[PATCH] D139993: [LoopFusion] Sorting of undominated FusionCandidates crashes

[Ramkrishnan Narayanan Komala via Phabricator via llvm-commits]

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This patch tries to fix issue <https://github.com/llvm/llvm-project/issues/56263>.

If two **FusionCandidates** are in same level of dominator tree then, they will not be dominates each other. But they are control flow equivalent. To sort those FusionCandidates **nonStrictlyPostDominate** check is needed.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D139993

Files:
  llvm/lib/Transforms/Scalar/LoopFuse.cpp
  llvm/test/Transforms/LoopFusion/undominated_loops.ll


Index: llvm/test/Transforms/LoopFusion/undominated_loops.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/LoopFusion/undominated_loops.ll
@@ -0,0 +1,26 @@
+; RUN: opt -S -passes=loop-simplify,newgvn,loop-fusion -pass-remarks-analysis=loop-fusion -disable-output < %s 2>&1 | FileCheck %s
+
+; CHECK: remark: <unknown>:0:0: [function_0]: Loop is not a candidate for fusion: Loop has unknown trip count
+
+define void @function_0() {
+entry_1:
+  br i1 false, label %bb_2, label %bb_3
+
+bb_2:                                         ; preds = %bb_7, %bb_5, %bb_2, %entry_1
+  br i1 false, label %bb_2, label %bb_5
+
+bb_3:                                         ; preds = %bb_3, %entry_1
+  br i1 false, label %bb_3, label %bb_4
+
+bb_4:                                         ; preds = %bb_3
+  br label %bb_6
+
+bb_5:                                         ; preds = %bb_2
+  br i1 undef, label %bb_2, label %bb_7
+
+bb_6:                                         ; preds = %bb_7, %bb_6, %bb_4
+  br i1 undef, label %bb_7, label %bb_6
+
+bb_7:                                         ; preds = %bb_6, %bb_5
+  br i1 undef, label %bb_2, label %bb_6
+}
Index: llvm/lib/Transforms/Scalar/LoopFuse.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/LoopFuse.cpp
+++ llvm/lib/Transforms/Scalar/LoopFuse.cpp
@@ -415,9 +415,29 @@
       return true;
     }
 
-    // If LHS does not dominate RHS and RHS does not dominate LHS then there is
-    // no dominance relationship between the two FusionCandidates. Thus, they
-    // should not be in the same set together.
+    // If two FusionCandidates are in same level of dominator tree then,
+    // they will not be dominates each other. But may be control flow
+    // equivalent. To sort those FusionCandidates nonStrictlyPostDominate is
+    // needed.
+    bool bWrongOrder =
+        nonStrictlyPostDominate(LHSEntryBlock, RHSEntryBlock, DT, LHS.PDT);
+    bool bRightOrder =
+        nonStrictlyPostDominate(RHSEntryBlock, LHSEntryBlock, DT, LHS.PDT);
+    if (bWrongOrder && bRightOrder) {
+      DomTreeNode *LNode = LHS.PDT->getNode(LHSEntryBlock);
+      DomTreeNode *RNode = LHS.PDT->getNode(RHSEntryBlock);
+      if (LNode->getLevel() > RNode->getLevel())
+        return true;
+      else
+        return false;
+    } else if (bWrongOrder)
+      return false;
+    else if (bRightOrder)
+      return true;
+
+    // If LHS does not non-strict Postdominate RHS and RHS does not non-strict
+    // Postdominate LHS then, there is no dominance relationship between the
+    // two FusionCandidates. Thus, they should not be in the same set together.
     llvm_unreachable(
         "No dominance relationship between these fusion candidates!");
   }


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