Merge up-cast, ops, down-cast sequences as minimal units of segments

csrc/fusion_segmenter.cpp

@@ -3586,6 +3586,210 @@ bool CombineReductions::shouldRun(
   return false;
 }
 
+// This preprocessing attempts to find groups of exprs consist of an
+// up-cast, followed by some ops and ended by a downcast. It is highly
+// likely that such sequences of ops should never be segmented
+// out. This is particularly commonly seen in fusions given by Thunder
+// as it inserts fine-grained downcasting and upcasting ops. Without
+// this preprocessing, a fusion may be segmented right after an
+// up-cast op, for example, and in fact it happened quitely frequently
+// in some of the RoPE cases. This preprocessing does not completely
+// avoid such segmentation boundaries, but it should become less
+// likely. See also https://github.com/NVIDIA/Fuser/pull/3699.
+class MergeUpAndDownCast {
+ public:
+  static void run(SegmentCandidateFinder* segment_candidate_finder) {
+    MergeUpAndDownCast group_cast(segment_candidate_finder);
+  }
+
+ private:
+  MergeUpAndDownCast(SegmentCandidateFinder* segment_candidate_finder)
+      : segment_candidate_finder_(segment_candidate_finder) {
+    merge();
+  }
+
+  void merge() {
+    bool merged = true;
+    while (merged) {
+      merged = false;
+      std::unordered_set<SegmentedGroup*> considered_groups;
+
+      for (SegmentedGroup* group : segment_candidate_finder_->groups()) {
+        // If the group is an up-cast group, see if there's a
+        // candidate group starting with the group.
+        if (!isUpCast(group) || considered_groups.count(group)) {
+          continue;
+        }
+
+        auto groups_to_merge = getCandidateCastGroup(group);
+        if (groups_to_merge.size() < 2) {
+          continue;
+        }
+
+        for (auto group : groups_to_merge) {
+          considered_groups.insert(group);
+        }
+
+        // Try merging the detected group
+        if (mergeCastGroup(groups_to_merge)) {
+          merged = true;
+          break;
+        }
+      }
+    }
+  }
+
+  // Try to detect a set of groups that could be merged as a cast
+  // group. The analysis starts with an initial group that solely
+  // consists of an up-cast expression. From the initial group, it
+  // traverses its neighbor groups. If the group is an down-cast group,
+  // it only traverses through the consumer edges. If it's an up-cast
+  // group, it only traverses through the producer edges.
+  //
+  // Additionaly, this traversal has several safeguards to keep the
+  // DAG property intact:
+  //
+  // - For a given group, it does not visit its consumers if it has
+  //   multiple consumers, even if the group is not a down-cast
+  //   group.
+  // - Similarly, it does not visit a producer if the producer has
+  //   multiple cosumers.
+  //
+  // The basic form of this set of groups should look like an up-cast
+  // group, followed by some op groups and ended by a down-cast
+  // group. However, it is not always the case because of the above
+  // safeguards. For example, the following groups would be detected
+  // as a cast group.
+  //
+  // t1 = bf16ToFp32(t0)
+  // t2 = neg(t1)
+  // t3 = sin(t2)
+  // t4 = cos(t2)
+  // t5 = fp32ToBf16(t3)
+  // t6 = fp32ToBf16(t4)
+  //
+  // In this case, t1 and t2 would be detected as a candidate group,
+  // but t3 and t4 would not be included. While we could certainly
+  // extend the analysis, it would need to make sure the DAG property
+  // is not violated.
+  std::vector<SegmentedGroup*> getCandidateCastGroup(
+      SegmentedGroup* initial_group) {
+    std::vector<SegmentedGroup*> groups_to_merge;
+    std::unordered_set<SegmentedGroup*> groups_to_merge_set;
+
+    std::deque<SegmentedGroup*> to_visit;
+    to_visit.push_back(initial_group);
+
+    while (!to_visit.empty()) {
+      SegmentedGroup* group = to_visit.front();
+      to_visit.pop_front();
+
+      if (groups_to_merge_set.count(group)) {
+        continue;
+      }
+
+      // For simplicity, all groups are assumed to be the initial
+      // single-expr groups. Skip if not
+
+      groups_to_merge.push_back(group);
