JIT: revise how synthesis treats handler regions (dotnet#100899)

Instead of giving hander regions a fraction of the entry weight, give them a small
fixed weight. This is intended to combat the lack of profile propagation out of
handler regions, where there are currently sometimes weight discontinuities large
enough to cause profile check asserts.

Contributes to dotnet#93020.

src/coreclr/jit/fgprofilesynthesis.cpp

@@ -1039,29 +1039,28 @@ void ProfileSynthesis::AssignInputWeights(ProfileSynthesisOption option)
 
     // Determine input weight for EH regions, if any.
     //
-    weight_t exceptionScaleFactor = exceptionScale;
+    weight_t ehWeight = exceptionWeight;
 
 #ifdef DEBUG
-    if (JitConfig.JitSynthesisExceptionScale() != nullptr)
+    if (JitConfig.JitSynthesisExceptionWeight() != nullptr)
     {
-        ConfigDoubleArray JitSynthesisExceptionScaleArray;
-        JitSynthesisExceptionScaleArray.EnsureInit(JitConfig.JitSynthesisExceptionScale());
-        weight_t newFactor = JitSynthesisExceptionScaleArray.GetData()[0];
+        ConfigDoubleArray JitSynthesisExceptionWeightArray;
+        JitSynthesisExceptionWeightArray.EnsureInit(JitConfig.JitSynthesisExceptionWeight());
+        weight_t newFactor = JitSynthesisExceptionWeightArray.GetData()[0];
 
         if ((newFactor >= 0) && (newFactor <= 1.0))
         {
-            exceptionScaleFactor = newFactor;
+            ehWeight = newFactor;
         }
     }
 #endif
 
-    JITDUMP("Synthesis: exception scale factor " FMT_WT "\n", exceptionScaleFactor);
-    const weight_t ehWeight = entryWeight * exceptionScaleFactor;
+    JITDUMP("Synthesis: exception weight " FMT_WT "\n", ehWeight);
 
     if (ehWeight != 0)
     {
-        // We can't inline methods with EH, also inlinees share the parent
-        // EH tab, so we can't rely on this being empty.
+        // We can't inline methods with EH. Inlinees share the parent
+        // EH tab, so we can't rely on the EH table being empty.
         //
         if (!m_comp->compIsForInlining())
         {

src/coreclr/jit/fgprofilesynthesis.h

@@ -45,7 +45,7 @@ class ProfileSynthesis
     {
     }
 
-    static constexpr weight_t exceptionScale        = 0.001;
+    static constexpr weight_t exceptionWeight       = 0.00001;
     static constexpr weight_t initialBlendFactor    = 0.05;
     static constexpr weight_t blendFactorGrowthRate = 3;
     static constexpr weight_t cappedLikelihood      = 0.999;

src/coreclr/jit/jitconfigvalues.h

@@ -719,18 +719,15 @@ CONFIG_INTEGER(JitRandomlyCollect64BitCounts, W("JitRandomlyCollect64BitCounts")
 // 3: profile synthesis for root methods, blend with existing PGO data
 CONFIG_INTEGER(JitSynthesizeCounts, W("JitSynthesizeCounts"), 0)
 
-// Check if synthesis left consistent counts
-CONFIG_INTEGER(JitCheckSynthesizedCounts, W("JitCheckSynthesizedCounts"), 0)
-
 // If instrumenting the method, run synthesis and save the synthesis results
 // as edge or block profile data. Do not actually instrument.
 CONFIG_INTEGER(JitPropagateSynthesizedCountsToProfileData, W("JitPropagateSynthesizedCountsToProfileData"), 0)
 
 // Use general (gauss-seidel) solver
 CONFIG_INTEGER(JitSynthesisUseSolver, W("JitSynthesisUseSolver"), 1)
 
-// Relative likelihood of exceptions for synthesis
-CONFIG_STRING(JitSynthesisExceptionScale, W("JitSynthesisExceptionScale"))
+// Weight for exception regions for synthesis
+CONFIG_STRING(JitSynthesisExceptionWeight, W("JitSynthesisExceptionWeight"))
 
 // Devirtualize virtual calls with getExactClasses (NativeAOT only for now)
 RELEASE_CONFIG_INTEGER(JitEnableExactDevirtualization, W("JitEnableExactDevirtualization"), 1)