JIT: save generics context for late devirtualization (#63420)

If we have a non-inline candidate call with generics context, save
the context so it's available for late devirtualization.

Fixes a missing devirtualization reported in #63283.

I am deliberately leaving `LateDevirtualizationInfo` more general than
necessary as it may serve as a jumping-off point for enabling late
inlining.

src/coreclr/jit/fginline.cpp

@@ -717,11 +717,18 @@ Compiler::fgWalkResult Compiler::fgLateDevirtualization(GenTree** pTree, fgWalkD
             }
 #endif // DEBUG
 
+            CORINFO_CONTEXT_HANDLE context                = nullptr;
             CORINFO_METHOD_HANDLE  method                 = call->gtCallMethHnd;
             unsigned               methodFlags            = 0;
-            CORINFO_CONTEXT_HANDLE context                = nullptr;
             const bool             isLateDevirtualization = true;
-            bool explicitTailCall = (call->AsCall()->gtCallMoreFlags & GTF_CALL_M_EXPLICIT_TAILCALL) != 0;
+            const bool             explicitTailCall       = call->IsTailPrefixedCall();
+
+            if ((call->gtCallMoreFlags & GTF_CALL_M_LATE_DEVIRT) != 0)
+            {
+                context                          = call->gtLateDevirtualizationInfo->exactContextHnd;
+                call->gtLateDevirtualizationInfo = nullptr;
+            }
+
             comp->impDevirtualizeCall(call, nullptr, &method, &methodFlags, &context, nullptr, isLateDevirtualization,
                                       explicitTailCall);
             *madeChanges = true;

src/coreclr/jit/gentree.cpp

@@ -6151,7 +6151,7 @@ GenTree* Compiler::gtNewInlineCandidateReturnExpr(GenTree* inlineCandidate, var_
 
     if (varTypeIsStruct(inlineCandidate) && !inlineCandidate->OperIsBlkOp())
     {
-        node->AsRetExpr()->gtRetClsHnd = gtGetStructHandle(inlineCandidate);
+        node->gtRetClsHnd = gtGetStructHandle(inlineCandidate);
     }
 
     // GT_RET_EXPR node eventually might be bashed back to GT_CALL (when inlining is aborted for example).
@@ -7806,7 +7806,7 @@ GenTreeCall* Compiler::gtCloneExprCallHelper(GenTreeCall* tree,
     else
     {
         copy->gtCallMethHnd         = tree->gtCallMethHnd;
-        copy->gtInlineCandidateInfo = nullptr;
+        copy->gtInlineCandidateInfo = tree->gtInlineCandidateInfo;
     }
 
     copy->gtCallType   = tree->gtCallType;

src/coreclr/jit/gentree.h

@@ -155,6 +155,7 @@ enum BasicBlockFlags : unsigned __int64;
 struct InlineCandidateInfo;
 struct GuardedDevirtualizationCandidateInfo;
 struct ClassProfileCandidateInfo;
+struct LateDevirtualizationInfo;
 
 typedef unsigned short AssertionIndex;
 
@@ -3760,7 +3761,7 @@ enum GenTreeCallFlags : unsigned int
     GTF_CALL_M_EXP_RUNTIME_LOOKUP      = 0x02000000, // this call needs to be tranformed into CFG for the dynamic dictionary expansion feature.
     GTF_CALL_M_STRESS_TAILCALL         = 0x04000000, // the call is NOT "tail" prefixed but GTF_CALL_M_EXPLICIT_TAILCALL was added because of tail call stress mode
     GTF_CALL_M_EXPANDED_EARLY          = 0x08000000, // the Virtual Call target address is expanded and placed in gtControlExpr in Morph rather than in Lower
-
+    GTF_CALL_M_LATE_DEVIRT             = 0x10000000, // this call has late devirtualzation info
 };
 
 inline constexpr GenTreeCallFlags operator ~(GenTreeCallFlags a)
@@ -4742,7 +4743,7 @@ struct GenTreeCall final : public GenTree
         InlineCandidateInfo*                  gtInlineCandidateInfo;
         GuardedDevirtualizationCandidateInfo* gtGuardedDevirtualizationCandidateInfo;
         ClassProfileCandidateInfo*            gtClassProfileCandidateInfo;
-
+        LateDevirtualizationInfo*             gtLateDevirtualizationInfo;
         CORINFO_GENERIC_HANDLE compileTimeHelperArgumentHandle; // Used to track type handle argument of dynamic helpers
         void*                  gtDirectCallAddress; // Used to pass direct call address between lower and codegen
     };

src/coreclr/jit/importer.cpp

@@ -9950,6 +9950,22 @@ var_types Compiler::impImportCall(OPCODE                  opcode,
             }
             else
             {
+                // If the call is virtual, and has a generics context, and is not going to have a class probe,
+                // record the context for possible use during late devirt.
+                //
+                // If we ever want to devirt at Tier0, and/or see issues where OSR methods under PGO lose
+                // important devirtualizations, we'll want to allow both a class probe and a captured context.
+                //
+                if (origCall->IsVirtual() && (origCall->gtCallType != CT_INDIRECT) && (exactContextHnd != nullptr) &&
+                    (origCall->gtClassProfileCandidateInfo == nullptr))
+                {
+                    JITDUMP("\nSaving context %p for call [%06u]\n", exactContextHnd, dspTreeID(origCall));
+                    origCall->gtCallMoreFlags |= GTF_CALL_M_LATE_DEVIRT;
+                    LateDevirtualizationInfo* const info = new (this, CMK_Inlining) LateDevirtualizationInfo;
+                    info->exactContextHnd                = exactContextHnd;
+                    origCall->gtLateDevirtualizationInfo = info;
+                }
+
                 if (isFatPointerCandidate)
                 {
                     // fatPointer candidates should be in statements of the form call() or var = call().

src/coreclr/jit/inline.h

@@ -591,6 +591,15 @@ struct InlineCandidateInfo : public GuardedDevirtualizationCandidateInfo
     InlineContext*         inlinersContext;
 };
 
+// LateDevirtualizationInfo
+//
+// Used to fill in missing contexts during late devirtualization.
+//
+struct LateDevirtualizationInfo
+{
+    CORINFO_CONTEXT_HANDLE exactContextHnd;
+};
+
 // InlArgInfo describes inline candidate argument properties.
 
 struct InlArgInfo