[mlir][sparse] Enhancing sparse=>sparse conversion.

[wrengr]

Summary

Fixes: https://github.com/llvm/llvm-project/issues/51652

Depends On D122060

[wrengr]

rebase, and fixing build error

[wrengr]

redoing the diff

[wrengr]

re-renaming the old appendPointer to appendCurrentPointer (since I've realized the name "finalizeSegment" is better used elsewhere).

[wrengr]

Lots of reorganization, subsequent to all the recent changes in the parent revisions.

[wrengr]

rebase

[wrengr]

rebase

[wrengr]

Rebasing for D123166. Also removing a bunch of inline keywords, per MLIR style-guide.

[wrengr]

rebase

[wrengr]

Rebasing for the latest version of D122060 (2022-05-11T13:09)

This also introduces a minor change to the enumerator design. Namely, due to the splitting of SparseTensorEnumeratorBase<V> vs SparseTensorEnumerator<P,I,V>, I've had to add the new SparseTensorStorageBase::newEnumerator methods to allow constructing objects of the abstract base type without reintroducing the "<P,I> vs <Q,J> problem" for direct conversion.

[wrengr]

Arg! It looks like when I installed the github version of Arcanist to avoid the https://www.mail-archive.com/[e-mail]/msg1838310.html bug with Debian's version, the new version isn't automatically applying the formatting script :( So please bear with the overly-long lines for now

[wrengr]

(attempting to get the clang-formatter to run again)

[wrengr]

(Previous attempt didn't work, so fixing some things manually)

[wrengr]

Rerunning git-clang-format

[aartbik]

mlir/lib/ExecutionEngine/SparseTensorUtils.cpp

784

Yeah! All your abstraction work is paying off!

819

shall we make this an explicit null_ptr for clarity
(I am fine either way but perhaps making it explicit reads clearer)?

1046

Fine for this revision, but would it make sense to move this into its own file in the longer run?
Just to split the storage vs. supporting classes a bit more. Probably applies to some of the other classes too by now.

1328

yeah, direct! Sweet!

mlir/test/Dialect/SparseTensor/conversion.mlir

5

period at end

mlir/test/Integration/Dialect/SparseTensor/CPU/sparse_conversion_sparse2sparse.mlir

2

period at end of comment

[wrengr]

Rebasing for D125596

mlir/lib/Dialect/SparseTensor/Transforms/SparseTensorConversion.cpp

static void insertScalarIntoDenseTensor(ConversionPatternRewriter &rewriter,
   for (unsigned i = 0; i < rank; i++) {
     Value idx = constantIndex(rewriter, loc, i);
     ivs.push_back(rewriter.create<memref::LoadOp>(loc, ind, idx));
   }
   Value elemV = rewriter.create<memref::LoadOp>(loc, elemPtr);
   rewriter.create<memref::StoreOp>(loc, elemV, tensor, ivs);
 }
 
+/// Determine if the runtime library supports direct conversion to the
+/// given target `dimTypes`.
+static bool canUseDirectConversion(
+    ArrayRef<SparseTensorEncodingAttr::DimLevelType> dimTypes) {
+  bool alreadyCompressed = false;
+  for (uint64_t rank = dimTypes.size(), r = 0; r < rank; r++) {
+    switch (dimTypes[r]) {
+    case SparseTensorEncodingAttr::DimLevelType::Compressed:
+      if (alreadyCompressed)
+        return false; // Multiple compressed dimensions not yet supported.
+      alreadyCompressed = true;
+      break;
+    case SparseTensorEncodingAttr::DimLevelType::Dense:
+      if (alreadyCompressed)
+        return false; // Dense after Compressed not yet supported.
+      break;
+    case SparseTensorEncodingAttr::DimLevelType::Singleton:
+      // Although Singleton isn't generally supported yet, the direct
+      // conversion method doesn't have any particular problems with
+      // singleton after compressed.
+      break;
+    }
+  }
+  return true;
+}
+
 //===----------------------------------------------------------------------===//
 // Conversion rules.
 //===----------------------------------------------------------------------===//
 
 /// Sparse conversion rule for returns.
 class SparseReturnConverter : public OpConversionPattern<func::ReturnOp> {
 public:
   using OpConversionPattern::OpConversionPattern;
if (encDst && encSrc) {
       if (encDst == encSrc) {
         rewriter.replaceOp(op, adaptor.getOperands()); // hidden nop cast
         return success();
       }
       SmallVector<Value, 4> sizes;
       SmallVector<Value, 8> params;
       ShapedType stp = srcType.cast<ShapedType>();
       sizesFromPtr(rewriter, sizes, op, encSrc, stp, src);
+      bool useDirectConversion;
+      switch (options.sparseToSparseStrategy) {
+      case SparseToSparseConversionStrategy::kViaCOO:
+        useDirectConversion = false;
+        break;
+      case SparseToSparseConversionStrategy::kDirect:
+        useDirectConversion = true;
+        assert(canUseDirectConversion(encDst.getDimLevelType()) &&
+               "Unsupported target for direct sparse-to-sparse conversion");
+        break;
+      case SparseToSparseConversionStrategy::kAuto:
+        useDirectConversion = canUseDirectConversion(encDst.getDimLevelType());
+        break;
+      }
+      if (useDirectConversion) {
+        newParams(rewriter, params, op, stp, encDst, Action::kSparseToSparse,
+                  sizes, src);
+        rewriter.replaceOp(op, genNewCall(rewriter, op, params));
+      } else { // use via-COO conversion.
         // Set up encoding with right mix of src and dst so that the two
         // method calls can share most parameters, while still providing
         // the correct sparsity information to either of them.
         auto enc = SparseTensorEncodingAttr::get(
             op->getContext(), encDst.getDimLevelType(), encDst.getDimOrdering(),
             encSrc.getPointerBitWidth(), encSrc.getIndexBitWidth());
         newParams(rewriter, params, op, stp, enc, Action::kToCOO, sizes, src);
         Value coo = genNewCall(rewriter, op, params);
         params[3] = constantPointerTypeEncoding(rewriter, loc, encDst);
         params[4] = constantIndexTypeEncoding(rewriter, loc, encDst);
         params[6] = constantAction(rewriter, loc, Action::kFromCOO);
         params[7] = coo;
         rewriter.replaceOp(op, genNewCall(rewriter, op, params));
+      }
       return success();
     }
     if (!encDst && encSrc) {
       // This is sparse => dense conversion, which is handled as follows:
       //   dst = new Tensor(0);
       //   iter = src->toCOO();
       //   iter->startIterator();
       //   while (elem = iter->getNext()) {

mlir/lib/ExecutionEngine/SparseTensorUtils.cpp

 // only visible as an opaque pointer.
 //
 //===----------------------------------------------------------------------===//
 
 namespace {
 
 static constexpr int kColWidth = 1025;
 
