change lod tensor to absolute offsets (#4952)

paddle/framework/lod_tensor.cc

@@ -25,31 +25,50 @@ LoD SliceLevels(const LoD& in, size_t level_begin, size_t level_end) {
   for (size_t i = level_begin; i < level_end; i++) {
     new_lod.emplace_back(in.at(i));
   }
+  // transform the lowest level to absolute offset.
+  LoD abs_offset_lod = ToAbsOffset(in);
+  new_lod.back() = abs_offset_lod[level_end - 1];
   return new_lod;
 }
 
 LoD SliceInLevel(const LoD& in, size_t level, size_t elem_begin,
                  size_t elem_end) {
-  // slice the lod.
-  LoD new_lod;
-  new_lod.reserve(in.size() - level);
-  auto start = in.at(level)[elem_begin];
-  auto end = in.at(level)[elem_end];
-
-  for (auto it = in.begin() + level; it != in.end(); it++) {
-    auto it_begin = std::find(it->begin(), it->end(), start);
-    auto it_end = std::find(it_begin, it->end(), end);
-    PADDLE_ENFORCE(it_begin != it->end(), "error in parsing lod info");
-    PADDLE_ENFORCE(it_end != it->end(), "error in parsing lod info");
-    new_lod.emplace_back(it_begin, it_end + 1);
-    // reset offset if tensor is copyed and sliced.
-    std::transform(new_lod.back().begin(), new_lod.back().end(),
-                   new_lod.back().begin(),
-                   [start](int v) { return v - start; });
-    PADDLE_ENFORCE_EQ(new_lod.back().front(), 0, "error in slice LoD");
+  PADDLE_ENFORCE_LT(level, in.size());
+  PADDLE_ENFORCE_LT(elem_end, in[level].size());
+
+  LoD res;
+  res.resize(in.size() - level);
+  // copy the first level
+  res[0].assign(in[level].begin() + elem_begin,
+                in[level].begin() + elem_end + 1);
+  for (size_t lvl = 1; lvl < res.size(); lvl++) {
+    const auto& in_level = in[level + lvl];
+    const auto& above_level = res[lvl - 1];
+    auto& out_level = res[lvl];
+    out_level.assign(in_level.begin() + above_level.front(),
+                     in_level.begin() + above_level.back() + 1);
   }
-  PADDLE_ENFORCE_LE(new_lod.size(), in.size());
-  return new_lod;
+  for (size_t lvl = 0; lvl < res.size(); lvl++) {
+    // to make the first offset equals 0, all the elements minus the first
+    // element
+    size_t front = res[lvl].front();
+    for (auto& ele : res[lvl]) {
+      ele -= front;
+    }
+  }
+  return res;
+}
+
+LoD ToAbsOffset(const LoD& in) {
+  // the lowest level stores relative offsets
+  if (in.empty() || in.size() == 1) return in;
+  LoD result = in;
+  for (int level = result.size() - 2; level >= 0; level--) {
+    for (auto& ele : result[level]) {
+      ele = result[level + 1][ele];
+    }
+  }
+  return result;
 }
 
 bool operator==(const LoD& a, const LoD& b) {
@@ -75,17 +94,7 @@ bool operator==(const LoD& a, const LoD& b) {
 size_t LoDTensor::NumElements(size_t level, size_t idx) const {
   PADDLE_ENFORCE_LT(level, NumLevels());
   PADDLE_ENFORCE_LT(idx, NumElements(level));
-  // the last level of LoD, just return number of records in Tensor
-  if (level == NumLevels() - 1) {
-    return lod_[level][idx + 1] - lod_[level][idx];
-  }
-  // high level of LoD, and there is another lower level, return number of
-  // lower-level elements
-  auto tmp = SliceInLevel(lod_, level, idx, idx + 1);
-  PADDLE_ENFORCE_GE(tmp.size(), 2);
-  // there is a 0 as a placeholder stored in LoD, so the number of elements
-  // equals lod.size() - 1
-  return tmp[1].size() - 1;
+  return lod_[level][idx + 1] - lod_[level][idx];
 }
 
 void LoDTensor::ShrinkLevels(size_t level_begin, size_t level_end) {

paddle/framework/lod_tensor.h

@@ -39,23 +39,36 @@ using Vector = thrust::host_vector<
 #endif
 
 /*
- * 3-level LoD stores
+ * LoD is short for Level of Details.
  *
- * 0 10 20
- * 0 5 10 15 20
- * 0 2 5 7 10 12 15 20
- *
- * - in a level, each element indicates offset in the underlying Tensor
+ * - in a level, each element indicates relative offset of the lower level
  * - the first element should be 0 and that indicates that this sequence start
  * from 0
  * - each sequence's begin and end(no-inclusive) is level[id, id+1]
+ *
+ * For example:
+ *    3-level LoD stores
+ *
+ *    0 2 3
+ *    0 2 4 7
+ *    0 2 5 7 10 12 15 20
  */
 using LoD = std::vector<Vector<size_t>>;
 
+/*
+ * Slice levels from a LoD.
+ * NOTE the lowest level should always be the absolute offsets of the underlying
+ * tensor instances. So if higher layers are sliced without the lowest level,
+ * the lower level of the sliced LoD will be transformed to the absolute offset.
+ */
 LoD SliceLevels(const LoD& in, size_t level_begin, size_t level_end);
 
