fix assertion merge for degenerate flow

src/coreclr/jit/assertionprop.cpp

@@ -4648,14 +4648,36 @@ class AssertionPropFlowCallback
     }
 
     // During merge, perform the actual merging of the predecessor's (since this is a forward analysis) dataflow flags.
-    void Merge(BasicBlock* block, BasicBlock* predBlock, flowList* preds)
+    void Merge(BasicBlock* block, BasicBlock* predBlock, unsigned dupCount)
     {
-        ASSERT_TP pAssertionOut = ((predBlock->bbJumpKind == BBJ_COND) && (predBlock->bbJumpDest == block))
-                                      ? mJumpDestOut[predBlock->bbNum]
-                                      : predBlock->bbAssertionOut;
+        ASSERT_TP pAssertionOut;
+
+        if (predBlock->bbJumpKind == BBJ_COND && (predBlock->bbJumpDest == block))
+        {
+            pAssertionOut = mJumpDestOut[predBlock->bbNum];
+
+            if (dupCount > 1)
+            {
+                // Scenario where next block and conditional block, both point to the same block.
+                // In such case, intersect the assertions present on both the out edges of predBlock.
+                assert(predBlock->bbNext == block);
+                BitVecOps::IntersectionD(apTraits, pAssertionOut, predBlock->bbAssertionOut);
+
+                JITDUMP("AssertionPropCallback::Merge     : Duplicate flow, " FMT_BB " in -> %s, predBlock " FMT_BB
+                        " out1 -> %s, out2 -> %s\n",
+                        block->bbNum, BitVecOps::ToString(apTraits, block->bbAssertionIn), predBlock->bbNum,
+                        BitVecOps::ToString(apTraits, mJumpDestOut[predBlock->bbNum]),
+                        BitVecOps::ToString(apTraits, predBlock->bbAssertionOut));
+            }
+        }
+        else
+        {
+            pAssertionOut = predBlock->bbAssertionOut;
+        }
+
         JITDUMP("AssertionPropCallback::Merge     : " FMT_BB " in -> %s, predBlock " FMT_BB " out -> %s\n",
                 block->bbNum, BitVecOps::ToString(apTraits, block->bbAssertionIn), predBlock->bbNum,
-                BitVecOps::ToString(apTraits, predBlock->bbAssertionOut));
+                BitVecOps::ToString(apTraits, pAssertionOut));
         BitVecOps::IntersectionD(apTraits, block->bbAssertionIn, pAssertionOut);
     }
 

src/coreclr/jit/dataflow.h

@@ -16,7 +16,7 @@
 //  {
 //  public:
 //      void StartMerge(BasicBlock* block);
-//      void Merge(BasicBlock* block, BasicBlock* pred, flowList* preds);
+//      void Merge(BasicBlock* block, BasicBlock* pred, unsigned dupCount);
 //      bool EndMerge(BasicBlock* block);
 //  };
 #pragma once
@@ -61,7 +61,7 @@ void DataFlow::ForwardAnalysis(TCallback& callback)
             flowList* preds = m_pCompiler->BlockPredsWithEH(block);
             for (flowList* pred = preds; pred; pred = pred->flNext)
             {
-                callback.Merge(block, pred->getBlock(), preds);
+                callback.Merge(block, pred->getBlock(), pred->flDupCount);
             }
         }
 

src/coreclr/jit/morph.cpp

@@ -16245,9 +16245,9 @@ bool Compiler::fgFoldConditional(BasicBlock* block)
                 // no side effects, remove the jump entirely
                 fgRemoveStmt(block, lastStmt);
             }
-            // block is a BBJ_COND that we are folding the conditional for
-            // bTaken is the path that will always be taken from block
-            // bNotTaken is the path that will never be taken from block
+            // block is a BBJ_COND that we are folding the conditional for.
+            // bTaken is the path that will always be taken from block.
+            // bNotTaken is the path that will never be taken from block.
             //
             BasicBlock* bTaken;
             BasicBlock* bNotTaken;

src/coreclr/jit/optcse.cpp

@@ -1188,7 +1188,7 @@ class CSE_DataFlow
     }
 
     // Merge: perform the merging of each of the predecessor's liveness values (since this is a forward analysis)
-    void Merge(BasicBlock* block, BasicBlock* predBlock, flowList* preds)
+    void Merge(BasicBlock* block, BasicBlock* predBlock, unsigned dupCount)
     {
 #ifdef DEBUG
         if (m_comp->verbose)