JIT: refine types when creating arg temps to improve devirtualization

src/jit/flowgraph.cpp

@@ -22625,13 +22625,10 @@ GenTreePtr Compiler::fgInlinePrependStatements(InlineInfo* inlineInfo)
                     impAssignTempGen(tmpNum, argNode, structHnd, (unsigned)CHECK_SPILL_NONE, &afterStmt, callILOffset,
                                      block);
 
-                    // If we know the argument's value can't be
-                    // changed within the method body, try and improve
-                    // the type of the temp.
-                    if (argIsSingleDef && (argType == TYP_REF))
-                    {
-                        lvaUpdateClass(tmpNum, argNode);
-                    }
+                    // We used to refine the temp type here based on
+                    // the actual arg, but we now do this up front, when
+                    // creating the temp, over in impInlineFetchArg.
+                    CLANG_FORMAT_COMMENT_ANCHOR;
 
 #ifdef DEBUG
                     if (verbose)

src/jit/importer.cpp

@@ -18165,11 +18165,22 @@ GenTreePtr Compiler::impInlineFetchArg(unsigned lclNum, InlArgInfo* inlArgInfo,
 
             lvaTable[tmpNum].lvType = lclTyp;
 
-            // Copy over class handle for ref types. Note this may be
-            // further improved if it is a shared type and we know the exact context.
+            // For ref types, determine the type of the temp.
             if (lclTyp == TYP_REF)
             {
-                lvaSetClass(tmpNum, lclInfo.lclVerTypeInfo.GetClassHandleForObjRef());
+                if (!argCanBeModified)
+                {
+                    // If the arg can't be modified in the method
+                    // body, use the type of the value, if
+                    // known. Otherwise, use the declared type.
+                    lvaSetClass(tmpNum, argInfo.argNode, lclInfo.lclVerTypeInfo.GetClassHandleForObjRef());
+                }
+                else
+                {
+                    // Arg might be modified, use the delcared type of
+                    // the argument.
+                    lvaSetClass(tmpNum, lclInfo.lclVerTypeInfo.GetClassHandleForObjRef());
+                }
             }
 
             assert(lvaTable[tmpNum].lvAddrExposed == 0);