+// TODO(wrengr): adjust this so it can be used by `openSparseTensorCOO`
+// too.  That version doesn't have the permutation, and the `sizes`
+// are a pointer/C-array rather than `std::vector`.
+/// Precondition: `perm` and `shape` must be valid for `rank`.
+static inline void assertPermutedSizesMatchShape(
+    const std::vector<uint64_t> &sizes, // in target order.
+    const uint64_t rank,
+    const uint64_t *perm,    // semantic order -> target order.
+    const uint64_t *shape) { // in semantic order.
+  assert(rank == sizes.size() && "Rank mismatch");
+  for (uint64_t r = 0; r < rank; r++) {
+    const uint64_t s = shape[r];
+    assert((s == 0 || s == sizes[perm[r]]) && "Dimension size mismatch");
+  }
+}
+
 /// A sparse tensor element in coordinate scheme (value and indices).
 /// For example, a rank-1 vector element would look like
 ///   ({i}, a[i])
 /// and a rank-5 tensor element like
 ///   ({i,j,k,l,m}, a[i,j,k,l,m])
 template <typename V>
 struct Element {
   Element(const std::vector<uint64_t> &ind, V val) : indices(ind), value(val){};

 
 private:
   const std::vector<uint64_t> sizes; // per-dimension sizes
   std::vector<Element<V>> elements;
   bool iteratorLocked;
   unsigned iteratorPos;
 };
 
+//===----------------------------------------------------------------------===//
 /// Abstract base class of sparse tensor storage. Note that we use
 /// function overloading to implement "partial" method specialization.
 class SparseTensorStorageBase {
 public:
   /// Dimension size query.
   virtual uint64_t getDimSize(uint64_t) const = 0;
 
   /// Overhead storage.
if (d == getRank()) {
         cursor[reord[d]] = i;
         forallElements(yield, pstart + i, d + 1);
       }
     }
   }
 }; // class SparseTensorEnumerator
 
