Enable GDV with multiple guesses for NativeAOT (#87380)

Author: EgorBo

src/coreclr/jit/compiler.h

@@ -7150,6 +7150,28 @@ class Compiler
                                              unsigned              classAttr,
                                              unsigned              likelihood);
 
+    int getGDVMaxTypeChecks()
+    {
+        int typeChecks = JitConfig.JitGuardedDevirtualizationMaxTypeChecks();
+        if (typeChecks < 0)
+        {
+            // Negative value means "it's up to JIT to decide"
+            if (IsTargetAbi(CORINFO_NATIVEAOT_ABI) && !opts.jitFlags->IsSet(JitFlags::JIT_FLAG_SIZE_OPT))
+            {
+                return 3;
+            }
+
+            // We plan to use 3 for CoreCLR too, but we need to make sure it doesn't regress performance
+            // as CoreCLR heavily relies on Dynamic PGO while for NativeAOT we *usually* don't have it and
+            // can only perform the "exact" devirtualization.
+            return 1;
+        }
+
+        // MAX_GDV_TYPE_CHECKS is the upper limit. The constant can be changed, we just suspect that even
+        // 4 type checks is already too much.
+        return min(MAX_GDV_TYPE_CHECKS, typeChecks);
+    }
+
     bool doesMethodHaveExpRuntimeLookup()
     {
         return (optMethodFlags & OMF_HAS_EXPRUNTIMELOOKUP) != 0;

src/coreclr/jit/importercalls.cpp

@@ -5728,7 +5728,7 @@ void Compiler::pickGDV(GenTreeCall*           call,
     // Prefer class guess as it is cheaper
     if (numberOfClasses > 0)
     {
-        const int maxNumberOfGuesses = min(MAX_GDV_TYPE_CHECKS, JitConfig.JitGuardedDevirtualizationMaxTypeChecks());
+        const int maxNumberOfGuesses = getGDVMaxTypeChecks();
         if (maxNumberOfGuesses == 0)
         {
             // DOTNET_JitGuardedDevirtualizationMaxTypeChecks=0 means we don't want to do any guarded devirtualization
@@ -5929,7 +5929,7 @@ void Compiler::considerGuardedDevirtualization(GenTreeCall*            call,
     {
         pickGDV(call, ilOffset, isInterface, likelyClasses, likelyMethodes, &candidatesCount, likelihoods);
         assert((unsigned)candidatesCount <= MAX_GDV_TYPE_CHECKS);
-        assert((unsigned)candidatesCount <= (unsigned)JitConfig.JitGuardedDevirtualizationMaxTypeChecks());
+        assert((unsigned)candidatesCount <= (unsigned)getGDVMaxTypeChecks());
         if (candidatesCount == 0)
         {
             hasPgoData = false;
@@ -5942,7 +5942,7 @@ void Compiler::considerGuardedDevirtualization(GenTreeCall*            call,
     // from both.
     if (!hasPgoData && (baseClass != NO_CLASS_HANDLE) && JitConfig.JitEnableExactDevirtualization())
     {
-        int maxTypeChecks = min(JitConfig.JitGuardedDevirtualizationMaxTypeChecks(), MAX_GDV_TYPE_CHECKS);
+        int maxTypeChecks = min(getGDVMaxTypeChecks(), MAX_GDV_TYPE_CHECKS);
 
         CORINFO_CLASS_HANDLE exactClasses[MAX_GDV_TYPE_CHECKS];
         int numExactClasses = info.compCompHnd->getExactClasses(baseClass, MAX_GDV_TYPE_CHECKS, exactClasses);

src/coreclr/jit/jitconfigvalues.h

@@ -528,8 +528,8 @@ CONFIG_INTEGER(JitEnableGuardedDevirtualization, W("JitEnableGuardedDevirtualiza
 
 #define MAX_GDV_TYPE_CHECKS 5
 // Number of types to probe for polymorphic virtual call-sites to devirtualize them,
-// Max number is MAX_GDV_TYPE_CHECKS defined above ^
-CONFIG_INTEGER(JitGuardedDevirtualizationMaxTypeChecks, W("JitGuardedDevirtualizationMaxTypeChecks"), 1)
+// Max number is MAX_GDV_TYPE_CHECKS defined above ^. -1 means it's up to JIT to decide
+CONFIG_INTEGER(JitGuardedDevirtualizationMaxTypeChecks, W("JitGuardedDevirtualizationMaxTypeChecks"), -1)
 
 // Various policies for GuardedDevirtualization
 CONFIG_INTEGER(JitGuardedDevirtualizationChainLikelihood, W("JitGuardedDevirtualizationChainLikelihood"), 0x4B) // 75