From 39a4204d3525114836fd98d0572369065b812ade Mon Sep 17 00:00:00 2001
From: Steven He <hez2010@outlook.com>
Date: Wed, 29 Jan 2025 17:00:52 +0900
Subject: [PATCH 01/15] Teach the JIT about System.SZArrayHelper.GetEnumerator

---
 src/coreclr/jit/fgbasic.cpp  |  2 ++
 src/coreclr/jit/fginline.cpp | 18 ++++++++++++++++++
 2 files changed, 20 insertions(+)

diff --git a/src/coreclr/jit/fgbasic.cpp b/src/coreclr/jit/fgbasic.cpp
index a34e2622157ba3..68a8894f5b46ae 100644
--- a/src/coreclr/jit/fgbasic.cpp
+++ b/src/coreclr/jit/fgbasic.cpp
@@ -3562,6 +3562,8 @@ void Compiler::fgFindBasicBlocks()
                         lvaSetClass(lvaInlineeReturnSpillTemp, retClassHnd);
                     }
                 }
+
+                impInlineInfo->inlineCandidateInfo->preexistingSpillTemp = lvaInlineeReturnSpillTemp;
             }
         }
 
diff --git a/src/coreclr/jit/fginline.cpp b/src/coreclr/jit/fginline.cpp
index ba55f989961d8d..32aac52271c1d1 100644
--- a/src/coreclr/jit/fginline.cpp
+++ b/src/coreclr/jit/fginline.cpp
@@ -1416,6 +1416,24 @@ void Compiler::fgInvokeInlineeCompiler(GenTreeCall* call, InlineResult* inlineRe
     // Let's insert it to inliner's basic block list.
     fgInsertInlineeBlocks(&inlineInfo);
 
+    // If we are inlining System.SZArrayHelper.GetEnumerator, mark the temp as single def to allow updating
+    // class. This can enable further devirtualization and inlining of MoveNext, get_Current and Dispose
+    // methods.
+    if (lookupNamedIntrinsic(inlineInfo.fncHandle) == NI_System_SZArrayHelper_GetEnumerator &&
+        inlineInfo.inlineCandidateInfo->preexistingSpillTemp != BAD_VAR_NUM)
+    {
+        lvaGetDesc(inlineInfo.inlineCandidateInfo->preexistingSpillTemp)->lvSingleDef = 1;
+
+#ifdef DEBUG
+        if (verbose)
+        {
+            printf(
+                "\nInlinee is System.SZArrayHelper.GetEnumerator, mark the return spilling local V%02u as single def.\n",
+                inlineInfo.inlineCandidateInfo->preexistingSpillTemp);
+        }
+#endif // DEBUG
+    }
+
 #ifdef DEBUG
 
     if (verbose)

From 6ec367f7b33f039de74a53f07c82c7551d1c17f0 Mon Sep 17 00:00:00 2001
From: Steven He <hez2010@outlook.com>
Date: Wed, 29 Jan 2025 18:23:54 +0900
Subject: [PATCH 02/15] A better approach

---
 src/coreclr/jit/fginline.cpp | 18 ------------------
 src/coreclr/jit/fgopt.cpp    | 19 +++++++++++++++++--
 2 files changed, 17 insertions(+), 20 deletions(-)

diff --git a/src/coreclr/jit/fginline.cpp b/src/coreclr/jit/fginline.cpp
index 32aac52271c1d1..ba55f989961d8d 100644
--- a/src/coreclr/jit/fginline.cpp
+++ b/src/coreclr/jit/fginline.cpp
@@ -1416,24 +1416,6 @@ void Compiler::fgInvokeInlineeCompiler(GenTreeCall* call, InlineResult* inlineRe
     // Let's insert it to inliner's basic block list.
     fgInsertInlineeBlocks(&inlineInfo);
 