 //===----------------------------------------------------------------------===//
+// Forward declaration.
+template <typename P, typename I, typename V>
+class SparseTensorStorage;
+
+/// Statistics regarding the number of nonzero subtensors in
+/// a source tensor, for direct sparse=>sparse conversion a la
+/// <https://arxiv.org/abs/2001.02609>.
+///
+/// N.B., this class stores a reference to the dimension-sizes passed
+/// to the constructor; thus, objects of this class must not outlive
+/// the dimension-sizes they depend on.
+class SparseTensorNNZ {
+  // All of these are in the target storage-order.
+  const std::vector<uint64_t> &dimSizes;
+  const std::vector<DimLevelType> dimTypes;
+  std::vector<std::vector<uint64_t>> nnz;
+
+public:
+  SparseTensorNNZ(const SparseTensorNNZ &) = delete;
+  SparseTensorNNZ(SparseTensorNNZ &&) = delete;
+  SparseTensorNNZ &operator=(const SparseTensorNNZ &) = delete;
+  SparseTensorNNZ &operator=(SparseTensorNNZ &&) = delete;
+
+  /// Returns the rank of the target tensor.
+  uint64_t getRank() const { return dimSizes.size(); }
+
+  /// Allocate the statistics structure for the desired sizes and
+  /// sparsity (in the target tensor's storage-order).
+  ///
+  /// Preconditions:
+  /// (1) `sparsity` must be valid for `szs.size()`
+  /// (2) `szs` must not contain zeros.
+  SparseTensorNNZ(const std::vector<uint64_t> &szs,
+                  const DimLevelType *sparsity)
+      : dimSizes(szs), dimTypes(sparsity, sparsity + getRank()),
+        nnz(getRank()) {
+    bool uncompressed = true;
+    uint64_t sz = 1; // sz_r := product(dimSizes[0:r-1]);
+    for (uint64_t rank = getRank(), r = 0; r < rank; r++) {
+      switch (dimTypes[r]) {
+      case DimLevelType::kCompressed:
+        assert(uncompressed &&
+               "Multiple compressed layers not currently supported");
+        uncompressed = false;
+        nnz[r].resize(sz, 0); // Both allocate and zero-initialize.
+        break;
+      case DimLevelType::kDense:
+        assert(uncompressed &&
+               "Dense after compressed not currently supported");
+        break;
+      case DimLevelType::kSingleton:
+        // Singleton after Compressed causes no problems for allocating
+        // `nnz` nor for the yieldPos loop.  This remains true even
+        // when adding support for multiple compressed dimensions or
+        // for dense-after-compressed.
+        break;
+      }
+      // TODO(wrengr): this division is expensive, but replacing it
+      // with ffs/clz or intrinsics is non-portable; does mlir have
+      // any cpp macro to enable this only when we want "paranoia mode"
+      // for extra debugging info?  Or should we just rely on Clang's
+      // -fsanitize=integer ?
+      assert(dimSizes[r] <= std::numeric_limits<uint64_t>::max() / sz);
+      sz *= dimSizes[r];
+    }
+  }
+
+  /// Enumerate the source tensor to fill in the statistics.  The
+  /// enumerator should already incorporate the permutation of dimensions
+  /// from the semantic-order to the target tensor's storage-order.
+  template <typename V>
+  void initialize(SparseTensorEnumerator<V> &enumerator) {
+    assert(enumerator.getRank() == getRank() && "Tensor rank mismatch");
+    assert(enumerator.permutedSizes() == dimSizes && "Tensor size mismatch");
+    enumerator.forallElements(
+        [this](const std::vector<uint64_t> &ind, V) { add(ind); });
+  }
+
+private:
+  /// Adds a new element (i.e., increment its statistics).  We use
+  /// a method rather than inlining into the lambda in `initialize`,
+  /// to avoid spurious templating over `V`.  And this method is private
+  /// to avoid needing to re-assert validity of `ind` (which is guaranteed
+  /// by `forallElements`).
+  void add(const std::vector<uint64_t> &ind) {
+    uint64_t parentPos = 0;
+    for (uint64_t rank = getRank(), r = 0; r < rank; r++) {
+      if (dimTypes[r] == DimLevelType::kCompressed)
+        nnz[r][parentPos]++;
+      parentPos = parentPos * dimSizes[r] + ind[r];
+    }
+  }
+
+public:
+  /// The type of callback functions which receive an nnz-statistic.
+  typedef const std::function<void(uint64_t)> &NNZConsumer;
+
+  /// Lexicographically enumerates all indicies for dimensions strictly
+  /// less than `stopDim`, and passes their nnz statistic to the callback.
+  /// Since our use case only requires the statistic not the coordinates
+  /// themselves, we do not bother to construct them.
+  void forallIndices(uint64_t stopDim, NNZConsumer yield) const {
+    assert(stopDim < getRank() && "Stopping-dimension is out of bounds");
+    assert(dimTypes[stopDim] == DimLevelType::kCompressed &&
+           "Cannot look up non-compressed dimensions");
+    forallIndices(yield, stopDim, 0, 0);
+  }
+
+private:
+  /// Recursive component of the public `forallIndices`.
+  void forallIndices(NNZConsumer yield, uint64_t stopDim, uint64_t parentPos,
+                     uint64_t d) const {
+    assert(d <= stopDim);
+    if (d == stopDim) {
+      assert(parentPos < nnz[d].size() && "Cursor is out of range");
+      yield(nnz[d][parentPos]);
+    } else {
+      const uint64_t sz = dimSizes[d];
+      const uint64_t pstart = parentPos * sz;
+      for (uint64_t i = 0; i < sz; i++)
+        forallIndices(yield, stopDim, pstart + i, d + 1);
+    }
+  }
+}; // class SparseTensorNNZ
+
+//===----------------------------------------------------------------------===//
 /// A memory-resident sparse tensor using a storage scheme based on
 /// per-dimension sparse/dense annotations. This data structure provides a
 /// bufferized form of a sparse tensor type. In contrast to generating setup
 /// methods for each differently annotated sparse tensor, this method provides
 /// a convenient "one-size-fits-all" solution that simply takes an input tensor
 /// and annotations to implement all required setup in a general manner.
 template <typename P, typename I, typename V>
 class SparseTensorStorage : protected EnumerableSparseTensorStorage<V> {
-public:
-  /// Constructs a sparse tensor storage scheme with the given dimensions,
-  /// permutation, and per-dimension dense/sparse annotations, using
-  /// the coordinate scheme tensor for the initial contents if provided.
+private:
+  /// Partial constructor for sharing code between the public
+  /// constructors.  This only does the bare minimum setup for the
+  /// other constructors, thus the resulting object is not guaranteed
+  /// to be in a valid state.  In particular `isCompressedDim` (and
+  /// hence `appendCurrentPointer` etc) will not work.
   ///
   /// Precondition: `perm` and `sparsity` must be valid for `szs.size()`.
   SparseTensorStorage(const std::vector<uint64_t> &szs, const uint64_t *perm,
-                      const DimLevelType *sparsity,
-                      SparseTensorCOO<V> *tensor = nullptr)
+                      const DimLevelType *sparsity)
       : sizes(szs), rev(getRank()), idx(getRank()), pointers(getRank()),
         indices(getRank()) {
-    uint64_t rank = getRank();
+    // Validate parameters.
+    for (uint64_t rank = getRank(), r = 0; r < rank; r++) {
+      assert(sizes[r] > 0 && "Dimension size zero has trivial storage");
+      assert((sparsity[r] == DimLevelType::kDense ||
+              sparsity[r] == DimLevelType::kCompressed) &&
+             "Unsupported DimLevelType");
+    }
     // Store "reverse" permutation.
-    for (uint64_t r = 0; r < rank; r++)
+    for (uint64_t rank = getRank(), r = 0; r < rank; r++)
       rev[perm[r]] = r;
+  }
+
+public:
+  /// Constructs a sparse tensor storage scheme with the given dimensions,
+  /// permutation, and per-dimension dense/sparse annotations, using
+  /// the coordinate scheme tensor for the initial contents if provided.
+  ///
+  /// Precondition: `perm` and `sparsity` must be valid for `szs.size()`.
+  SparseTensorStorage(const std::vector<uint64_t> &szs, const uint64_t *perm,
+                      const DimLevelType *sparsity, SparseTensorCOO<V> *tensor)
+      : SparseTensorStorage(szs, perm, sparsity) {
     // Provide hints on capacity of pointers and indices.
     // TODO: needs fine-tuning based on sparsity
     bool allDense = true;
     uint64_t sz = 1;
-    for (uint64_t r = 0; r < rank; r++) {
-      assert(sizes[r] > 0 && "Dimension size zero has trivial storage");
+    for (uint64_t rank = getRank(), r = 0; r < rank; r++) {
+      // TODO(wrengr): this division is expensive, but replacing it
+      // with ffs/clz or intrinsics is non-portable; does mlir have
+      // any cpp macro to enable this only when we want "paranoia mode"
+      // for extra debugging info?  Or should we just rely on Clang's
+      // -fsanitize=integer ?
+      assert(sizes[r] <= std::numeric_limits<uint64_t>::max() / sz);
       sz *= sizes[r];
       if (sparsity[r] == DimLevelType::kCompressed) {
         pointers[r].reserve(sz + 1);
         indices[r].reserve(sz);
         sz = 1;
         allDense = false;
-        // Prepare the pointer structure.  We cannot use `appendPointer`
-        // here, because `isCompressedDim` won't work until after this
-        // preparation has been done.
+        // Can't use `isCompressedDim` until after we push this zero.
         pointers[r].push_back(0);
-      } else {
-        assert(sparsity[r] == DimLevelType::kDense &&
-               "singleton not yet supported");
       }
     }
     // Then assign contents from coordinate scheme tensor if provided.
     if (tensor) {
       // Ensure both preconditions of `fromCOO`.
       assert(tensor->getSizes() == sizes && "Tensor size mismatch");
       tensor->sort();
       // Now actually insert the `elements`.
       const std::vector<Element<V>> &elements = tensor->getElements();
       uint64_t nnz = elements.size();
       values.reserve(nnz);
       fromCOO(elements, 0, nnz, 0);
     } else if (allDense) {
       values.resize(sz, 0);
     }
   }
 
