[llvm] [ADT][NFC] Refactor/optimize DenseMap::copyFrom (PR #108377)

[Jakub Kuderski via llvm-commits]

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@@ -471,19 +471,22 @@ class DenseMapBase : public DebugEpochBase {
     setNumEntries(other.getNumEntries());
     setNumTombstones(other.getNumTombstones());
 
-    if (std::is_trivially_copyable<KeyT>::value &&
-        std::is_trivially_copyable<ValueT>::value)
-      memcpy(reinterpret_cast<void *>(getBuckets()), other.getBuckets(),
+    BucketT *Buckets = getBuckets();
+    const BucketT *OtherBuckets = other.getBuckets();
+    if constexpr (std::is_trivially_copyable_v<KeyT> &&
+                  std::is_trivially_copyable_v<ValueT>) {
+      memcpy(reinterpret_cast<void *>(Buckets), OtherBuckets,
              getNumBuckets() * sizeof(BucketT));
-    else
-      for (size_t i = 0; i < getNumBuckets(); ++i) {
-        ::new (&getBuckets()[i].getFirst())
-            KeyT(other.getBuckets()[i].getFirst());
-        if (!KeyInfoT::isEqual(getBuckets()[i].getFirst(), getEmptyKey()) &&
-            !KeyInfoT::isEqual(getBuckets()[i].getFirst(), getTombstoneKey()))
-          ::new (&getBuckets()[i].getSecond())
-              ValueT(other.getBuckets()[i].getSecond());
+    } else {
+      const KeyT EmptyKey = getEmptyKey();
+      const KeyT TombstoneKey = getTombstoneKey();
+      for (size_t I = 0; I < getNumBuckets(); ++I) {
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kuhar wrote:

If the trip count is constant here, this should only calculate it once. See https://llvm.org/docs/CodingStandards.html#don-t-evaluate-end-every-time-through-a-loop.

https://github.com/llvm/llvm-project/pull/108377