JIT: add option to choose guarded devirt class randomly (#53399)

Add a config setting to randomly choose one of the observed classes for
guarded devirtualization, rather than the most likely class.

src/coreclr/jit/compiler.h

@@ -5732,6 +5732,12 @@ class Compiler
     void        fgIncorporateBlockCounts();
     void        fgIncorporateEdgeCounts();
 
+    CORINFO_CLASS_HANDLE getRandomClass(ICorJitInfo::PgoInstrumentationSchema* schema,
+                                        UINT32                                 countSchemaItems,
+                                        BYTE*                                  pInstrumentationData,
+                                        int32_t                                ilOffset,
+                                        CLRRandom*                             random);
+
 public:
     const char*                            fgPgoFailReason;
     bool                                   fgPgoDisabled;

src/coreclr/jit/importer.cpp

@@ -1,4 +1,3 @@
-
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
@@ -21645,17 +21644,40 @@ void Compiler::considerGuardedDevirtualization(
     unsigned       likelihood          = 0;
     unsigned       numberOfClasses     = 0;
 
-    CORINFO_CLASS_HANDLE likelyClass =
-        getLikelyClass(fgPgoSchema, fgPgoSchemaCount, fgPgoData, ilOffset, &likelihood, &numberOfClasses);
+    CORINFO_CLASS_HANDLE likelyClass = NO_CLASS_HANDLE;
+
+    bool doRandomDevirt = false;
+
+#ifdef DEBUG
+    // Optional stress mode to pick a random known class, rather than
+    // the most likely known class.
+    //
+    doRandomDevirt = JitConfig.JitRandomGuardedDevirtualization() != 0;
+
+    if (doRandomDevirt)
+    {
+        // Reuse the random inliner's random state.
+        //
+        CLRRandom* const random =
+            impInlineRoot()->m_inlineStrategy->GetRandom(JitConfig.JitRandomGuardedDevirtualization());
+        likelihood      = 100;
+        numberOfClasses = 1;
+        likelyClass     = getRandomClass(fgPgoSchema, fgPgoSchemaCount, fgPgoData, ilOffset, random);
+    }
+    else
+#endif
+    {
+        likelyClass = getLikelyClass(fgPgoSchema, fgPgoSchemaCount, fgPgoData, ilOffset, &likelihood, &numberOfClasses);
+    }
 
     if (likelyClass == NO_CLASS_HANDLE)
     {
         JITDUMP("No likely class, sorry\n");
         return;
     }
 
-    JITDUMP("Likely class for %p (%s) is %p (%s) [likelihood:%u classes seen:%u]\n", dspPtr(objClass), objClassName,
-            likelyClass, eeGetClassName(likelyClass), likelihood, numberOfClasses);
+    JITDUMP("%s class for %p (%s) is %p (%s) [likelihood:%u classes seen:%u]\n", doRandomDevirt ? "Random" : "Likely",
+            dspPtr(objClass), objClassName, likelyClass, eeGetClassName(likelyClass), likelihood, numberOfClasses);
 
     // Todo: a more advanced heuristic using likelihood, number of
     // classes, and the profile count for this block.

src/coreclr/jit/inline.cpp

@@ -1665,6 +1665,9 @@ void InlineStrategy::FinalizeXml(FILE* file)
 //------------------------------------------------------------------------
 // GetRandom: setup or access random state
 //
+// Arguments:
+//   seed -- seed value to use if not doing random inlines
+//
 // Return Value:
 //    New or pre-existing random state.
 //
@@ -1673,11 +1676,11 @@ void InlineStrategy::FinalizeXml(FILE* file)
 //    specified externally (via stress or policy setting) and partially
 //    specified internally via method hash.
 
-CLRRandom* InlineStrategy::GetRandom()
+CLRRandom* InlineStrategy::GetRandom(int optionalSeed)
 {
     if (m_Random == nullptr)
     {
-        int externalSeed = 0;
+        int externalSeed = optionalSeed;
 
 #ifdef DEBUG
 
@@ -1707,6 +1710,8 @@ CLRRandom* InlineStrategy::GetRandom()
 
         int seed = externalSeed ^ internalSeed;
 
+        JITDUMP("\n*** Using random seed ext(%u) ^ int(%u) = %u\n", externalSeed, internalSeed, seed);
+
         m_Random = new (m_Compiler, CMK_Inlining) CLRRandom();
         m_Random->Init(seed);
     }

src/coreclr/jit/inline.h

@@ -912,7 +912,7 @@ class InlineStrategy
     }
 
     // Set up or access random state (for use by RandomPolicy)
-    CLRRandom* GetRandom();
+    CLRRandom* GetRandom(int optionalSeed = 0);
 
 #endif // defined(DEBUG) || defined(INLINE_DATA)
 

src/coreclr/jit/jitconfigvalues.h

@@ -469,6 +469,7 @@ CONFIG_INTEGER(JitGuardedDevirtualizationChainLikelihood, W("JitGuardedDevirtual
 CONFIG_INTEGER(JitGuardedDevirtualizationChainStatements, W("JitGuardedDevirtualizationChainStatements"), 4)
 #if defined(DEBUG)
 CONFIG_STRING(JitGuardedDevirtualizationRange, W("JitGuardedDevirtualizationRange"))
+CONFIG_INTEGER(JitRandomGuardedDevirtualization, W("JitRandomGuardedDevirtualization"), 0)
 #endif // DEBUG
 
 // Enable insertion of patchpoints into Tier0 methods with loops.