+  /// Constructs a sparse tensor storage scheme with the given dimensions,
+  /// permutation, and per-dimension dense/sparse annotations, using
+  /// the given sparse tensor for the initial contents.
+  ///
+  /// Precondition: `perm` and `sparsity` must be valid for `szs.size()`.
+  SparseTensorStorage(const std::vector<uint64_t> &szs, const uint64_t *perm,
+                      const DimLevelType *sparsity,
+                      const EnumerableSparseTensorStorage<V> &tensor)
+      : SparseTensorStorage(szs, perm, sparsity) {
+    SparseTensorEnumerator<V> enumerator(tensor, getRank(), perm);
+    {
+      // Initialize the statistics structure.
+      SparseTensorNNZ nnz(sizes, sparsity);
+      nnz.initialize(enumerator);
+      // Initialize "pointers" overhead (and allocate "indices", "values").
+      uint64_t parentSz = 1; // assembled-size (not dimension-size) of `r-1`.
+      for (uint64_t rank = getRank(), r = 0; r < rank; r++) {
+        if (sparsity[r] == DimLevelType::kCompressed) {
+          pointers[r].reserve(parentSz + 1);
+          // Can't use `isCompressedDim` until after we push this zero.
+          pointers[r].push_back(0);
+          uint64_t currentPos = 0;
+          nnz.forallIndices(r, [this, &currentPos, r](uint64_t n) {
+            currentPos += n;
+            appendPointer(r, currentPos);
+          });
+          assert(pointers[r].size() == parentSz + 1 &&
+                 "Final pointers size doesn't match allocated size");
+          // That assertion entails `assembledSize(parentSz, r)`
+          // is now in a valid state.  That is, `pointers[r][parentSz]`
+          // equals the present value of `currentPos`, which is the
+          // correct assembled-size for `indices[r]`.
+        }
+        // Update assembled-size for the next iteration.
+        parentSz = assembledSize(parentSz, r);
+        // Ideally we need only `indices[r].reserve(parentSz)`, however
+        // the `std::vector` implementation forces us to initialize it too.
+        // That is, in the yieldPos loop we need random-access assignment
+        // to `indices[r]`; however, `std::vector`'s subscript-assignment
+        // only allows assigning to already-initialized positions.
+        if (isCompressedDim(r))
+          indices[r].resize(parentSz, 0);
+      }
+      values.resize(parentSz, 0); // Both allocate and zero-initialize.
+    }                             // end initializing "pointers" overhead
+    // The yieldPos loop
+    enumerator.forallElements([this](const std::vector<uint64_t> &ind, V val) {
+      uint64_t parentSz = 1, parentPos = 0;
+      for (uint64_t rank = getRank(), r = 0; r < rank; r++) {
+        if (isCompressedDim(r)) {
+          // If `parentPos == parentSz` then it's valid as an array-lookup;
+          // however, it's semantically invalid here since that entry
+          // does not represent a segment of `indices[r]`.  Moreover, that
+          // entry must be immutable for `assembledSize` to remain valid.
+          assert(parentPos < parentSz && "Pointers position is out of bounds");
+          const uint64_t currentPos = pointers[r][parentPos];
+          // This increment won't overflow the `P` type, since it can't
+          // exceed the original value of `pointers[r][parentPos+1]`
+          // which was already verified to be within bounds for `P`
+          // when it was written to the array.
+          pointers[r][parentPos]++;
+          writeIndex(r, currentPos, ind[r]);
+          parentPos = currentPos;
+        } else { // if dense
+          parentPos = parentPos * sizes[r] + ind[r];
+        } // else if singleton: the new `parentPos` is the same as the old one.
+        parentSz = assembledSize(parentSz, r);
+      }
+      assert(parentPos < values.size() && "Value position is out of bounds");
+      values[parentPos] = val;
+    }); // end yieldPos loop
+    // The finalizeYieldPos loop
+    for (uint64_t parentSz = 1, rank = getRank(), r = 0; r < rank; r++) {
+      if (isCompressedDim(r)) {
+        assert(parentSz == pointers[r].size() - 1 &&
+               "Actual pointers size doesn't match the expected size");
+        // Can't check all of them, but at least we can check the last one.
+        assert(pointers[r][parentSz - 1] == pointers[r][parentSz] &&
+               "Pointers got corrupted");
+        // TODO: optimize this by using `memmove` or similar.
+        for (uint64_t n = 0; n < parentSz; n++) {
+          uint64_t parentPos = parentSz - n;
+          pointers[r][parentPos] = pointers[r][parentPos - 1];
+        }
+        pointers[r][0] = 0;
+      }
+      parentSz = assembledSize(parentSz, r);
+    } // end finalizeYieldPos loop
+  }
+
+private:
+  /// Computes the assembled-size associated with the `d`-th dimension,
+  /// given the assembled-size associated with the `(d-1)`-th dimension.
+  /// "Assembled-sizes" correspond to the (nominal) sizes of overhead
+  /// storage, as opposed to "dimension-sizes" which are the cardinality
+  /// of coordinates for that dimension.
+  ///
+  /// Precondition: the `pointers[d]` array must be fully initialized
+  /// before calling this method.
+  inline uint64_t assembledSize(uint64_t parentSz, uint64_t d) const {
+    if (isCompressedDim(d))
+      return pointers[d][parentSz];
+    // else if dense:
+    return parentSz * sizes[d];
+    // else if singleton: `return parentSz;`
+  }
+
+public:
   ~SparseTensorStorage() override = default;
 
   /// Get the rank of the tensor.
   uint64_t getRank() const { return sizes.size(); }
 
   /// Get the size of the given dimension of the tensor.
   uint64_t getDimSize(uint64_t d) const override {
     assert(d < getRank());
public:
   }
 
   /// Returns this sparse tensor storage scheme as a new memory-resident
   /// sparse tensor in coordinate scheme with the given dimension order.
   ///
   /// Precondition: `perm` must be valid for `getRank()`.
   SparseTensorCOO<V> *toCOO(const uint64_t *perm) const {
     SparseTensorEnumerator<V> enumerator(*this, getRank(), perm);
     SparseTensorCOO<V> *coo =
         new SparseTensorCOO<V>(enumerator.permutedSizes(), values.size());
     enumerator.forallElements([&coo](const std::vector<uint64_t> &ind, V val) {
       coo->add(ind, val);
     });
     // TODO: This assertion assumes there are no stored zeros,
     // or if there are then that we don't filter them out.
     // Cf., <https://github.com/llvm/llvm-project/issues/54179>
     assert(coo->getElements().size() == values.size());
     return coo;
   }
 
