MemoryOverlap.cpp

#include <ATen/MemoryOverlap.h>
#include <ATen/core/TensorBase.h>
#include <c10/core/Layout.h>
#include <c10/util/irange.h>

namespace at {

MemOverlap has_internal_overlap(const TensorBase& tensor) {
  return has_internal_overlap(tensor.unsafeGetTensorImpl());
}

MemOverlap has_internal_overlap(TensorImpl* t) {
  TORCH_INTERNAL_ASSERT_DEBUG_ONLY(t->layout() == kStrided);

  if (t->is_non_overlapping_and_dense()) {
    return MemOverlap::No;
  }

  auto strides = t->sym_strides();
  auto sizes = t->sym_sizes();
  for (const auto i : c10::irange(strides.size())) {
    // NB: The size oblivious test is written very carefully here.  When
    // unbacked SymInts are involved, we should try to conservatively report
    // if memory overlap /could/ happen under some setting of unbacked
    // SymInts.  Thus, if I have u0 size, we should assume that this has > 1
    // elements (first expression), but if I have a u0 stride, I should NOT
    // assume that it is not zero (second expression)
    if (TORCH_GUARD_SIZE_OBLIVIOUS(sizes[i].sym_gt(1)) && strides[i] == 0) {
      return MemOverlap::Yes;
    }
  }

  return MemOverlap::TooHard;
}

void assert_no_internal_overlap(const TensorBase& t) {
  assert_no_internal_overlap(t.unsafeGetTensorImpl());
}

void assert_no_internal_overlap(TensorImpl* t) {
  TORCH_CHECK(has_internal_overlap(t) != MemOverlap::Yes,
    "unsupported operation: more than one element of the written-to tensor "
    "refers to a single memory location. Please clone() the tensor before "
    "performing the operation.");
}

MemOverlapStatus get_overlap_status(const TensorBase& a, const TensorBase& b) {
  return get_overlap_status(a.unsafeGetTensorImpl(), b.unsafeGetTensorImpl());
}

MemOverlapStatus get_overlap_status(const TensorImpl* a, const TensorImpl* b) {
  if (a == b) return MemOverlapStatus::Full;
  if (a->numel() == 0 || b->numel() == 0) {
    return MemOverlapStatus::No;
  }
  if (!a->is_non_overlapping_and_dense() || !b->is_non_overlapping_and_dense()) {
    return MemOverlapStatus::TooHard;
  }
  // Test for storage equality, rather than pointer equality.
  // This reduces precision, but if people are aliasing the
  // same pointer across multiple storages there are many
  // similar situations (e.g., storage().data() == storage().data()+1)
  // which we will miss.
  auto a_storage = a->unsafe_storage();
  if (a_storage && a_storage.is_alias_of(b->unsafe_storage())) {
    const auto a_begin = static_cast<const char*>(a->data());
    const auto a_end = a_begin + a->numel() * a->itemsize();
    const auto b_begin = static_cast<const char*>(b->data());
    const auto b_end = b_begin + b->numel() * b->itemsize();

    if (a_begin == b_begin && a_end == b_end) {
      return (a->strides() == b->strides()) ?
          MemOverlapStatus::Full : MemOverlapStatus::Partial;
    }
    if (a_begin < b_end && b_begin < a_end) {
      return MemOverlapStatus::Partial;
    }
  }
  return MemOverlapStatus::No;
}

void assert_no_partial_overlap(const TensorBase& a, const TensorBase& b) {
  assert_no_partial_overlap(a.unsafeGetTensorImpl(), b.unsafeGetTensorImpl());
}

void assert_no_partial_overlap(TensorImpl* a, TensorImpl* b) {
  TORCH_CHECK(get_overlap_status(a, b) != MemOverlapStatus::Partial,
    "unsupported operation: some elements of the input tensor and "
    "the written-to tensor refer to a single memory location. "
    "Please clone() the tensor before performing the operation.");
}

void assert_no_overlap(const TensorBase& a, const TensorBase& b) {
  assert_no_overlap(a.unsafeGetTensorImpl(), b.unsafeGetTensorImpl());
}

void assert_no_overlap(TensorImpl* a, TensorImpl* b) {
  const auto lap = get_overlap_status(a, b);
  TORCH_CHECK(lap != MemOverlapStatus::Partial && lap != MemOverlapStatus::Full,
    "unsupported operation: some elements of the input tensor and "
    "the written-to tensor refer to a single memory location. "
    "Please clone() the tensor before performing the operation.");
}

}