JIT: Skip old promotion for retbuf defined locals (#104439)

These locals end up being dependently promoted. Skip them and allow
physical promotion to handle them instead.

src/coreclr/jit/fginline.cpp

@@ -387,6 +387,19 @@ class SubstitutePlaceholdersAndDevirtualizeWalker : public GenTreeVisitor<Substi
 #endif // DEBUG
         }
 
+        // If the inline was rejected and returns a retbuffer, then mark that
+        // local as DNER now so that promotion knows to leave it up to physical
+        // promotion.
+        if ((*use)->IsCall())
+        {
+            CallArg* retBuffer = (*use)->AsCall()->gtArgs.GetRetBufferArg();
+            if ((retBuffer != nullptr) && retBuffer->GetNode()->OperIs(GT_LCL_ADDR))
+            {
+                m_compiler->lvaSetVarDoNotEnregister(retBuffer->GetNode()->AsLclVarCommon()->GetLclNum()
+                                                         DEBUGARG(DoNotEnregisterReason::HiddenBufferStructArg));
+            }
+        }
+
 #if FEATURE_MULTIREG_RET
         // If an inline was rejected and the call returns a struct, we may
         // have deferred some work when importing call for cases where the

src/coreclr/jit/importer.cpp

@@ -851,6 +851,12 @@ GenTree* Compiler::impStoreStruct(GenTree*         store,
             GenTree*     destAddr   = impGetNodeAddr(store, CHECK_SPILL_ALL, &indirFlags);
             NewCallArg   newArg     = NewCallArg::Primitive(destAddr).WellKnown(wellKnownArgType);
 
+            if (destAddr->OperIs(GT_LCL_ADDR))
+            {
+                lvaSetVarDoNotEnregister(destAddr->AsLclVarCommon()->GetLclNum()
+                                             DEBUGARG(DoNotEnregisterReason::HiddenBufferStructArg));
+            }
+
 #if !defined(TARGET_ARM)
             // Unmanaged instance methods on Windows or Unix X86 need the retbuf arg after the first (this) parameter
             if ((TargetOS::IsWindows || compUnixX86Abi()) && srcCall->IsUnmanaged())

src/coreclr/jit/lclvars.cpp

@@ -2570,9 +2570,11 @@ bool Compiler::StructPromotionHelper::CanPromoteStructVar(unsigned lclNum)
         return false;
     }
 
-    if (varDsc->IsAddressExposed())
+    if (varDsc->lvDoNotEnregister)
     {
-        JITDUMP("  struct promotion of V%02u is disabled because it has already been marked address exposed\n", lclNum);
+        // Promoting structs that are marked DNER will result in dependent
+        // promotion. Allow physical promotion to handle these.
+        JITDUMP("  struct promotion of V%02u is disabled because it has already been marked DNER\n", lclNum);
         return false;
     }
 
@@ -3173,7 +3175,6 @@ void Compiler::lvaSetVarDoNotEnregister(unsigned varNum DEBUGARG(DoNotEnregister
             break;
         case DoNotEnregisterReason::HiddenBufferStructArg:
             JITDUMP("it is hidden buffer struct arg\n");
-            assert(varDsc->IsHiddenBufferStructArg());
             break;
         case DoNotEnregisterReason::DontEnregStructs:
             JITDUMP("struct enregistration is disabled\n");