-    // If we are inlining System.SZArrayHelper.GetEnumerator, mark the temp as single def to allow updating
-    // class. This can enable further devirtualization and inlining of MoveNext, get_Current and Dispose
-    // methods.
-    if (lookupNamedIntrinsic(inlineInfo.fncHandle) == NI_System_SZArrayHelper_GetEnumerator &&
-        inlineInfo.inlineCandidateInfo->preexistingSpillTemp != BAD_VAR_NUM)
-    {
-        lvaGetDesc(inlineInfo.inlineCandidateInfo->preexistingSpillTemp)->lvSingleDef = 1;
-
-#ifdef DEBUG
-        if (verbose)
-        {
-            printf(
-                "\nInlinee is System.SZArrayHelper.GetEnumerator, mark the return spilling local V%02u as single def.\n",
-                inlineInfo.inlineCandidateInfo->preexistingSpillTemp);
-        }
-#endif // DEBUG
-    }
-
 #ifdef DEBUG
 
     if (verbose)
diff --git a/src/coreclr/jit/fgopt.cpp b/src/coreclr/jit/fgopt.cpp
index 6187274c70332f..98e2f0846dedd0 100644
--- a/src/coreclr/jit/fgopt.cpp
+++ b/src/coreclr/jit/fgopt.cpp
@@ -317,9 +317,24 @@ PhaseStatus Compiler::fgPostImportationCleanup()
             {
                 LclVarDsc* returnSpillVarDsc = lvaGetDesc(lvaInlineeReturnSpillTemp);
 
-                if ((returnSpillVarDsc->lvType == TYP_REF) && returnSpillVarDsc->lvSingleDef)
+                if ((returnSpillVarDsc->lvType == TYP_REF))
                 {
-                    lvaUpdateClass(lvaInlineeReturnSpillTemp, retExprClassHnd, impInlineInfo->retExprClassHndIsExact);
+                    if (returnSpillVarDsc->lvSingleDef)
+                    {
+                        lvaUpdateClass(lvaInlineeReturnSpillTemp, retExprClassHnd,
+                                       impInlineInfo->retExprClassHndIsExact);
+                    }
+                    else if (lookupNamedIntrinsic(info.compMethodHnd) == NI_System_SZArrayHelper_GetEnumerator)
+                    {
+                        // If we are inlining System.SZArrayHelper.GetEnumerator, update the type of the return spill
+                        // temp regardless of whether it is single def or not.
+                        returnSpillVarDsc->lvType = TYP_REF;
+                        returnSpillVarDsc->lvClassHnd = retExprClassHnd;
+                        returnSpillVarDsc->lvClassIsExact = impInlineInfo->retExprClassHndIsExact;
+#if DEBUG
+                        returnSpillVarDsc->lvClassInfoUpdated = true;
+#endif
+                    }
                 }
             }
         }

From 126a04287d516a5f929b731ae12aee36759101bc Mon Sep 17 00:00:00 2001
From: Steven He <hez2010@outlook.com>
Date: Wed, 29 Jan 2025 18:24:54 +0900
Subject: [PATCH 03/15] Format JIT

---
 src/coreclr/jit/fgopt.cpp | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/src/coreclr/jit/fgopt.cpp b/src/coreclr/jit/fgopt.cpp
index 98e2f0846dedd0..6bd46ce1e5faeb 100644
--- a/src/coreclr/jit/fgopt.cpp
+++ b/src/coreclr/jit/fgopt.cpp
@@ -328,8 +328,7 @@ PhaseStatus Compiler::fgPostImportationCleanup()
                     {
                         // If we are inlining System.SZArrayHelper.GetEnumerator, update the type of the return spill
                         // temp regardless of whether it is single def or not.
-                        returnSpillVarDsc->lvType = TYP_REF;
-                        returnSpillVarDsc->lvClassHnd = retExprClassHnd;
+                        returnSpillVarDsc->lvClassHnd     = retExprClassHnd;
                         returnSpillVarDsc->lvClassIsExact = impInlineInfo->retExprClassHndIsExact;
 #if DEBUG
                         returnSpillVarDsc->lvClassInfoUpdated = true;

From 0e91fd0fd7e96e929eb32375e60201b471f04955 Mon Sep 17 00:00:00 2001
From: Steven He <hez2010@outlook.com>
Date: Wed, 29 Jan 2025 18:44:08 +0900
Subject: [PATCH 04/15] Use lvaInlineeReturnSpillTempFreshlyCreated

---
 src/coreclr/jit/compiler.h  | 2 ++
 src/coreclr/jit/fgbasic.cpp | 2 +-
 src/coreclr/jit/fgopt.cpp   | 6 +++---
 3 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/src/coreclr/jit/compiler.h b/src/coreclr/jit/compiler.h
index 442dd8f17a5a24..dc5b00f176065d 100644
--- a/src/coreclr/jit/compiler.h
+++ b/src/coreclr/jit/compiler.h
@@ -4091,6 +4091,8 @@ class Compiler
     unsigned lvaInlineeReturnSpillTemp = BAD_VAR_NUM; // The temp to spill the non-VOID return expression
                                         // in case there are multiple BBJ_RETURN blocks in the inlinee
                                         // or if the inlinee has GC ref locals.
+    
+    bool lvaInlineeReturnSpillTempFreshlyCreated = false; // True if the temp was freshly created for the inlinee return
 
