JIT: use synthesis to repair some reconstruction issues (#84312)

In particular, run synthesis in repair mode for cases where there are profile
counts within the method but zero counts in `fgFirstBB`.

Recall that sparse profiling effectively probes return blocks to determine the
method entry count.

So the zero-entry but not zero-everywhere case can happen if we have a method
with a very long running loop plus sparse profiling plus OSR -- we will only
get profile counts from the instrumented Tier0 method, and it will never return
(instead it will always escape to an OSR version which will eventually return,
but that version won't be instrumented).

I originally was a bit more ambitious and ran repair for a broader set of
reconstruction issues, but lead to a large number of diffs, in part because
repair doesn't cope well with irreducible loops.

Leaving the entry count zero can have fairly disastrous impact on the quality
of optimizations done in the method.

Addresses quite a few of the worst-performing benchmarks in #84264.

src/coreclr/jit/compiler.h

@@ -5703,8 +5703,8 @@ class Compiler
     PhaseStatus fgPrepareToInstrumentMethod();
     PhaseStatus fgInstrumentMethod();
     PhaseStatus fgIncorporateProfileData();
-    void        fgIncorporateBlockCounts();
-    void        fgIncorporateEdgeCounts();
+    bool        fgIncorporateBlockCounts();
+    bool        fgIncorporateEdgeCounts();
 
 public:
     const char*                            fgPgoFailReason;

src/coreclr/jit/fgprofile.cpp

@@ -2606,18 +2606,35 @@ PhaseStatus Compiler::fgIncorporateProfileData()
 
     fgPgoHaveWeights = haveBlockCounts || haveEdgeCounts;
 
-    // We expect not to have both block and edge counts. We may have other
-    // forms of profile data even if we do not have any counts.
-    //
-    assert(!haveBlockCounts || !haveEdgeCounts);
-
-    if (haveBlockCounts)
+    if (fgPgoHaveWeights)
     {
-        fgIncorporateBlockCounts();
-    }
-    else if (haveEdgeCounts)
-    {
-        fgIncorporateEdgeCounts();
+        // We expect not to have both block and edge counts. We may have other
+        // forms of profile data even if we do not have any counts.
+        //
+        assert(!haveBlockCounts || !haveEdgeCounts);
+
+        bool dataIsGood = false;
+
+        if (haveBlockCounts)
+        {
+            dataIsGood = fgIncorporateBlockCounts();
+        }
+        else if (haveEdgeCounts)
+        {
+            dataIsGood = fgIncorporateEdgeCounts();
+        }
+
+        // Profile incorporation may have tossed out all PGO data if it
+        // encountered major issues. This is perhaps too drastic. Consider
+        // at least keeping the class profile data, or perhaps enable full synthesis.
+        //
+        // If profile incorporation hit fixable problems, run synthesis in repair mode.
+        //
+        if (fgPgoHaveWeights && !dataIsGood)
+        {
+            JITDUMP("\nIncorporated count data had inconsistencies; repairing profile...\n");
+            ProfileSynthesis::Run(this, ProfileSynthesisOption::RepairLikelihoods);
+        }
     }
 
 #ifdef DEBUG
@@ -2667,6 +2684,9 @@ void Compiler::fgSetProfileWeight(BasicBlock* block, weight_t profileWeight)
 // fgIncorporateBlockCounts: read block count based profile data
 //   and set block weights
 //
+// Returns:
+//   True if data is in good shape
+//
 // Notes:
 //   Since we are now running before the importer, we do not know which
 //   blocks will be imported, and we should not see any internal blocks.
@@ -2680,7 +2700,7 @@ void Compiler::fgSetProfileWeight(BasicBlock* block, weight_t profileWeight)
 //   Find some other mechanism for handling cases where handler entry
 //   blocks must be in the hot section.
 //
-void Compiler::fgIncorporateBlockCounts()
+bool Compiler::fgIncorporateBlockCounts()
 {
     for (BasicBlock* const block : Blocks())
     {
@@ -2691,6 +2711,10 @@ void Compiler::fgIncorporateBlockCounts()
             fgSetProfileWeight(block, profileWeight);
         }
     }
+
+    // For now assume data is always good.
+    //
+    return true;
 }
 
 //------------------------------------------------------------------------
@@ -2871,6 +2895,7 @@ class EfficientEdgeCountReconstructor : public SpanningTreeVisitor
     bool m_negativeCount;
     bool m_failedToConverge;
     bool m_allWeightsZero;
+    bool m_entryWeightZero;
 
 public:
     EfficientEdgeCountReconstructor(Compiler* comp)
@@ -2889,6 +2914,7 @@ class EfficientEdgeCountReconstructor : public SpanningTreeVisitor
         , m_negativeCount(false)
         , m_failedToConverge(false)
         , m_allWeightsZero(true)
+        , m_entryWeightZero(false)
     {
     }
 