   /// Factory method. Constructs a sparse tensor storage scheme with the given
   /// dimensions, permutation, and per-dimension dense/sparse annotations,
   /// using the coordinate scheme tensor for the initial contents if provided.
   /// In the latter case, the coordinate scheme must respect the same
   /// permutation as is desired for the new sparse tensor storage.
   ///
   /// Precondition: `shape`, `perm`, and `sparsity` must be valid for `rank`.
   static SparseTensorStorage<P, I, V> *
   newSparseTensor(uint64_t rank, const uint64_t *shape, const uint64_t *perm,
-                  const DimLevelType *sparsity, SparseTensorCOO<V> *tensor) {
+                  const DimLevelType *sparsity, SparseTensorCOO<V> *coo) {
     SparseTensorStorage<P, I, V> *n = nullptr;
-    if (tensor) {
-      assert(tensor->getRank() == rank && "Tensor rank mismatch");
-      for (uint64_t r = 0; r < rank; r++)
-        assert(shape[r] == 0 || shape[r] == tensor->getSizes()[perm[r]]);
-      n = new SparseTensorStorage<P, I, V>(tensor->getSizes(), perm, sparsity,
-                                           tensor);
-      delete tensor;
+    if (coo) {
+      const auto &coosz = coo->getSizes();
+      assertPermutedSizesMatchShape(coosz, rank, perm, shape);
+      n = new SparseTensorStorage<P, I, V>(coosz, perm, sparsity, coo);
+      delete coo;
     } else {
       std::vector<uint64_t> permsz(rank);
       for (uint64_t r = 0; r < rank; r++) {
         assert(shape[r] > 0 && "Dimension size zero has trivial storage");
         permsz[perm[r]] = shape[r];
       }
-      n = new SparseTensorStorage<P, I, V>(permsz, perm, sparsity);
+      // We pass the null `coo` to ensure we select the intended constructor.
+      n = new SparseTensorStorage<P, I, V>(permsz, perm, sparsity, coo);
     }
     return n;
   }
 
+  /// Factory method. Constructs a sparse tensor storage scheme with
+  /// the given dimensions, permutation, and per-dimension dense/sparse
+  /// annotations, using the sparse tensor for the initial contents.
+  ///
+  /// Precondition: `shape`, `perm`, and `sparsity` must be valid for `rank`.
+  static SparseTensorStorage<P, I, V> *
+  newSparseTensor(uint64_t rank, const uint64_t *shape, const uint64_t *perm,
+                  const DimLevelType *sparsity,
+                  const EnumerableSparseTensorStorage<V> *source) {
+    assert(source && "Got nullptr for source");
+    SparseTensorEnumerator<V> enumerator(*source, rank, perm);
+    const auto &permsz = enumerator.permutedSizes();
+    assertPermutedSizesMatchShape(permsz, rank, perm, shape);
+    // N.B., do *not* `delete source`.  The other `newSparseTensor`
+    // only deletes the COO because the end-user has no way to do so
+    // themselves.  Whereas if we delete the source here, then we'll
+    // get a double-free error in `delSparseTensor`.
+    return new SparseTensorStorage<P, I, V>(permsz, perm, sparsity, *source);
+  }
+
 private:
   /// Appends the next free position of `indices[d]` to `pointers[d]`.
   /// Thus, when called after inserting the last element of a segment,
   /// it will append the position where the next segment begins.
-  inline void appendPointer(uint64_t d) {
+  inline void appendCurrentPointer(uint64_t d) {
     assert(isCompressedDim(d)); // Entails `d < getRank()`.
-    uint64_t p = indices[d].size();
-    assert(p <= std::numeric_limits<P>::max() &&
+    appendPointer(d, indices[d].size());
+  }
+
+  /// Appends an arbitrary new position to `pointers[d]`.  This method
+  /// checks that `pos` is representable in the `P` type; however, it
+  /// does not check that `pos` is semantically valid (i.e., larger than
+  /// the previous position and smaller than `indices[d].capacity()`).
+  inline void appendPointer(uint64_t d, uint64_t pos) {
+    // This first assertion is trivially satisfied by both use sites;
+    // but we leave it in just to be safe.
+    assert(isCompressedDim(d)); // Entails `d < getRank()`.
+    assert(pos <= std::numeric_limits<P>::max() &&
            "Pointer value is too large for the P-type");
-    pointers[d].push_back(p); // Here is where we convert to `P`.
+    pointers[d].push_back(pos); // Here is where we convert to `P`.
   }
 
-  /// Appends the given index to `indices[d]`.
+  /// Appends the given index to `indices[d]`.  This method checks that
+  /// `i` is representable in the `I` type; however, it does not check
+  /// that `i` is semantically valid (i.e., in bounds for `sizes[d]`
+  /// and not previously occurring in the same segment).
   inline void appendIndex(uint64_t d, uint64_t i) {
     assert(isCompressedDim(d)); // Entails `d < getRank()`.
     assert(i <= std::numeric_limits<I>::max() &&
            "Index value is too large for the I-type");
     indices[d].push_back(i); // Here is where we convert to `I`.
   }
 
+  /// Writes the given coordinate to `indices[d][pos]`.  This method
+  /// checks that `i` is representable in the `I` type; however, it
+  /// does not check that `i` is semantically valid (i.e., in bounds
+  /// for `sizes[d]` and not elsewhere occurring in the same segment).
+  inline void writeIndex(uint64_t d, uint64_t pos, uint64_t i) {
+    // This first assertion trivially entailed at the one call-site;
+    // but just to be safe.
+    assert(isCompressedDim(d)); // Entails `d < getRank()`.
+    // Subscript assignment to `std::vector` requires that the `pos`-th
+    // entry has been initialized; thus we must be sure to check `size()`
+    // here, instead of `capacity()` as would be ideal.
+    assert(pos < indices[d].size() && "Index position is out of bounds");
+    assert(i <= std::numeric_limits<I>::max() &&
+           "Index value is too large for the I-type");
+    indices[d][pos] = i; // Here is where we convert to `I`.
+  }
+
   /// Initializes sparse tensor storage scheme from a memory-resident sparse
   /// tensor in coordinate scheme. This method prepares the pointers and
   /// indices arrays under the given per-dimension dense/sparse annotations.
   ///
   /// Preconditions:
   /// (1) the `elements` must be lexicographically sorted.
   /// (2) the indices of every element are valid for `sizes` (equal rank
   ///     and pointwise less-than).
while (lo < hi) { // If `hi` is unchanged, then `lo < elements.size()`.
         full++;
       }
       fromCOO(elements, lo, seg, d + 1);
       // And move on to next segment in interval.
       lo = seg;
     }
     // Finalize the sparse pointer structure at this dimension.
     if (isCompressedDim(d)) {
-      appendPointer(d);
+      appendCurrentPointer(d);
     } else {
       // For dense storage we must fill in all the zero values after
       // the last element.
       for (uint64_t sz = sizes[d]; full < sz; full++)
         endDim(d + 1);
     }
   }
 