 #if FEATURE_FIXED_OUT_ARGS
     unsigned            lvaOutgoingArgSpaceVar = BAD_VAR_NUM;  // var that represents outgoing argument space
diff --git a/src/coreclr/jit/fgbasic.cpp b/src/coreclr/jit/fgbasic.cpp
index 68a8894f5b46ae..6772977aed91a4 100644
--- a/src/coreclr/jit/fgbasic.cpp
+++ b/src/coreclr/jit/fgbasic.cpp
@@ -3563,7 +3563,7 @@ void Compiler::fgFindBasicBlocks()
                     }
                 }
 
-                impInlineInfo->inlineCandidateInfo->preexistingSpillTemp = lvaInlineeReturnSpillTemp;
+                lvaInlineeReturnSpillTempFreshlyCreated = true;
             }
         }
 
diff --git a/src/coreclr/jit/fgopt.cpp b/src/coreclr/jit/fgopt.cpp
index 6bd46ce1e5faeb..61c68d12d6aa80 100644
--- a/src/coreclr/jit/fgopt.cpp
+++ b/src/coreclr/jit/fgopt.cpp
@@ -324,10 +324,10 @@ PhaseStatus Compiler::fgPostImportationCleanup()
                         lvaUpdateClass(lvaInlineeReturnSpillTemp, retExprClassHnd,
                                        impInlineInfo->retExprClassHndIsExact);
                     }
-                    else if (lookupNamedIntrinsic(info.compMethodHnd) == NI_System_SZArrayHelper_GetEnumerator)
+                    else if (lvaInlineeReturnSpillTempFreshlyCreated)
                     {
-                        // If we are inlining System.SZArrayHelper.GetEnumerator, update the type of the return spill
-                        // temp regardless of whether it is single def or not.
+                        // If the inlinee return spill temp was freshly created in fgFindBasicBlocks, we can update the
+                        // type regardless of whether it is single def or not.
                         returnSpillVarDsc->lvClassHnd     = retExprClassHnd;
                         returnSpillVarDsc->lvClassIsExact = impInlineInfo->retExprClassHndIsExact;
 #if DEBUG

From c43cf22007ffb5f074d172a40bef17716667d0e8 Mon Sep 17 00:00:00 2001
From: Steve <hez2010@outlook.com>
Date: Wed, 29 Jan 2025 18:53:05 +0900
Subject: [PATCH 05/15] Update src/coreclr/jit/fgopt.cpp

Co-authored-by: Jakob Botsch Nielsen <Jakob.botsch.nielsen@gmail.com>
---
 src/coreclr/jit/fgopt.cpp | 12 +-----------
 1 file changed, 1 insertion(+), 11 deletions(-)

diff --git a/src/coreclr/jit/fgopt.cpp b/src/coreclr/jit/fgopt.cpp
index 61c68d12d6aa80..99f93d0003de23 100644
--- a/src/coreclr/jit/fgopt.cpp
+++ b/src/coreclr/jit/fgopt.cpp
@@ -319,21 +319,11 @@ PhaseStatus Compiler::fgPostImportationCleanup()
 
                 if ((returnSpillVarDsc->lvType == TYP_REF))
                 {
-                    if (returnSpillVarDsc->lvSingleDef)
+                    if (returnSpillVarDsc->lvSingleDef || lvaInlineeReturnSpillTempFreshlyCreated)
                     {
                         lvaUpdateClass(lvaInlineeReturnSpillTemp, retExprClassHnd,
                                        impInlineInfo->retExprClassHndIsExact);
                     }
-                    else if (lvaInlineeReturnSpillTempFreshlyCreated)
-                    {
-                        // If the inlinee return spill temp was freshly created in fgFindBasicBlocks, we can update the
-                        // type regardless of whether it is single def or not.
-                        returnSpillVarDsc->lvClassHnd     = retExprClassHnd;
-                        returnSpillVarDsc->lvClassIsExact = impInlineInfo->retExprClassHndIsExact;
-#if DEBUG
-                        returnSpillVarDsc->lvClassInfoUpdated = true;
-#endif
-                    }
                 }
             }
         }