+      groups_to_merge_set.insert(group);
+
+      // Consumer traversal. Stop if this group is a down cast
+      // group. Also stop if there are multiple consumer edges to
+      // simplify keeping the DAG property.
+      if (!isDownCast(group) && group->consumer_edges.size() == 1) {
+        auto consumer_edge = group->consumer_edges.at(0);
+        SegmentedGroup* consumer_group = consumer_edge->to;
+        if (!groups_to_merge_set.count(consumer_group)) {
+          to_visit.push_back(consumer_group);
+        }
+      }
+
+      if (!isUpCast(group)) {
+        for (const auto producer_edge : group->producer_edges) {
+          SegmentedGroup* producer_group = producer_edge->from;
+          // Don't add producers that have more than multiple consumers
+          if (producer_group->consumer_edges.size() > 1) {
+            continue;
+          }
+          if (!groups_to_merge_set.count(producer_group)) {
+            to_visit.push_back(producer_group);
+          }
+        }
+      }
+    }
+
+    return groups_to_merge;
+  }
+
+  // Try merging a candidate cast group. Return a merged group if merged.
+  SegmentedGroup* mergeCastGroup(const std::vector<SegmentedGroup*>& groups) {
+    auto sched_type = tryMerge(
+        segment_candidate_finder_->segmented_fusion_.get(),
+        segment_candidate_finder_->runtimeInfo(),
+        groups);
+
+    if (sched_type == SchedulerType::None) {
+      return nullptr;
+    }
+
+    auto joined_group = segment_candidate_finder_->mergeAllGivenGroups(groups);
+
+    return joined_group;
+  }
+
+  bool isUpCast(SegmentedGroup* group) const {
+    if (auto precision_bits = getProducerConsumerPrecision(group);
+        precision_bits.has_value()) {
+      return precision_bits->first < precision_bits->second;
+    } else {
+      return false;
+    }
+  }
+
+  bool isDownCast(SegmentedGroup* group) const {
+    if (auto precision_bits = getProducerConsumerPrecision(group);
+        precision_bits.has_value()) {
+      return precision_bits->first > precision_bits->second;
+    } else {
+      return false;
+    }
+  }
+
+  std::optional<std::pair<int64_t, int64_t>> getProducerConsumerPrecision(
+      SegmentedGroup* group) const {
+    if (group->exprs().size() != 1) {
+      return std::nullopt;
+    }
+
+    auto uop = dynamic_cast<UnaryOp*>(group->exprs().front());
+    if (uop == nullptr || uop->getUnaryOpType() != UnaryOpType::Cast) {
+      return std::nullopt;
+    }
+
+    auto inp_tv = ir_utils::getTvInput(uop);
+    auto out_tv = ir_utils::getTvOutput(uop);
+    if (inp_tv == nullptr || out_tv == nullptr) {
+      return std::nullopt;
+    }
+
+    auto inp_dtype = inp_tv->dtype().type;
+    auto out_dtype = out_tv->dtype().type;
+    auto inp_prim_type = std::get_if<PrimDataType>(&inp_dtype);
+    auto out_prim_type = std::get_if<PrimDataType>(&out_dtype);
+
+    if (inp_prim_type == nullptr || out_prim_type == nullptr) {
+      return std::nullopt;
+    }
+
+    return std::make_pair(
+        primDataTypeSize(*inp_prim_type), primDataTypeSize(*out_prim_type));
+  }
+
+ private:
+  SegmentCandidateFinder* segment_candidate_finder_ = nullptr;
+};
+
 namespace {
 
 //! Returns true if group1 and group2 are an immediate producer-consumer pair.
@@ -3945,6 +4149,9 @@ void SegmentCandidateFinder::findSegments() {
   removeScalarEdges();
 
   // Run pre-merge heuristics
+  MergeUpAndDownCast::run(this);
+  segmented_fusion_->validateIfDebug(true);
+
   if (options_.run_combine_reductions && CombineReductions::shouldRun(this)) {
     CombineReductions::run(this);
   }

csrc/fusion_segmenter.h

@@ -488,6 +488,7 @@ class GroupDependencyAnalysis;
 
 // Manual node merging passes
 class CombineReductions;
+class MergeUpAndDownCast;
 
 //! Options to configure/debug candidate finder
 struct SegmentCandidateFinderOptions {
@@ -691,6 +692,7 @@ class SegmentCandidateFinder {
   //!  eventually should have a dedicated interface
   //!  instead of keeping adding friends
   friend class CombineReductions;
+  friend class MergeUpAndDownCast;
 
   //! options to configure and debug the segment process
   SegmentCandidateFinderOptions options_;