@@ -2916,6 +2942,24 @@ class EfficientEdgeCountReconstructor : public SpanningTreeVisitor
         m_failedToConverge = true;
     }
 
+    void EntryWeightZero()
+    {
+        m_entryWeightZero = true;
+    }
+
+    // Are there are reparable issues with the reconstruction?
+    //
+    // Ideally we'd also have || !m_negativeCount here, but this
+    // leads to lots of diffs in async methods.
+    //
+    // Looks like we might first need to resolve reconstruction
+    // shortcomings with irreducible loops.
+    //
+    bool IsGood() const
+    {
+        return !m_entryWeightZero;
+    }
+
     void VisitBlock(BasicBlock*) override
     {
     }
@@ -3381,52 +3425,15 @@ void EfficientEdgeCountReconstructor::Solve()
 
     JITDUMP("\nSolver: converged in %u passes\n", nPasses);
 
-    // If, after solving, the entry weight ends up as zero, set it to
-    // the max of the weight of successor edges or join-free successor
-    // block weight. We do this so we can determine a plausible scale
-    // count.
-    //
-    // This can happen for methods that do not return (say they always
-    // throw, or had not yet returned when we snapped the counts).
-    //
-    // Note we know there are nonzero counts elsewhere in the method, otherwise
-    // m_allWeightsZero would be true and we would have bailed out above.
+    // If, after solving, the entry weight ends up as zero, note
+    // this so we can run a profile repair immediately.
     //
     BlockInfo* const firstInfo = BlockToInfo(m_comp->fgFirstBB);
     if (firstInfo->m_weight == BB_ZERO_WEIGHT)
     {
         assert(!m_allWeightsZero);
-
-        weight_t newWeight = BB_ZERO_WEIGHT;
-
-        for (Edge* edge = firstInfo->m_outgoingEdges; edge != nullptr; edge = edge->m_nextOutgoingEdge)
-        {
-            if (edge->m_weightKnown)
-            {
-                newWeight = max(newWeight, edge->m_weight);
-            }
-
-            BlockInfo* const targetBlockInfo  = BlockToInfo(edge->m_targetBlock);
-            Edge* const      targetBlockEdges = targetBlockInfo->m_incomingEdges;
-
-            if (targetBlockInfo->m_weightKnown && (targetBlockEdges->m_nextIncomingEdge == nullptr))
-            {
-                newWeight = max(newWeight, targetBlockInfo->m_weight);
-            }
-        }
-
-        if (newWeight == BB_ZERO_WEIGHT)
-        {
-            // TODO -- throw out profile data or trigger repair/synthesis.
-            //
-            JITDUMP("Entry block weight and neighborhood was zero\n");
-        }
-        else
-        {
-            JITDUMP("Entry block weight was zero, setting entry weight to neighborhood max " FMT_WT "\n", newWeight);
-        }
-
-        firstInfo->m_weight = newWeight;
+        JITDUMP("\nSolver: entry block weight is zero\n");
+        EntryWeightZero();
     }
 }
 
@@ -3436,9 +3443,6 @@ void EfficientEdgeCountReconstructor::Solve()
 //
 void EfficientEdgeCountReconstructor::Propagate()
 {
-    // We don't expect mismatches or convergence failures.
-    //
-
     // Mismatches are currently expected as the flow for static pgo doesn't prevent them now.
     //    assert(!m_mismatch);
 
@@ -3939,6 +3943,10 @@ void EfficientEdgeCountReconstructor::MarkInterestingSwitches(BasicBlock* block,
 // fgIncorporateEdgeCounts: read sparse edge count based profile data
 //   and set block weights
 //
+// Returns:
+//    true if incorporated profile is in good shape (consistent, etc).
+//    false if some repair seems necessary
+//
 // Notes:
 //   Because edge counts are sparse, we need to solve for the missing
 //   edge counts; in the process, we also determine block counts.
@@ -3948,7 +3956,7 @@ void EfficientEdgeCountReconstructor::MarkInterestingSwitches(BasicBlock* block,
 //   Since we have edge weights here, we might as well set them
 //   (or likelihoods)
 //
-void Compiler::fgIncorporateEdgeCounts()
+bool Compiler::fgIncorporateEdgeCounts()
 {
     JITDUMP("\nReconstructing block counts from sparse edge instrumentation\n");
 
@@ -3957,6 +3965,8 @@ void Compiler::fgIncorporateEdgeCounts()
     WalkSpanningTree(&e);
     e.Solve();
     e.Propagate();
+
+    return e.IsGood();
 }
 
 //------------------------------------------------------------------------

src/coreclr/jit/fgprofilesynthesis.cpp

@@ -64,6 +64,10 @@ void ProfileSynthesis::Run(ProfileSynthesisOption option)
             RandomizeLikelihoods();
             break;
 
+        case ProfileSynthesisOption::RepairLikelihoods:
+            RepairLikelihoods();
+            break;
+
         default:
             assert(!"unexpected profile synthesis option");
             break;