From 1c45b3eee102ecf469f3bd9684c9d53fba31dae8 Mon Sep 17 00:00:00 2001
From: Steven He <hez2010@outlook.com>
Date: Wed, 29 Jan 2025 18:53:22 +0900
Subject: [PATCH 06/15] Update assertion

---
 src/coreclr/jit/lclvars.cpp | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/src/coreclr/jit/lclvars.cpp b/src/coreclr/jit/lclvars.cpp
index ad9b203b454e3b..63bdd1d8ca61cb 100644
--- a/src/coreclr/jit/lclvars.cpp
+++ b/src/coreclr/jit/lclvars.cpp
@@ -3631,8 +3631,9 @@ void Compiler::lvaUpdateClass(unsigned varNum, CORINFO_CLASS_HANDLE clsHnd, bool
     // We should already have a class
     assert(varDsc->lvClassHnd != NO_CLASS_HANDLE);
 
-    // We should only be updating classes for single-def locals.
-    assert(varDsc->lvSingleDef);
+    // We should only be updating classes for single-def locals,
+    // or for the inlinee return spill temp that is freshly created.
+    assert(varDsc->lvSingleDef || (lvaInlineeReturnSpillTempFreshlyCreated && varNum == lvaInlineeReturnSpillTemp));
 
     // Now see if we should update.
     //
@@ -3679,7 +3680,7 @@ void Compiler::lvaUpdateClass(unsigned varNum, CORINFO_CLASS_HANDLE clsHnd, bool
 }
 
 //------------------------------------------------------------------------
-// lvaUpdateClass: Uupdate class information for a local var from a tree
+// lvaUpdateClass: Update class information for a local var from a tree
 //  or stack type
 //
 // Arguments:

From 02c808b88ee022cb5df932f185306df41e22b685 Mon Sep 17 00:00:00 2001
From: Steven He <hez2010@outlook.com>
Date: Wed, 29 Jan 2025 18:57:03 +0900
Subject: [PATCH 07/15] Nit

---
 src/coreclr/jit/fgopt.cpp | 9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)

diff --git a/src/coreclr/jit/fgopt.cpp b/src/coreclr/jit/fgopt.cpp
index 99f93d0003de23..3760b8745f4f51 100644
--- a/src/coreclr/jit/fgopt.cpp
+++ b/src/coreclr/jit/fgopt.cpp
@@ -317,13 +317,10 @@ PhaseStatus Compiler::fgPostImportationCleanup()
             {
                 LclVarDsc* returnSpillVarDsc = lvaGetDesc(lvaInlineeReturnSpillTemp);
 
-                if ((returnSpillVarDsc->lvType == TYP_REF))
+                if (returnSpillVarDsc->lvType == TYP_REF &&
+                    (returnSpillVarDsc->lvSingleDef || lvaInlineeReturnSpillTempFreshlyCreated))
                 {
-                    if (returnSpillVarDsc->lvSingleDef || lvaInlineeReturnSpillTempFreshlyCreated)
-                    {
-                        lvaUpdateClass(lvaInlineeReturnSpillTemp, retExprClassHnd,
-                                       impInlineInfo->retExprClassHndIsExact);
-                    }
+                    lvaUpdateClass(lvaInlineeReturnSpillTemp, retExprClassHnd, impInlineInfo->retExprClassHndIsExact);
                 }
             }
         }

From 3ae7287345a7146535cfa416cffbb407613e234a Mon Sep 17 00:00:00 2001
From: Steven He <hez2010@outlook.com>
Date: Wed, 29 Jan 2025 18:58:37 +0900
Subject: [PATCH 08/15] Nit

---
 src/coreclr/jit/lclvars.cpp | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/coreclr/jit/lclvars.cpp b/src/coreclr/jit/lclvars.cpp
index 63bdd1d8ca61cb..3e6625f4c52a72 100644
--- a/src/coreclr/jit/lclvars.cpp
+++ b/src/coreclr/jit/lclvars.cpp
@@ -3632,7 +3632,7 @@ void Compiler::lvaUpdateClass(unsigned varNum, CORINFO_CLASS_HANDLE clsHnd, bool
     assert(varDsc->lvClassHnd != NO_CLASS_HANDLE);
 
     // We should only be updating classes for single-def locals,
-    // or for the inlinee return spill temp that is freshly created.
+    // or for the inlinee return spill temps that are freshly created.
     assert(varDsc->lvSingleDef || (lvaInlineeReturnSpillTempFreshlyCreated && varNum == lvaInlineeReturnSpillTemp));
 
     // Now see if we should update.

