JIT: fix count reconstruction problem (#100385)

In large methods with lots of irreducible loops we may find
reconstructed counts reaching very large values.

Since profile counts in practice won't ever be much larger than say
10^12, detect when reconstructed counts exceed this value, and stop
the algorithm.

We may eventually decide to rerun in "hard blend" mode where we
intentionally limit the edge likelihood ranges. But this should do for
now.

Closes #100350.

src/coreclr/jit/fgprofilesynthesis.cpp

@@ -1289,6 +1289,12 @@ void ProfileSynthesis::GaussSeidelSolver()
                 residual      = change;
                 residualBlock = block;
             }
+
+            if (newWeight >= maxCount)
+            {
+                JITDUMP("count overflow in " FMT_BB ": " FMT_WT "\n", block->bbNum, newWeight);
+                m_overflow = true;
+            }
         }
 
         // If there were no improper headers, we will have converged in one pass.
@@ -1312,6 +1318,11 @@ void ProfileSynthesis::GaussSeidelSolver()
             break;
         }
 
+        if (m_overflow)
+        {
+            break;
+        }
+
         // If we have been iterating for a bit, estimate the dominant GS
         // eigenvalue. (we might want to start with Jacobi iterations
         // to get the Jacobi eigenvalue instead).

src/coreclr/jit/fgprofilesynthesis.h

@@ -51,6 +51,7 @@ class ProfileSynthesis
     static constexpr weight_t ilNextLikelihood   = 0.52;
     static constexpr weight_t loopBackLikelihood = 0.9;
     static constexpr weight_t loopExitLikelihood = 0.9;
+    static constexpr weight_t maxCount           = 1e12;
 
     void Run(ProfileSynthesisOption option);
 
@@ -84,6 +85,7 @@ class ProfileSynthesis
     unsigned               m_improperLoopHeaders       = 0;
     unsigned               m_cappedCyclicProbabilities = 0;
     bool                   m_approximate               = false;
+    bool                   m_overflow                  = false;
 };
 
 #endif // !_FGPROFILESYNTHESIS_H_