   /// Ends a deeper, never seen before dimension.
   void endDim(uint64_t d) {
     assert(d <= getRank());
     if (d == getRank()) {
       values.push_back(0);
     } else if (isCompressedDim(d)) {
-      appendPointer(d);
+      appendCurrentPointer(d);
     } else {
       for (uint64_t full = 0, sz = sizes[d]; full < sz; full++)
         endDim(d + 1);
     }
   }
 
   /// Wraps up a single insertion path, inner to outer.
   void endPath(uint64_t diff) {
     uint64_t rank = getRank();
     assert(diff <= rank);
     for (uint64_t i = 0; i < rank - diff; i++) {
       uint64_t d = rank - i - 1;
       if (isCompressedDim(d)) {
-        appendPointer(d);
+        appendCurrentPointer(d);
       } else {
         for (uint64_t full = idx[d] + 1, sz = sizes[d]; full < sz; full++)
           endDim(d + 1);
       }
     }
   }
 
   /// Continues a single insertion path, outer to inner.
private:
   const std::vector<uint64_t> sizes; // per-dimension sizes
   std::vector<uint64_t> rev;   // "reverse" permutation
   std::vector<uint64_t> idx;   // index cursor
   std::vector<std::vector<P>> pointers;
   std::vector<std::vector<I>> indices;
   std::vector<V> values;
 };
 
+//===----------------------------------------------------------------------===//
 /// Helper to convert string to lower case.
 static char *toLower(char *token) {
   for (char *c = token; *c; c++)
     *c = tolower(*c);
   return token;
 }
 
 /// Read the MME header of a general sparse matrix of type real.
 static void readMMEHeader(FILE *file, char *filename, char *line,
                           uint64_t *idata, bool *isSymmetric) {
   char header[64];
   char object[64];
   char format[64];
   char field[64];
   char symmetry[64];
static SparseTensorCOO<V> *openSparseTensorCOO(char *filename, uint64_t rank,
   // and the number of nonzeros as initial capacity.
   assert(rank == idata[0] && "rank mismatch");
   uint64_t nnz = idata[1];
   for (uint64_t r = 0; r < rank; r++)
     assert((shape[r] == 0 || shape[r] == idata[2 + r]) &&
            "dimension size mismatch");
   SparseTensorCOO<V> *tensor =
       SparseTensorCOO<V>::newSparseTensorCOO(rank, idata + 2, perm, nnz);
-  //  Read all nonzero elements.
+  // Read all nonzero elements.
   std::vector<uint64_t> indices(rank);
   for (uint64_t k = 0; k < nnz; k++) {
     if (!fgets(line, kColWidth, file)) {
       fprintf(stderr, "Cannot find next line of data in %s\n", filename);
       exit(1);
     }
     char *linePtr = line;
     for (uint64_t r = 0; r < rank; r++) {

 //
 // Some macro magic is used to generate implementations for all required type
 // combinations that can be called from MLIR compiler-generated code.
 //
 //===----------------------------------------------------------------------===//
 
 #define CASE(p, i, v, P, I, V)                                                 \
   if (ptrTp == (p) && indTp == (i) && valTp == (v)) {                          \
-    SparseTensorCOO<V> *tensor = nullptr;                                      \
+    SparseTensorCOO<V> *coo = nullptr;                                         \
     if (action <= Action::kFromCOO) {                                          \
       if (action == Action::kFromFile) {                                       \
         char *filename = static_cast<char *>(ptr);                             \
-        tensor = openSparseTensorCOO<V>(filename, rank, shape, perm);          \
+        coo = openSparseTensorCOO<V>(filename, rank, shape, perm);             \
       } else if (action == Action::kFromCOO) {                                 \
-        tensor = static_cast<SparseTensorCOO<V> *>(ptr);                       \
+        coo = static_cast<SparseTensorCOO<V> *>(ptr);                          \
       } else {                                                                 \
         assert(action == Action::kEmpty);                                      \
       }                                                                        \
       return SparseTensorStorage<P, I, V>::newSparseTensor(rank, shape, perm,  \
+                                                           sparsity, coo);     \
+    }                                                                          \
+    if (action == Action::kSparseToSparse) {                                   \
+      EnumerableSparseTensorStorage<V> *tensor =                               \
+          static_cast<EnumerableSparseTensorStorage<V> *>(ptr);                \
+      return SparseTensorStorage<P, I, V>::newSparseTensor(rank, shape, perm,  \
                                                            sparsity, tensor);  \
     }                                                                          \
     if (action == Action::kEmptyCOO)                                           \
       return SparseTensorCOO<V>::newSparseTensorCOO(rank, shape, perm);        \
-    tensor = static_cast<SparseTensorStorage<P, I, V> *>(ptr)->toCOO(perm);    \
+    coo = static_cast<SparseTensorStorage<P, I, V> *>(ptr)->toCOO(perm);       \
     if (action == Action::kToIterator) {                                       \
-      tensor->startIterator();                                                 \
+      coo->startIterator();                                                    \
     } else {                                                                   \
       assert(action == Action::kToCOO);                                        \
     }                                                                          \
-    return tensor;                                                             \
+    return coo;                                                                \
   }
 
 #define CASE_SECSAME(p, v, P, V) CASE(p, p, v, P, P, V)
 