From f5fd84ed6921519a0496e27ea32d98ae677abcfe Mon Sep 17 00:00:00 2001
From: Steven He <hez2010@outlook.com>
Date: Thu, 30 Jan 2025 01:18:49 +0900
Subject: [PATCH 09/15] Address review feedbacks

---
 src/coreclr/jit/compiler.h  | 2 +-
 src/coreclr/jit/fgopt.cpp   | 5 +++--
 src/coreclr/jit/lclvars.cpp | 8 ++++----
 3 files changed, 8 insertions(+), 7 deletions(-)

diff --git a/src/coreclr/jit/compiler.h b/src/coreclr/jit/compiler.h
index dc5b00f176065d..bb24937e794faf 100644
--- a/src/coreclr/jit/compiler.h
+++ b/src/coreclr/jit/compiler.h
@@ -4357,7 +4357,7 @@ class Compiler
     // If the local is TYP_REF, set or update the associated class information.
     void lvaSetClass(unsigned varNum, CORINFO_CLASS_HANDLE clsHnd, bool isExact = false);
     void lvaSetClass(unsigned varNum, GenTree* tree, CORINFO_CLASS_HANDLE stackHandle = nullptr);
-    void lvaUpdateClass(unsigned varNum, CORINFO_CLASS_HANDLE clsHnd, bool isExact = false);
+    void lvaUpdateClass(unsigned varNum, CORINFO_CLASS_HANDLE clsHnd, bool isExact = false DEBUGARG(bool mustBeSingleDef = true));
     void lvaUpdateClass(unsigned varNum, GenTree* tree, CORINFO_CLASS_HANDLE stackHandle = nullptr);
 
 #define MAX_NumOfFieldsInPromotableStruct 4 // Maximum number of fields in promotable struct
diff --git a/src/coreclr/jit/fgopt.cpp b/src/coreclr/jit/fgopt.cpp
index 3760b8745f4f51..91ad0de4d8b62f 100644
--- a/src/coreclr/jit/fgopt.cpp
+++ b/src/coreclr/jit/fgopt.cpp
@@ -309,7 +309,7 @@ PhaseStatus Compiler::fgPostImportationCleanup()
     {
         // Update type of return spill temp if we have gathered
         // better info when importing the inlinee, and the return
-        // spill temp is single def.
+        // spill temp is single def or was freshly created for this inlinee
         if (fgNeedReturnSpillTemp())
         {
             CORINFO_CLASS_HANDLE retExprClassHnd = impInlineInfo->retExprClassHnd;
@@ -320,7 +320,8 @@ PhaseStatus Compiler::fgPostImportationCleanup()
                 if (returnSpillVarDsc->lvType == TYP_REF &&
                     (returnSpillVarDsc->lvSingleDef || lvaInlineeReturnSpillTempFreshlyCreated))
                 {
-                    lvaUpdateClass(lvaInlineeReturnSpillTemp, retExprClassHnd, impInlineInfo->retExprClassHndIsExact);
+                    lvaUpdateClass(lvaInlineeReturnSpillTemp, retExprClassHnd,
+                                   impInlineInfo->retExprClassHndIsExact DEBUGARG(false));
                 }
             }
         }
diff --git a/src/coreclr/jit/lclvars.cpp b/src/coreclr/jit/lclvars.cpp
index 3e6625f4c52a72..11d822e8654a21 100644
--- a/src/coreclr/jit/lclvars.cpp
+++ b/src/coreclr/jit/lclvars.cpp
@@ -3601,6 +3601,7 @@ void Compiler::lvaSetClass(unsigned varNum, GenTree* tree, CORINFO_CLASS_HANDLE
 //    varNum -- number of the variable
 //    clsHnd -- class handle to use in set or update
 //    isExact -- true if class is known exactly
+//    mustBeSingleDef -- true if we should only update single-def locals
 //
 // Notes:
 //
@@ -3618,7 +3619,7 @@ void Compiler::lvaSetClass(unsigned varNum, GenTree* tree, CORINFO_CLASS_HANDLE
 //    for shared code, so ensuring this is so is currently not
 //    possible.
 
