JIT: Reduce heuristics-derived edge likelihoods into throw blocks found by morph

src/coreclr/jit/block.h

@@ -594,6 +594,9 @@ struct FlowEdge
     // Convenience flag for phases that need to track edge visitation
     bool m_visited;
 
+    // Indicates if m_likelihood was determined using profile synthesis's heuristics
+    bool m_heuristicBasedLikelihood;
+
     // True if likelihood has been set
     INDEBUG(bool m_likelihoodSet);
 
@@ -605,6 +608,7 @@ struct FlowEdge
         , m_likelihood(0)
         , m_dupCount(0)
         , m_visited(false)
+        , m_heuristicBasedLikelihood(false)
 #ifdef DEBUG
         , m_likelihoodSet(false)
 #endif // DEBUG
@@ -661,7 +665,8 @@ struct FlowEdge
 
     void clearLikelihood()
     {
-        m_likelihood = 0.0;
+        m_likelihood               = 0.0;
+        m_heuristicBasedLikelihood = false;
         INDEBUG(m_likelihoodSet = false);
     }
 
@@ -706,6 +711,16 @@ struct FlowEdge
         assert(visited());
         m_visited = false;
     }
+
+    bool isHeuristicBased() const
+    {
+        return m_heuristicBasedLikelihood;
+    }
+
+    void setHeuristicBased()
+    {
+        m_heuristicBasedLikelihood = true;
+    }
 };
 
 //------------------------------------------------------------------------

src/coreclr/jit/fgbasic.cpp

@@ -173,6 +173,36 @@ void Compiler::fgConvertBBToThrowBB(BasicBlock* block)
         }
     }
 
+    // Reduce the heuristics-derived edge likelihoods into 'block' to indicate exceptional behavior
+    for (FlowEdge* const predEdge : block->PredEdges())
+    {
+        if (predEdge->isHeuristicBased() && predEdge->getSourceBlock()->KindIs(BBJ_COND))
+        {
+            BasicBlock* const condBlock = predEdge->getSourceBlock();
+            FlowEdge* const   otherEdge =
+                condBlock->TrueEdgeIs(predEdge) ? condBlock->GetFalseEdge() : condBlock->GetTrueEdge();
+
+            // We should not have degenerate branches
+            assert(predEdge != otherEdge);
+            assert(otherEdge->isHeuristicBased());
+
+            // If the predecessor can jump to a non-throw block, bias the likelihoods to that path
+            if (!otherEdge->getDestinationBlock()->KindIs(BBJ_THROW))
+            {
+                predEdge->setLikelihood(0.0);
+                otherEdge->setLikelihood(1.0);
+            }
+            // If both branches are to throw blocks, consider them equally likely
+            else
+            {
+                predEdge->setLikelihood(0.5);
+                otherEdge->setLikelihood(0.5);
+            }
+
+            profileInconsistent = true;
+        }
+    }
+
     if (profileInconsistent)
     {
         JITDUMP("Flow removal of " FMT_BB " needs to be propagated. Data %s inconsistent.\n", block->bbNum,

src/coreclr/jit/fgprofilesynthesis.cpp

@@ -252,6 +252,8 @@ void ProfileSynthesis::AssignLikelihoods()
 
             case BBJ_COND:
                 // Two successor cases
+                block->GetTrueEdge()->setHeuristicBased();
+                block->GetFalseEdge()->setHeuristicBased();
                 AssignLikelihoodCond(block);
                 break;