 #define IMPL_SPARSEVALUES(NAME, TYPE, LIB)                                     \
   void _mlir_ciface_##NAME(StridedMemRefType<TYPE, 1> *ref, void *tensor) {    \
     assert(ref &&tensor);                                                      \
     std::vector<TYPE> *v;                                                      \

mlir/test/Dialect/SparseTensor/conversion.mlir

-// RUN: mlir-opt %s --sparse-tensor-conversion --canonicalize --cse | FileCheck %s
+// First use with `kViaCOO` for sparse2sparse conversion (the old way).
+// RUN: mlir-opt %s --sparse-tensor-conversion="s2s-strategy=1" \
+// RUN:    --canonicalize --cse | FileCheck %s
+//
+// Now again with `kAuto` (the new default)
+// RUN: mlir-opt %s --sparse-tensor-conversion="s2s-strategy=0" \
+// RUN:    --canonicalize --cse | FileCheck %s -check-prefix=CHECKAUTO
 
 #SparseVector = #sparse_tensor.encoding<{
   dimLevelType = ["compressed"]
 }>
 
 #SparseVector64 = #sparse_tensor.encoding<{
   dimLevelType = ["compressed"],
   pointerBitWidth = 64,

 //       CHECK: return %[[T]] : !llvm.ptr<i8>
 func @sparse_convert_1d(%arg0: tensor<?xi32>) -> tensor<?xi32, #SparseVector> {
   %0 = sparse_tensor.convert %arg0 : tensor<?xi32> to tensor<?xi32, #SparseVector>
   return %0 : tensor<?xi32, #SparseVector>
 }
 
 // CHECK-LABEL: func @sparse_convert_1d_ss(
 //  CHECK-SAME: %[[A:.*]]: !llvm.ptr<i8>)
+//   CHECK-DAG: %[[ToCOO:.*]] = arith.constant 5 : i32
+//   CHECK-DAG: %[[FromCOO:.*]] = arith.constant 2 : i32
 //   CHECK-DAG: %[[P:.*]] = memref.alloca() : memref<1xi8>
 //   CHECK-DAG: %[[Q:.*]] = memref.alloca() : memref<1xindex>
 //   CHECK-DAG: %[[R:.*]] = memref.alloca() : memref<1xindex>
 //   CHECK-DAG: %[[X:.*]] = memref.cast %[[P]] : memref<1xi8> to memref<?xi8>
 //   CHECK-DAG: %[[Y:.*]] = memref.cast %[[Q]] : memref<1xindex> to memref<?xindex>
 //   CHECK-DAG: %[[Z:.*]] = memref.cast %[[R]] : memref<1xindex> to memref<?xindex>
-//       CHECK: %[[C:.*]] = call @newSparseTensor(%[[X]], %[[Y]], %[[Z]], %{{.*}}, %{{.*}}, %{{.*}}, %{{.*}}, %[[A]])
-//       CHECK: %[[T:.*]] = call @newSparseTensor(%[[X]], %[[Y]], %[[Z]], %{{.*}}, %{{.*}}, %{{.*}}, %{{.*}}, %[[C]])
+//       CHECK: %[[C:.*]] = call @newSparseTensor(%[[X]], %[[Y]], %[[Z]], %{{.*}}, %{{.*}}, %{{.*}}, %[[ToCOO]], %[[A]])
+//       CHECK: %[[T:.*]] = call @newSparseTensor(%[[X]], %[[Y]], %[[Z]], %{{.*}}, %{{.*}}, %{{.*}}, %[[FromCOO]], %[[C]])
 //       CHECK: return %[[T]] : !llvm.ptr<i8>
+// CHECKAUTO-LABEL: func @sparse_convert_1d_ss(
+//  CHECKAUTO-SAME: %[[A:.*]]: !llvm.ptr<i8>)
+//   CHECKAUTO-DAG: %[[SparseToSparse:.*]] = arith.constant 3 : i32
+//   CHECKAUTO-DAG: %[[P:.*]] = memref.alloca() : memref<1xi8>
+//   CHECKAUTO-DAG: %[[Q:.*]] = memref.alloca() : memref<1xindex>
+//   CHECKAUTO-DAG: %[[R:.*]] = memref.alloca() : memref<1xindex>
+//   CHECKAUTO-DAG: %[[X:.*]] = memref.cast %[[P]] : memref<1xi8> to memref<?xi8>
+//   CHECKAUTO-DAG: %[[Y:.*]] = memref.cast %[[Q]] : memref<1xindex> to memref<?xindex>
+//   CHECKAUTO-DAG: %[[Z:.*]] = memref.cast %[[R]] : memref<1xindex> to memref<?xindex>
+//       CHECKAUTO: %[[T:.*]] = call @newSparseTensor(%[[X]], %[[Y]], %[[Z]], %{{.*}}, %{{.*}}, %{{.*}}, %[[SparseToSparse]], %[[A]])
+//       CHECKAUTO: return %[[T]] : !llvm.ptr<i8>
 func @sparse_convert_1d_ss(%arg0: tensor<?xf32, #SparseVector64>) -> tensor<?xf32, #SparseVector32> {
   %0 = sparse_tensor.convert %arg0 : tensor<?xf32, #SparseVector64> to tensor<?xf32, #SparseVector32>
   return %0 : tensor<?xf32, #SparseVector32>
 }
 
 // CHECK-LABEL: func @sparse_convert_2d(
 //  CHECK-SAME: %[[A:.*]]: tensor<2x4xf64>) -> !llvm.ptr<i8>
 //   CHECK-DAG: %[[C0:.*]] = arith.constant 0 : index

mlir/test/Integration/Dialect/SparseTensor/CPU/sparse_conversion_sparse2sparse.mlir

