Adding sibling path for MaxInfoSpanningTree (#1776)

The sibling path is required to generate consistent replay for some cases where `MaxInfoSpanningTree` is used with a selector. For example, when the producer of a Welford is excluded from the propagation section. See test `FusionTransformPropagateSelectorSibling_CUDA` for a detailed example. Besides, since we know that siblings should be transformed exactly the same, the sibling path is a perfect next hop for preserving information.

If you want a spanning tree without a sibling path, you can override `allowSibling` as `return false` in your selector;

torch/csrc/jit/codegen/cuda/ir_utils.cpp

@@ -529,6 +529,22 @@ TORCH_CUDA_CU_API std::vector<Val*> consumerValsOf(Val* val) {
   return uniqueEntries<Val>(consumer_vals);
 }
 
+// Return immediate siblings of val
+TORCH_CUDA_CU_API std::vector<Val*> siblingValsOf(Val* val) {
+  std::vector<Val*> sibling_vals;
+  auto def = val->definition();
+  if (def != nullptr) {
+    auto outs = def->outputs();
+    for (auto sibling_val : outs) {
+      if (sibling_val == val) {
+        continue;
+      }
+      sibling_vals.emplace_back(sibling_val);
+    }
+  }
+  return sibling_vals;
+}
+
 // Return immediate producers of val
 TORCH_CUDA_CU_API std::vector<Val*> producerValsOf(
     const std::vector<Val*>& vals) {
@@ -556,22 +572,21 @@ TORCH_CUDA_CU_API std::vector<Val*> consumerValsOf(
 }
 
 std::vector<TensorView*> producerTvsOf(TensorView* tv) {
-  if (tv->definition() == nullptr) {
-    return {};
-  }
-  auto producer_vals =
-      ir_utils::filterByType<TensorView>(tv->definition()->inputs());
-  return uniqueEntries<TensorView>(
-      {producer_vals.begin(), producer_vals.end()});
+  auto producer_vals = producerValsOf(tv);
+  auto producer_tvs = ir_utils::filterByType<TensorView>(producer_vals);
+  return {producer_tvs.begin(), producer_tvs.end()};
 }
 
 std::vector<TensorView*> consumerTvsOf(TensorView* tv) {
-  std::vector<TensorView*> consumer_tvs;
-  for (auto use_expr : tv->uses()) {
-    auto outputs = ir_utils::filterByType<TensorView>(use_expr->outputs());
-    consumer_tvs.insert(consumer_tvs.end(), outputs.begin(), outputs.end());
-  }
-  return uniqueEntries<TensorView>(consumer_tvs);
+  auto consumer_vals = consumerValsOf(tv);
+  auto consumer_tvs = ir_utils::filterByType<TensorView>(consumer_vals);
+  return {consumer_tvs.begin(), consumer_tvs.end()};
+}
+
+TORCH_CUDA_CU_API std::vector<TensorView*> siblingTvsOf(TensorView* tv) {
+  auto sibling_vals = siblingValsOf(tv);
+  auto sibling_tvs = ir_utils::filterByType<TensorView>(sibling_vals);
+  return {sibling_tvs.begin(), sibling_tvs.end()};
 }
 
 std::vector<TensorView*> producerTvsOf(const std::vector<TensorView*>& tvs) {

torch/csrc/jit/codegen/cuda/ir_utils.h

@@ -181,6 +181,16 @@ TORCH_CUDA_CU_API std::vector<Val*> producerValsOf(Val* val);
 // code.
 TORCH_CUDA_CU_API std::vector<Val*> consumerValsOf(Val* val);
 
+// Return immediate siblings of val, this function can be used on any Val and
+// will return siblings through Exprs.
+//
+// Warning: returned val's are not guaranteed to be between fusion inputs and
+// outputs. This function simply uses val->definition() or val->uses() which is
+// limited to not go through fusion inputs/outputs, but if on a path that isn't
+// strictly between fusion inputs/outputs, it could effectively return dead
+// code.
+TORCH_CUDA_CU_API std::vector<Val*> siblingValsOf(Val* val);
+
 // Return immediate producers of vals, this function can be used on any vals and
 // will return producers through Exprs.
 //
@@ -223,6 +233,16 @@ TORCH_CUDA_CU_API std::vector<TensorView*> producerTvsOf(TensorView* tv);
 // code.
 TORCH_CUDA_CU_API std::vector<TensorView*> consumerTvsOf(TensorView* tv);
 
+// Return immediate siblings of tv, this function will return all immediate
+// siblings of tv through Exprs.
+//
+// Warning: returned tv's are not guaranteed to be between fusion inputs and
+// outputs. This function simply uses tv->definition() or tv->uses() which is
+// limited to not go through fusion inputs/outputs, but if on a path that isn't
+// strictly between fusion inputs/outputs, it could effectively return dead
+// code.
+TORCH_CUDA_CU_API std::vector<TensorView*> siblingTvsOf(TensorView* tv);
+
 // Return immediate producers of tvs, this function will return all immediate
 // producers of tvs through Exprs.
 //

torch/csrc/jit/codegen/cuda/maxinfo_propagator.cpp

@@ -80,6 +80,13 @@ void MaxInfoSpanningTree::compute_spanning_tree() {
     return selector_->allowCasP(from, to);
   };
 
