[ntuple] fix up vector resize for over-aligned values

Author: jblomer

tree/ntuple/inc/ROOT/RField/RFieldSequenceContainer.hxx

@@ -260,6 +260,9 @@ protected:
    void CommitClusterImpl() final { fNWritten = 0; }
 
 public:
+   /// Maximum alignment of the vector's value type
+   static constexpr std::size_t kMaxItemAlignment = 4096;
+
    RVectorField(std::string_view fieldName, std::unique_ptr<RFieldBase> itemField);
    RVectorField(RVectorField &&other) = default;
    RVectorField &operator=(RVectorField &&other) = default;

tree/ntuple/src/RFieldSequenceContainer.cxx

@@ -567,7 +567,38 @@ void ROOT::RVectorField::ResizeVector(void *vec, std::size_t nItems, std::size_t
          itemDeleter->operator()(typedValue->data() + (i * itemSize), true /* dtorOnly */);
       }
    }
-   typedValue->resize(nItems * itemSize);
+
+   // Resize the vector with correct alignment
+   const auto itemAlignment = itemField.GetAlignment();
+   const auto nbytes = nItems * itemSize;
+   if ((oldNItems >= nItems) || (itemAlignment <= sizeof(std::max_align_t))) {
+      typedValue->resize(nbytes);
+   } else {
+      auto fnAlignedResize = [](auto &vecOfAlignedStorage, std::size_t targetSize) {
+         constexpr auto valueTypeSize = sizeof(typename std::decay_t<decltype(vecOfAlignedStorage)>::value_type);
+         constexpr auto valueTypeAlignment = alignof(typename std::decay_t<decltype(vecOfAlignedStorage)>::value_type);
+         static_assert(valueTypeSize == valueTypeAlignment);
+         assert(targetSize % valueTypeAlignment == 0);
+         vecOfAlignedStorage.resize(targetSize / valueTypeSize);
+      };
+
+      static_assert(sizeof(std::max_align_t) >= 8 && kMaxItemAlignment == 4096);
+      // clang-format off
+      switch (itemAlignment) {
+      case   16: fnAlignedResize(*static_cast<std::vector<Internal::RAlignedStorage<  16>> *>(vec), nbytes); break;
+      case   32: fnAlignedResize(*static_cast<std::vector<Internal::RAlignedStorage<  32>> *>(vec), nbytes); break;
+      case   64: fnAlignedResize(*static_cast<std::vector<Internal::RAlignedStorage<  64>> *>(vec), nbytes); break;
+      case  128: fnAlignedResize(*static_cast<std::vector<Internal::RAlignedStorage< 128>> *>(vec), nbytes); break;
+      case  256: fnAlignedResize(*static_cast<std::vector<Internal::RAlignedStorage< 256>> *>(vec), nbytes); break;
+      case  512: fnAlignedResize(*static_cast<std::vector<Internal::RAlignedStorage< 512>> *>(vec), nbytes); break;
+      case 1024: fnAlignedResize(*static_cast<std::vector<Internal::RAlignedStorage<1024>> *>(vec), nbytes); break;
+      case 2048: fnAlignedResize(*static_cast<std::vector<Internal::RAlignedStorage<2048>> *>(vec), nbytes); break;
+      case 4096: fnAlignedResize(*static_cast<std::vector<Internal::RAlignedStorage<4096>> *>(vec), nbytes); break;
+      default: throw RException(R__FAIL(std::string("Unsupported alignment: ") + std::to_string(itemAlignment)));
+      }
+      // clang-format on
+   }
+
    if (!(itemField.GetTraits() & kTraitTriviallyConstructible)) {
       for (std::size_t i = allDeallocated ? 0 : oldNItems; i < nItems; ++i) {
          CallConstructValueOn(itemField, typedValue->data() + (i * itemSize));

tree/ntuple/test/rfield_class.cxx

@@ -540,6 +540,5 @@ TEST(RNTuple, AlignmentCornerCases)
 
    auto view = reader->GetView<AlignmentEnvelope>("f");
    EXPECT_EQ(1u, view(0).fVec.size());
-   // TODO(jblomer): FIXME
-   // EXPECT_EQ(0, reinterpret_cast<std::uintptr_t>(view(0).fVec.data()) % alignof(OverAligned));
+   EXPECT_EQ(0, reinterpret_cast<std::uintptr_t>(view(0).fVec.data()) % alignof(OverAligned));
 }