JIT: profile synthesis blend and repair modes (#83567)

Implement blend and repair modes for synthesis. Blend merges a bit of
synthesized data into an existing PGO data set; repair tries to fix any
local inconsistencies (via heuristics). Both run count construction
afterwards.

Trust blended data like we trust dynamic data. Probably will want more
nuance here (eg trust dynamic blend, but not static blend) but this is
sufficent for now.

Also implement random and reverse modes; these will ultimately be used
for stress testing (not called anywhere yet).

Parameterize some of the magic constants that have cropped up.

Add blend mode as a new weekend pgo stress mode; fix the other synthesis
mode I added recently to pgo stress to set the config properly.

Contributes to #82964.

eng/pipelines/common/templates/runtimes/run-test-job.yml

@@ -542,6 +542,7 @@ jobs:
             - fullpgo_random_gdv_edge
             - fullpgo_methodprofiling_always_optimized
             - syntheticpgo
+            - syntheticpgo_blend
         ${{ if in(parameters.testGroup, 'gc-longrunning') }}:
           longRunningGcTests: true
           scenarios:

eng/pipelines/libraries/run-test-job.yml

@@ -201,3 +201,5 @@ jobs:
                 - jitosr_stress
                 - jitosr_stress_random
                 - syntheticpgo
+                - syntheticpgo_blend
+

src/coreclr/jit/block.h

@@ -1867,6 +1867,12 @@ struct FlowEdge
         m_likelihood = likelihood;
     }
 
+    void clearLikelihood()
+    {
+        m_likelihood = 0.0;
+        INDEBUG(m_likelihoodSet = false);
+    }
+
 #ifdef DEBUG
     bool hasLikelihood() const
     {

src/coreclr/jit/compiler.h

@@ -5561,7 +5561,7 @@ class Compiler
 
 #endif // DEBUG
 
-    static bool fgProfileWeightsEqual(weight_t weight1, weight_t weight2);
+    static bool fgProfileWeightsEqual(weight_t weight1, weight_t weight2, weight_t epsilon = 0.01);
     static bool fgProfileWeightsConsistent(weight_t weight1, weight_t weight2);
 
     static GenTree* fgGetFirstNode(GenTree* tree);

src/coreclr/jit/fgprofile.cpp

@@ -78,10 +78,10 @@ bool Compiler::fgHaveSufficientProfileWeights()
     {
         case ICorJitInfo::PgoSource::Dynamic:
         case ICorJitInfo::PgoSource::Text:
+        case ICorJitInfo::PgoSource::Blend:
             return true;
 
         case ICorJitInfo::PgoSource::Static:
-        case ICorJitInfo::PgoSource::Blend:
         {
             // We sometimes call this very early, eg evaluating the prejit root.
             //
@@ -123,6 +123,7 @@ bool Compiler::fgHaveTrustedProfileWeights()
     switch (fgPgoSource)
     {
         case ICorJitInfo::PgoSource::Dynamic:
+        case ICorJitInfo::PgoSource::Blend:
         case ICorJitInfo::PgoSource::Text:
             return true;
         default:
@@ -2624,6 +2625,16 @@ PhaseStatus Compiler::fgIncorporateProfileData()
         fgIncorporateEdgeCounts();
     }
 
+#ifdef DEBUG
+    // Optionally synthesize & blend
+    //
+    if ((JitConfig.JitSynthesizeCounts() == 3) && !compIsForInlining())
+    {
+        JITDUMP("Synthesizing profile data and blending it with the actual profile data\n");
+        ProfileSynthesis::Run(this, ProfileSynthesisOption::BlendLikelihoods);
+    }
+#endif
+
     // Scale data as appropriate
     //
     fgApplyProfileScale();
@@ -4890,14 +4901,15 @@ EARLY_EXIT:;
 // Arguments:
 //   weight1 -- first weight
 //   weight2 -- second weight
+//   epsilon -- maximum absolute difference for weights to be considered equal
 //
 // Notes:
 //   In most cases you should probably call fgProfileWeightsConsistent instead
 //   of this method.
 //
-bool Compiler::fgProfileWeightsEqual(weight_t weight1, weight_t weight2)
+bool Compiler::fgProfileWeightsEqual(weight_t weight1, weight_t weight2, weight_t epsilon)
 {
-    return fabs(weight1 - weight2) < 0.01;
+    return fabs(weight1 - weight2) <= epsilon;
 }
 
 //------------------------------------------------------------------------