-void Compiler::lvaUpdateClass(unsigned varNum, CORINFO_CLASS_HANDLE clsHnd, bool isExact)
+void Compiler::lvaUpdateClass(unsigned varNum, CORINFO_CLASS_HANDLE clsHnd, bool isExact DEBUGARG(bool mustBeSingleDef))
 {
     assert(varNum < lvaCount);
 
@@ -3631,9 +3632,8 @@ void Compiler::lvaUpdateClass(unsigned varNum, CORINFO_CLASS_HANDLE clsHnd, bool
     // We should already have a class
     assert(varDsc->lvClassHnd != NO_CLASS_HANDLE);
 
-    // We should only be updating classes for single-def locals,
-    // or for the inlinee return spill temps that are freshly created.
-    assert(varDsc->lvSingleDef || (lvaInlineeReturnSpillTempFreshlyCreated && varNum == lvaInlineeReturnSpillTemp));
+    // We should only be updating classes for single-def locals if requested
+    assert(!mustBeSingleDef || varDsc->lvSingleDef);
 
     // Now see if we should update.
     //

From 22bd9cf32b8c6fa3ca24930fa312048caf6ff039 Mon Sep 17 00:00:00 2001
From: Steven He <hez2010@outlook.com>
Date: Thu, 30 Jan 2025 01:32:03 +0900
Subject: [PATCH 10/15] Make the check always available

---
 src/coreclr/jit/compiler.h  |  2 +-
 src/coreclr/jit/fgopt.cpp   |  4 ++--
 src/coreclr/jit/lclvars.cpp | 10 +++++++---
 3 files changed, 10 insertions(+), 6 deletions(-)

diff --git a/src/coreclr/jit/compiler.h b/src/coreclr/jit/compiler.h
index bb24937e794faf..ab95317e184cb5 100644
--- a/src/coreclr/jit/compiler.h
+++ b/src/coreclr/jit/compiler.h
@@ -4357,7 +4357,7 @@ class Compiler
     // If the local is TYP_REF, set or update the associated class information.
     void lvaSetClass(unsigned varNum, CORINFO_CLASS_HANDLE clsHnd, bool isExact = false);
     void lvaSetClass(unsigned varNum, GenTree* tree, CORINFO_CLASS_HANDLE stackHandle = nullptr);
-    void lvaUpdateClass(unsigned varNum, CORINFO_CLASS_HANDLE clsHnd, bool isExact = false DEBUGARG(bool mustBeSingleDef = true));
+    void lvaUpdateClass(unsigned varNum, CORINFO_CLASS_HANDLE clsHnd, bool isExact = false, bool singleDefOnly = true);
     void lvaUpdateClass(unsigned varNum, GenTree* tree, CORINFO_CLASS_HANDLE stackHandle = nullptr);
 
 #define MAX_NumOfFieldsInPromotableStruct 4 // Maximum number of fields in promotable struct
diff --git a/src/coreclr/jit/fgopt.cpp b/src/coreclr/jit/fgopt.cpp
index 91ad0de4d8b62f..a9442342b50436 100644
--- a/src/coreclr/jit/fgopt.cpp
+++ b/src/coreclr/jit/fgopt.cpp
@@ -320,8 +320,8 @@ PhaseStatus Compiler::fgPostImportationCleanup()
                 if (returnSpillVarDsc->lvType == TYP_REF &&
                     (returnSpillVarDsc->lvSingleDef || lvaInlineeReturnSpillTempFreshlyCreated))
                 {
-                    lvaUpdateClass(lvaInlineeReturnSpillTemp, retExprClassHnd,
-                                   impInlineInfo->retExprClassHndIsExact DEBUGARG(false));
+                    lvaUpdateClass(lvaInlineeReturnSpillTemp, retExprClassHnd, impInlineInfo->retExprClassHndIsExact,
+                                   false);
                 }
             }
         }
diff --git a/src/coreclr/jit/lclvars.cpp b/src/coreclr/jit/lclvars.cpp
index 11d822e8654a21..dc5144b0431659 100644
--- a/src/coreclr/jit/lclvars.cpp
+++ b/src/coreclr/jit/lclvars.cpp
@@ -3601,7 +3601,7 @@ void Compiler::lvaSetClass(unsigned varNum, GenTree* tree, CORINFO_CLASS_HANDLE
 //    varNum -- number of the variable
 //    clsHnd -- class handle to use in set or update
 //    isExact -- true if class is known exactly
-//    mustBeSingleDef -- true if we should only update single-def locals
+//    singleDefOnly -- true if we should only update single-def locals
 //
 // Notes:
 //
@@ -3619,7 +3619,7 @@ void Compiler::lvaSetClass(unsigned varNum, GenTree* tree, CORINFO_CLASS_HANDLE
 //    for shared code, so ensuring this is so is currently not
 //    possible.
 
