 8;
+    assert(*StartBit <= TotalNumBits);
+    return std::min(*StartBit ? NumSubtrieBits : NumRootBits,
+                    TotalNumBits - *StartBit);
+  }
+
+  // Get the index of the object in the next level of trie.
+  size_t next() {
+    if (!StartBit) {
+      // Compute index for root when StartBit is not set.
+      StartBit = 0;
+      return getIndex(Bytes, *StartBit, NumRootBits);
+    }
+    if (*StartBit < Bytes.size() * 8) {
+      // Compute index for sub-trie.
+      *StartBit += *StartBit ? NumSubtrieBits : NumRootBits;
+      assert((*StartBit - NumRootBits) % NumSubtrieBits == 0);
+      return getIndex(Bytes, *StartBit, NumSubtrieBits);
+    }
+    // All the bits are consumed.
+    return end();
+  }
+
+  // Provide a hint to speed up the index generation by providing the
+  // information of the hash in current level. For example, if the object is
+  // known to have \c Index on a level that already consumes first n \c Bits of
+  // the hash, it can start index generation from this level by calling \c hint
+  // function.
+  size_t hint(unsigned Index, unsigned Bit) {
+    assert(Bit < Bytes.size() * 8);
+    assert(Bit == 0 || (Bit - NumRootBits) % NumSubtrieBits == 0);
+    StartBit = Bit;
+    return Index;
+  }
+
+  // Utility function for looking up the index in the trie for an object that
+  // has colliding hash bits in the front as the hash of the object that is
+  // currently being computed.
+  size_t getCollidingBits(ArrayRef<uint8_t> CollidingBits) const {
+    assert(StartBit);
+    return getIndex(CollidingBits, *StartBit, NumSubtrieBits);
+  }
+
+  size_t end() const { return SIZE_MAX; }
+
+  // Compute the index for the object from its hash, current start bits, and
+  // the number of bits used for current level.
+  static size_t getIndex(ArrayRef<uint8_t> Bytes, size_t StartBit,
+                         size_t NumBits) {
+    assert(StartBit < Bytes.size() * 8);
+    // Drop all the bits before StartBit.
+    Bytes = Bytes.drop_front(StartBit / 8u);
+    StartBit %= 8u;
+    size_t Index = 0;
+    // Compute the index using the bits in range [StartBit, StartBit + NumBits),
+    // note the range can spread across few `uint8_t` in the array.
+    for (uint8_t Byte : Bytes) {
+      size_t ByteStart = 0, ByteEnd = 8;
+      if (StartBit) {
+        ByteStart = StartBit;
+        Byte &= (1u << (8 - StartBit)) - 1u;
+        StartBit = 0;
+      }
+      size_t CurrentNumBits = ByteEnd - ByteStart;
+      if (CurrentNumBits > NumBits) {
+        Byte >>= CurrentNumBits - NumBits;
+        CurrentNumBits = NumBits;
+      }
+      Index <<= CurrentNumBits;
+      Index |= Byte & ((1u << CurrentNumBits) - 1u);
+
+      assert(NumBits >= CurrentNumBits);
+      NumBits -= CurrentNumBits;
+      if (!NumBits)
+        break;
+    }
+    return Index;
+  }
+};
+
+} // namespace llvm
+
+#endif // LLVM_ADT_TRIEHASHINDEXGENERATOR_H