[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;
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MaskRay wrote:

unneeded user-providied default ctor

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