+// Force this file to use the kDirect method for sparse2sparse
+// RUN: mlir-opt %s --sparse-compiler="s2s-strategy=2" | \
+// RUN: mlir-cpu-runner -e entry -entry-point-result=void \
+// RUN:  -shared-libs=%mlir_integration_test_dir/libmlir_c_runner_utils%shlibext | \
+// RUN: FileCheck %s
+
+#Tensor1 = #sparse_tensor.encoding<{
+  dimLevelType = [ "dense", "dense", "compressed" ]
+}>
+
+// NOTE: dense after compressed is not currently supported for the target
+// of direct-sparse2sparse conversion.  (It's fine for the source though.)
+#Tensor2 = #sparse_tensor.encoding<{
+  dimLevelType = [ "dense", "compressed", "dense" ]
+}>
+
+#Tensor3 = #sparse_tensor.encoding<{
+  dimLevelType = [ "dense", "dense", "compressed" ],
+  dimOrdering = affine_map<(i,j,k) -> (i,k,j)>
+}>
+
+module {
+  //
+  // Utilities for output and releasing memory.
+  //
+  func @dump(%arg0: tensor<2x3x4xf64>) {
+    %c0 = arith.constant 0 : index
+    %d0 = arith.constant -1.0 : f64
+    %0 = vector.transfer_read %arg0[%c0, %c0, %c0], %d0: tensor<2x3x4xf64>, vector<2x3x4xf64>
+    vector.print %0 : vector<2x3x4xf64>
+    return
+  }
+  func @dumpAndRelease_234(%arg0: tensor<2x3x4xf64>) {
+    call @dump(%arg0) : (tensor<2x3x4xf64>) -> ()
+    %1 = bufferization.to_memref %arg0 : memref<2x3x4xf64>
+    memref.dealloc %1 : memref<2x3x4xf64>
+    return
+  }
+
+  //
+  // Main driver.
+  //
+  func @entry() {
+    //
+    // Initialize a 3-dim dense tensor.
+    //
+    %src = arith.constant dense<[
+       [  [  1.0,  2.0,  3.0,  4.0 ],
+          [  5.0,  6.0,  7.0,  8.0 ],
+          [  9.0, 10.0, 11.0, 12.0 ] ],
+       [  [ 13.0, 14.0, 15.0, 16.0 ],
+          [ 17.0, 18.0, 19.0, 20.0 ],
+          [ 21.0, 22.0, 23.0, 24.0 ] ]
+    ]> : tensor<2x3x4xf64>
+
+    //
+    // Convert dense tensor directly to various sparse tensors.
+    //
+    %s1 = sparse_tensor.convert %src : tensor<2x3x4xf64> to tensor<2x3x4xf64, #Tensor1>
+    %s2 = sparse_tensor.convert %src : tensor<2x3x4xf64> to tensor<2x3x4xf64, #Tensor2>
+    %s3 = sparse_tensor.convert %src : tensor<2x3x4xf64> to tensor<2x3x4xf64, #Tensor3>
+
+    //
+    // Convert sparse tensor directly to another sparse format.
+    //
+    %t13 = sparse_tensor.convert %s1 : tensor<2x3x4xf64, #Tensor1> to tensor<2x3x4xf64, #Tensor3>
+    %t21 = sparse_tensor.convert %s2 : tensor<2x3x4xf64, #Tensor2> to tensor<2x3x4xf64, #Tensor1>
+    %t23 = sparse_tensor.convert %s2 : tensor<2x3x4xf64, #Tensor2> to tensor<2x3x4xf64, #Tensor3>
+    %t31 = sparse_tensor.convert %s3 : tensor<2x3x4xf64, #Tensor3> to tensor<2x3x4xf64, #Tensor1>
+
+    //
+    // Convert sparse tensor back to dense.
+    //
+    %d13 = sparse_tensor.convert %t13 : tensor<2x3x4xf64, #Tensor3> to tensor<2x3x4xf64>
+    %d21 = sparse_tensor.convert %t21 : tensor<2x3x4xf64, #Tensor1> to tensor<2x3x4xf64>
+    %d23 = sparse_tensor.convert %t23 : tensor<2x3x4xf64, #Tensor3> to tensor<2x3x4xf64>
+    %d31 = sparse_tensor.convert %t31 : tensor<2x3x4xf64, #Tensor1> to tensor<2x3x4xf64>
+
+    //
+    // Check round-trip equality.  And release dense tensors.
+    //
+    // CHECK-COUNT-5: ( ( ( 1, 2, 3, 4 ), ( 5, 6, 7, 8 ), ( 9, 10, 11, 12 ) ), ( ( 13, 14, 15, 16 ), ( 17, 18, 19, 20 ), ( 21, 22, 23, 24 ) ) )
+    call @dump(%src) : (tensor<2x3x4xf64>) -> ()
+    call @dumpAndRelease_234(%d13) : (tensor<2x3x4xf64>) -> ()
+    call @dumpAndRelease_234(%d21) : (tensor<2x3x4xf64>) -> ()
+    call @dumpAndRelease_234(%d23) : (tensor<2x3x4xf64>) -> ()
+    call @dumpAndRelease_234(%d31) : (tensor<2x3x4xf64>) -> ()
+
+    //
+    // Release sparse tensors.
+    //
+    sparse_tensor.release %t13 : tensor<2x3x4xf64, #Tensor3>
+    sparse_tensor.release %t21 : tensor<2x3x4xf64, #Tensor1>
+    sparse_tensor.release %t23 : tensor<2x3x4xf64, #Tensor3>
+    sparse_tensor.release %t31 : tensor<2x3x4xf64, #Tensor1>
+    sparse_tensor.release %s1 : tensor<2x3x4xf64, #Tensor1>
+    sparse_tensor.release %s2 : tensor<2x3x4xf64, #Tensor2>
+    sparse_tensor.release %s3 : tensor<2x3x4xf64, #Tensor3>
+
+    return
+  }
+}

mlir/test/Integration/Dialect/SparseTensor/python/test_stress.py

def main():
 
   # CHECK-LABEL: TEST: test_stress
   print("\nTEST: test_stress")
   with ir.Context() as ctx, ir.Location.unknown():
     par = 0
     vec = 0
     vl = 1
     e = False
+    # Disable direct sparse2sparse conversion, because it doubles the time!
+    # TODO: While direct s2s is far too slow for per-commit testing,
+    # we should have some framework ensure that we run this test with
+    # `s2s=0` on a regular basis, to ensure that it does continue to work.
+    s2s = 1
     sparsification_options = (
         f'parallelization-strategy={par} '
         f'vectorization-strategy={vec} '
         f'vl={vl} '
-        f'enable-simd-index32={e}')
+        f'enable-simd-index32={e} '
+        f's2s-strategy={s2s}')
     compiler = sparse_compiler.SparseCompiler(options=sparsification_options)
     f64 = ir.F64Type.get()
     # Be careful about increasing this because
     #     len(types) = 1 + 2^rank * rank! * len(bitwidths)^2
     shape = range(2, 6)
     rank = len(shape)
     # All combinations.
     levels = list(itertools.product(*itertools.repeat(