-void Compiler::lvaUpdateClass(unsigned varNum, CORINFO_CLASS_HANDLE clsHnd, bool isExact DEBUGARG(bool mustBeSingleDef))
+void Compiler::lvaUpdateClass(unsigned varNum, CORINFO_CLASS_HANDLE clsHnd, bool isExact, bool singleDefOnly)
 {
     assert(varNum < lvaCount);
 
@@ -3633,7 +3633,11 @@ void Compiler::lvaUpdateClass(unsigned varNum, CORINFO_CLASS_HANDLE clsHnd, bool
     assert(varDsc->lvClassHnd != NO_CLASS_HANDLE);
 
     // We should only be updating classes for single-def locals if requested
-    assert(!mustBeSingleDef || varDsc->lvSingleDef);
+    if (singleDefOnly && !varDsc->lvSingleDef)
+    {
+        assert(!"Updating class for multi-def local");
+        return;
+    }
 
     // Now see if we should update.
     //

From e94839bf462b636e07e13e3d14553de4281e781a Mon Sep 17 00:00:00 2001
From: Steven He <hez2010@outlook.com>
Date: Thu, 30 Jan 2025 17:52:20 +0900
Subject: [PATCH 11/15] Deploy a fix for COM objects

---
 src/coreclr/jit/importer.cpp | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/src/coreclr/jit/importer.cpp b/src/coreclr/jit/importer.cpp
index 0435ecf0b514e4..a031aecebd8ec5 100644
--- a/src/coreclr/jit/importer.cpp
+++ b/src/coreclr/jit/importer.cpp
@@ -11075,6 +11075,18 @@ bool Compiler::impReturnInstruction(int prefixFlags, OPCODE& opcode)
                             // of this return value.
                             impInlineInfo->retExprClassHnd        = returnClsHnd;
                             impInlineInfo->retExprClassHndIsExact = isExact;
+#ifdef FEATURE_COMINTEROP
+                            if (returnClsHnd != nullptr)
+                            {
+                                const char* className = eeGetClassName(returnClsHnd);
+                                if (strcmp(className, "System.Dynamic.DynamicMetaObject") == 0)
+                                {
+                                    // We don't want to set DynamicMetaObject as exact
+                                    // because of dynamic runtime binding on COM objects.
+                                    impInlineInfo->retExprClassHndIsExact = false;
+                                }
+                            }
+#endif // FEATURE_COMINTEROP
                         }
                         else if (impInlineInfo->retExprClassHnd != returnClsHnd)
                         {

From 7844b8f7130fde125bc1a8475d8244dd3149b16d Mon Sep 17 00:00:00 2001
From: Steven He <hez2010@outlook.com>
Date: Thu, 30 Jan 2025 22:46:16 +0900
Subject: [PATCH 12/15] Revert "Deploy a fix for COM objects"

This reverts commit e94839bf462b636e07e13e3d14553de4281e781a.
---
 src/coreclr/jit/importer.cpp | 12 ------------
 1 file changed, 12 deletions(-)

diff --git a/src/coreclr/jit/importer.cpp b/src/coreclr/jit/importer.cpp
index a031aecebd8ec5..0435ecf0b514e4 100644
--- a/src/coreclr/jit/importer.cpp
+++ b/src/coreclr/jit/importer.cpp
@@ -11075,18 +11075,6 @@ bool Compiler::impReturnInstruction(int prefixFlags, OPCODE& opcode)
                             // of this return value.
                             impInlineInfo->retExprClassHnd        = returnClsHnd;
                             impInlineInfo->retExprClassHndIsExact = isExact;
-#ifdef FEATURE_COMINTEROP
-                            if (returnClsHnd != nullptr)
-                            {
-                                const char* className = eeGetClassName(returnClsHnd);
-                                if (strcmp(className, "System.Dynamic.DynamicMetaObject") == 0)
-                                {
-                                    // We don't want to set DynamicMetaObject as exact
-                                    // because of dynamic runtime binding on COM objects.
-                                    impInlineInfo->retExprClassHndIsExact = false;
-                                }
-                            }
-#endif // FEATURE_COMINTEROP
                         }
                         else if (impInlineInfo->retExprClassHnd != returnClsHnd)
                         {