 LoD SliceInLevel(const LoD& in, size_t level, size_t elem_begin,
                  size_t elem_end);
+/*
+ * Transform an LoD from relative offsets to absolute offsets.
+ */
+LoD ToAbsOffset(const LoD& in);
 
 bool operator==(const LoD& a, const LoD& b);
 

paddle/framework/lod_tensor_test.cc

@@ -30,8 +30,8 @@ class LoDTensorTester : public ::testing::Test {
     // 0 5 10 15 20
     // 0 2 5 7 10 12 15 20
     LoD lod;
-    lod.push_back(std::vector<size_t>{0, 10, 20});
-    lod.push_back(std::vector<size_t>{0, 5, 10, 15, 20});
+    lod.push_back(std::vector<size_t>{0, 2, 3});
+    lod.push_back(std::vector<size_t>{0, 2, 5, 8});
     lod.push_back(std::vector<size_t>{0, 2, 5, 7, 10, 12, 15, 17, 20});
 
     ASSERT_EQ(lod.size(), 3UL);
@@ -52,14 +52,14 @@ TEST_F(LoDTensorTester, NumLevels) { ASSERT_EQ(lod_tensor_.NumLevels(), 3UL); }
 
 TEST_F(LoDTensorTester, NumElements) {
   ASSERT_EQ(lod_tensor_.NumElements(0), 2UL);
-  ASSERT_EQ(lod_tensor_.NumElements(1), 4UL);
+  ASSERT_EQ(lod_tensor_.NumElements(1), 3UL);
   ASSERT_EQ(lod_tensor_.NumElements(2), 8UL);
 }
 
 TEST_F(LoDTensorTester, NumElements2) {
   ASSERT_EQ(lod_tensor_.NumElements(0, 0), 2UL);
-  ASSERT_EQ(lod_tensor_.NumElements(0, 1), 2UL);
-  ASSERT_EQ(lod_tensor_.NumElements(1, 1), 2UL);
+  ASSERT_EQ(lod_tensor_.NumElements(0, 1), 1UL);
+  ASSERT_EQ(lod_tensor_.NumElements(1, 1), 3UL);
 }
 
 TEST_F(LoDTensorTester, ShrinkLevels) {
@@ -68,37 +68,36 @@ TEST_F(LoDTensorTester, ShrinkLevels) {
     LoDTensor new_lod_tensor = lod_tensor_;
     new_lod_tensor.ShrinkLevels(level, level + 1);
     ASSERT_EQ(new_lod_tensor.NumLevels(), 1UL);
-    ASSERT_EQ(new_lod_tensor.NumElements(0), lod_tensor_.NumElements(level));
     ASSERT_EQ(new_lod_tensor.data<float>(), lod_tensor_.data<float>());
   }
   // shrink 2 level
   for (size_t level = 0; level < 2UL; ++level) {
     LoDTensor new_lod_tensor = lod_tensor_;
     new_lod_tensor.ShrinkLevels(level, level + 2);
+    // the lowest level's last element should be the tensor's batch_size.
+    ASSERT_EQ(new_lod_tensor.lod().back().back(),
+              lod_tensor_.lod().back().back());
     ASSERT_EQ(new_lod_tensor.NumLevels(), 2UL);
-    ASSERT_EQ(new_lod_tensor.NumElements(0), lod_tensor_.NumElements(level));
-    ASSERT_EQ(new_lod_tensor.NumElements(1),
-              lod_tensor_.NumElements(level + 1));
     ASSERT_EQ(new_lod_tensor.data<float>(), lod_tensor_.data<float>());
   }
 }
 
 TEST_F(LoDTensorTester, ShrinkInLevel) {
   size_t level = 0;
   LoDTensor new_lod_tensor = lod_tensor_;
-  new_lod_tensor.ShrinkInLevel(level, 0, 2);
+  new_lod_tensor.ShrinkInLevel(level, 0, 1);
   EXPECT_EQ(new_lod_tensor.NumLevels(), 3UL);
-  EXPECT_EQ(new_lod_tensor.NumElements(0), 2UL);
-  EXPECT_EQ(new_lod_tensor.NumElements(1), 4UL);
-  EXPECT_EQ(new_lod_tensor.NumElements(2), 8UL);
+  EXPECT_EQ(new_lod_tensor.NumElements(0), 1UL);
+  EXPECT_EQ(new_lod_tensor.NumElements(1), 2UL);
+  EXPECT_EQ(new_lod_tensor.NumElements(2), 5UL);
   ASSERT_EQ(new_lod_tensor.data<float>(), lod_tensor_.data<float>());
 
   level = 1;
   new_lod_tensor = lod_tensor_;
-  new_lod_tensor.ShrinkInLevel(level, 0, 2);
+  new_lod_tensor.ShrinkInLevel(level, 1, 2);
   ASSERT_EQ(new_lod_tensor.NumLevels(), 2UL);
-  ASSERT_EQ(new_lod_tensor.NumElements(0), 2UL);
-  ASSERT_EQ(new_lod_tensor.NumElements(1), 4UL);
+  ASSERT_EQ(new_lod_tensor.NumElements(0), 1UL);
+  ASSERT_EQ(new_lod_tensor.NumElements(1), 3UL);
   ASSERT_EQ(new_lod_tensor.data<float>(), lod_tensor_.data<float>());
 }