+  auto allowSibling = [this](TensorView* from, TensorView* to) {
+    if (selector_ == nullptr) {
+      return true;
+    }
+    return selector_->allowSibling(from, to);
+  };
+
   while (!candidates.empty()) {
     const auto next_hop_info = candidates.back();
     const auto& next_hop = next_hop_info.next_hop;
@@ -91,6 +98,21 @@ void MaxInfoSpanningTree::compute_spanning_tree() {
     }
     replayed.emplace(next_hop.to);
 
+    for (auto sibling_tv : ir_utils::siblingTvsOf(next_hop.to)) {
+      if (replayed.count(sibling_tv) ||
+          !allowSibling(next_hop.to, sibling_tv)) {
+        continue;
+      }
+      insertNextHop(
+          {.next_hop =
+               {.type = NextHopType::SIBLING,
+                .from = next_hop.to,
+                .to = sibling_tv},
+           .info_from = next_hop_info.info_to,
+           .info_to = computeInfoSibling(
+               next_hop.to, sibling_tv, next_hop_info.info_to)});
+    }
+
     for (auto consumer_tv : ir_utils::consumerTvsOf(next_hop.to)) {
       if (replayed.count(consumer_tv) || !allowCasP(next_hop.to, consumer_tv)) {
         continue;
@@ -127,6 +149,9 @@ void MaxInfoSpanningTree::traverse(Propagator* propagator) {
   }
   for (const auto& next_hop : path_) {
     switch (next_hop.type) {
+      case NextHopType::SIBLING:
+        propagator->propagateTvSibling(next_hop.from, next_hop.to);
+        break;
       case NextHopType::C_AS_P:
         propagator->propagateTvCasP(next_hop.from, next_hop.to);
         break;
@@ -380,6 +405,17 @@ MaxRootDomainInfoSpanningTree::getReferenceRootIDInfo(
   return std::make_shared<RootDomainInfo>(std::move(result));
 }
 
+// Given the preserved reference root ID info of a tensor, compute
+// the corresponding info in its sibling. Since info has nothing to do with
+// replay state, so sibling info is always identical by definition.
+std::shared_ptr<MaxInfoSpanningTree::Information> MaxRootDomainInfoSpanningTree::
+    computeInfoSibling(
+        TensorView* from,
+        TensorView* to,
+        std::shared_ptr<Information> from_info) const {
+  return from_info;
+}
+
 } // namespace cuda
 } // namespace fuser
 } // namespace jit

torch/csrc/jit/codegen/cuda/maxinfo_propagator.h

@@ -29,8 +29,9 @@ namespace cuda {
  * MaxInfoSpanningTree::Information and implement `operator<` which is used to
  * tell which path contains more information, and `operator bool` which is used
  * to tell if there is any information stored. You also need to implement
- * computeInfoPasC and computeInfoCasP, which are the functions that compute
- * information of the `to` tensor from the information of the `from` tensor.
+ * computeInfoPasC, computeInfoCasP, and computeInfoSibling, which are the
+ * functions that compute information of the `to` tensor from the information of
+ * the `from` tensor.
  */
 // NOLINTNEXTLINE(cppcoreguidelines-pro-type-member-init)
 class TORCH_CUDA_CU_API MaxInfoSpanningTree {
@@ -40,12 +41,14 @@ class TORCH_CUDA_CU_API MaxInfoSpanningTree {
   struct Selector {
     virtual bool allowPasC(TensorView* from, TensorView* to) = 0;
     virtual bool allowCasP(TensorView* from, TensorView* to) = 0;
+    virtual bool allowSibling(TensorView* from, TensorView* to) = 0;
   };
 
   // This is the interface to implement the actual propagation
   struct Propagator {
     virtual void propagateTvPasC(TensorView* from, TensorView* to) = 0;
     virtual void propagateTvCasP(TensorView* from, TensorView* to) = 0;
+    virtual void propagateTvSibling(TensorView* from, TensorView* to) = 0;
   };
 
   // This is the interface that specifies the structure of information used to
@@ -71,6 +74,7 @@ class TORCH_CUDA_CU_API MaxInfoSpanningTree {
 
  private:
   enum class NextHopType {
+    SIBLING,
     C_AS_P,
     P_AS_C,
   };
@@ -109,6 +113,10 @@ class TORCH_CUDA_CU_API MaxInfoSpanningTree {
       TensorView* from,
       TensorView* to,
       std::shared_ptr<Information> from_info) const = 0;
+  virtual std::shared_ptr<Information> computeInfoSibling(
+      TensorView* from,
+      TensorView* to,
+      std::shared_ptr<Information> from_info) const = 0;
 
  public:
   MaxInfoSpanningTree(
@@ -190,6 +198,10 @@ class TORCH_CUDA_CU_API MaxRootDomainInfoSpanningTree
       TensorView* from,
       TensorView* to,
       std::shared_ptr<Information> from_info) const override;
+  virtual std::shared_ptr<Information> computeInfoSibling(
+      TensorView* from,
+      TensorView* to,
+      std::shared_ptr<Information> from_info) const override;
 
  private:
   static std::shared_ptr<RootDomainInfo> getReferenceRootIDInfo(TensorView* tv);