src/coreclr/jit/likelyclass.cpp

@@ -33,7 +33,6 @@ struct LikelyClassHistogramEntry
 };
 
 // Summarizes a ClassProfile table by forming a Histogram
-
 //
 struct LikelyClassHistogram
 {
@@ -54,6 +53,13 @@ struct LikelyClassHistogram
     }
 };
 
+//------------------------------------------------------------------------
+// LikelyClassHistogram::LikelyClassHistgram: construct a new histogram
+//
+// Arguments:
+//    histogramEntries - pointer to the table portion of a ClassProfile* object (see corjit.h)
+//    entryCount - number of entries in the table to examine
+//
 LikelyClassHistogram::LikelyClassHistogram(INT_PTR* histogramEntries, unsigned entryCount)
 {
     m_unknownTypes                 = 0;
@@ -97,8 +103,29 @@ LikelyClassHistogram::LikelyClassHistogram(INT_PTR* histogramEntries, unsigned e
     }
 }
 
-// This is used by the devirtualization logic below, and by crossgen2 when producing the R2R image (to reduce the size
-// cost of carrying the type histogram)
+//------------------------------------------------------------------------
+// getLikelyClass: find class profile data for an IL offset, and return the most likely class
+//
+// Arguments:
+//    schema - profile schema
+//    countSchemaItems - number of items in the schema
+//    pInstrumentationData - associated data
+//    ilOffset - il offset of the callvirt
+//    pLikelihood - [OUT] likelihood of observing that entry [0...100]
+//    pNumberOfClasses - [OUT] estimated number of classes seen at runtime
+//
+// Returns:
+//    Class handle for the most likely class, or nullptr
+//
+// Notes:
+//    A "monomorphic" call site will return likelihood 100 and number of entries = 1.
+//
+//   This is used by the devirtualization logic below, and by crossgen2 when producing
+//   the R2R image (to reduce the sizecost of carrying the type histogram)
+//
+//   This code can runs without a jit instance present, so JITDUMP and related
+//   cannot be used.
+//
 extern "C" DLLEXPORT CORINFO_CLASS_HANDLE WINAPI getLikelyClass(ICorJitInfo::PgoInstrumentationSchema* schema,
                                                                 UINT32                                 countSchemaItems,
                                                                 BYTE*   pInstrumentationData,
@@ -220,3 +247,82 @@ extern "C" DLLEXPORT CORINFO_CLASS_HANDLE WINAPI getLikelyClass(ICorJitInfo::Pgo
     //
     return NULL;
 }
+
+//------------------------------------------------------------------------
+// getRandomClass: find class profile data for an IL offset, and return
+//   one of the possible classes at random
+//
+// Arguments:
+//    schema - profile schema
+//    countSchemaItems - number of items in the schema
+//    pInstrumentationData - associated data
+//    ilOffset - il offset of the callvirt
+//    random - randomness generator
+//
+// Returns:
+//    Randomly observed class, or nullptr.
+//
+CORINFO_CLASS_HANDLE Compiler::getRandomClass(ICorJitInfo::PgoInstrumentationSchema* schema,
+                                              UINT32                                 countSchemaItems,
+                                              BYTE*                                  pInstrumentationData,
+                                              int32_t                                ilOffset,
+                                              CLRRandom*                             random)
+{
+    if (schema == nullptr)
+    {
+        return NO_CLASS_HANDLE;
+    }
+
+    for (COUNT_T i = 0; i < countSchemaItems; i++)
+    {
+        if (schema[i].ILOffset != (int32_t)ilOffset)
+        {
+            continue;
+        }
+
+        if ((schema[i].InstrumentationKind == ICorJitInfo::PgoInstrumentationKind::GetLikelyClass) &&
+            (schema[i].Count == 1))
+        {
+            INT_PTR result = *(INT_PTR*)(pInstrumentationData + schema[i].Offset);
+            if (ICorJitInfo::IsUnknownTypeHandle(result))
+            {
+                return NO_CLASS_HANDLE;
+            }
+            else
+            {
+                return (CORINFO_CLASS_HANDLE)result;
+            }
+        }
+
+        bool isHistogramCount =
+            (schema[i].InstrumentationKind == ICorJitInfo::PgoInstrumentationKind::TypeHandleHistogramIntCount) ||
+            (schema[i].InstrumentationKind == ICorJitInfo::PgoInstrumentationKind::TypeHandleHistogramLongCount);
+
+        if (isHistogramCount && (schema[i].Count == 1) && ((i + 1) < countSchemaItems) &&
+            (schema[i + 1].InstrumentationKind == ICorJitInfo::PgoInstrumentationKind::TypeHandleHistogramTypeHandle))
+        {
+            // Form a histogram
+            //
+            LikelyClassHistogram h((INT_PTR*)(pInstrumentationData + schema[i + 1].Offset), schema[i + 1].Count);
+
+            if (h.countHistogramElements == 0)
+            {
+                return NO_CLASS_HANDLE;
+            }
+
+            // Choose an entry at random.
+            //
+            unsigned                  randomEntryIndex = random->Next(0, h.countHistogramElements - 1);
+            LikelyClassHistogramEntry randomEntry      = h.HistogramEntryAt(randomEntryIndex);
+
+            if (ICorJitInfo::IsUnknownTypeHandle(randomEntry.m_mt))
+            {
+                return NO_CLASS_HANDLE;
+            }
+
+            return (CORINFO_CLASS_HANDLE)randomEntry.m_mt;
+        }
+    }
+
+    return NO_CLASS_HANDLE;
+}