[llvm] [ADT] Extract DenseMapStorage from DenseMap (NFC) (PR #226664)

[Fangrui Song via llvm-commits]

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@@ -192,6 +192,101 @@ LLVM_ABI void *growRelocatable(void *OldBuckets, const UsedT *OldUsed,
                                size_t BucketSize, size_t Align,
                                BucketHasher Hasher, bool FreeOld);
 
+// A snapshot of the three fields the hot lookup paths need. Fetching them
+// together lets SmallDenseMap test its Small discriminator once rather than
+// once per accessor; for plain DenseMap it is three member loads either way.
+template <typename BucketT> struct StorageRep {
+  const BucketT *Buckets;
+  const UsedT *Used;
+  unsigned NumBuckets;
+};
+
+template <typename BucketT> class DenseMapStorage {
+  BucketT *Buckets = nullptr;
+  UsedT *Used = nullptr;
+  unsigned NumEntries = 0;
+  unsigned NumBuckets = 0;
+
+public:
+  DenseMapStorage() = default;
+
+  unsigned getNumEntries() const { return NumEntries; }
+  void setNumEntries(unsigned Num) { NumEntries = Num; }
+
+  BucketT *getBuckets() const { return Buckets; }
+  UsedT *getUsed() const { return Used; }
+  unsigned getNumBuckets() const { return NumBuckets; }
+  StorageRep<BucketT> getRep() const { return {Buckets, Used, NumBuckets}; }
+
+  void swap(DenseMapStorage &RHS) {
+    std::swap(Buckets, RHS.Buckets);
+    std::swap(Used, RHS.Used);
+    std::swap(NumEntries, RHS.NumEntries);
+    std::swap(NumBuckets, RHS.NumBuckets);
+  }
+
+  void setStorage(void *Storage, unsigned Num) {
+    Buckets = static_cast<BucketT *>(Storage);
+    Used = usedFor(Storage, sizeof(BucketT), Num);
+    NumBuckets = Num;
+  }
+
+  void growShared(unsigned MinNumBuckets, BucketHasher Hasher) {
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MaskRay wrote:

Just `grow`

https://github.com/llvm/llvm-project/pull/226664