From 3c5a41b5ab35e228103d722c713eddc07ba27dc4 Mon Sep 17 00:00:00 2001
From: Steven He <hez2010@outlook.com>
Date: Fri, 31 Jan 2025 00:45:11 +0900
Subject: [PATCH 13/15] The real fix

---
 src/coreclr/jit/importer.cpp | 21 +++++++++++++++------
 1 file changed, 15 insertions(+), 6 deletions(-)

diff --git a/src/coreclr/jit/importer.cpp b/src/coreclr/jit/importer.cpp
index 0435ecf0b514e4..8447d06ba4db0c 100644
--- a/src/coreclr/jit/importer.cpp
+++ b/src/coreclr/jit/importer.cpp
@@ -11076,13 +11076,22 @@ bool Compiler::impReturnInstruction(int prefixFlags, OPCODE& opcode)
                             impInlineInfo->retExprClassHnd        = returnClsHnd;
                             impInlineInfo->retExprClassHndIsExact = isExact;
                         }
-                        else if (impInlineInfo->retExprClassHnd != returnClsHnd)
+                        else
                         {
-                            // This return site type differs from earlier seen sites,
-                            // so reset the info and we'll fall back to using the method's
-                            // declared return type for the return spill temp.
-                            impInlineInfo->retExprClassHnd        = nullptr;
-                            impInlineInfo->retExprClassHndIsExact = false;
+                            if (impInlineInfo->retExprClassHndIsExact != isExact)
+                            {
+                                // This return site type exactness differs from earlier seen sites,
+                                // so reset the exactness info.
+                                impInlineInfo->retExprClassHndIsExact = false;
+                            }
+                            if (impInlineInfo->retExprClassHnd != returnClsHnd)
+                            {
+                                // This return site type differs from earlier seen sites,
+                                // so reset the info and we'll fall back to using the method's
+                                // declared return type for the return spill temp.
+                                impInlineInfo->retExprClassHnd        = nullptr;
+                                impInlineInfo->retExprClassHndIsExact = false;
+                            }
                         }
                     }
 

From b177f777c55acbad451fa65bbd0b82bf3ebd263c Mon Sep 17 00:00:00 2001
From: Steve <hez2010@outlook.com>
Date: Fri, 31 Jan 2025 15:49:05 +0900
Subject: [PATCH 14/15] Address review feedback

---
 src/coreclr/jit/importer.cpp | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/src/coreclr/jit/importer.cpp b/src/coreclr/jit/importer.cpp
index 8447d06ba4db0c..41983fb77c3ef4 100644
--- a/src/coreclr/jit/importer.cpp
+++ b/src/coreclr/jit/importer.cpp
@@ -11078,12 +11078,6 @@ bool Compiler::impReturnInstruction(int prefixFlags, OPCODE& opcode)
                         }
                         else
                         {
-                            if (impInlineInfo->retExprClassHndIsExact != isExact)
-                            {
-                                // This return site type exactness differs from earlier seen sites,
-                                // so reset the exactness info.
-                                impInlineInfo->retExprClassHndIsExact = false;
-                            }
                             if (impInlineInfo->retExprClassHnd != returnClsHnd)
                             {
                                 // This return site type differs from earlier seen sites,
@@ -11092,6 +11086,11 @@ bool Compiler::impReturnInstruction(int prefixFlags, OPCODE& opcode)
                                 impInlineInfo->retExprClassHnd        = nullptr;
                                 impInlineInfo->retExprClassHndIsExact = false;
                             }
+                            else
+                            {
+                                // Same return type, but we may need to update exactness.
+                                impInlineInfo->regExprClassHndIsExact &= isExact;
+                            }
                         }
                     }
 

From 1ea346a2f9fe8cf38572efc5ea4cdea0e6068e9d Mon Sep 17 00:00:00 2001
From: Steven He <hez2010@outlook.com>
Date: Fri, 31 Jan 2025 18:49:12 +0900
Subject: [PATCH 15/15] Fix typo

---
 src/coreclr/jit/importer.cpp | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/coreclr/jit/importer.cpp b/src/coreclr/jit/importer.cpp
index 41983fb77c3ef4..4246e58f9fd869 100644
--- a/src/coreclr/jit/importer.cpp
+++ b/src/coreclr/jit/importer.cpp
@@ -11089,7 +11089,7 @@ bool Compiler::impReturnInstruction(int prefixFlags, OPCODE& opcode)
                             else
                             {
                                 // Same return type, but we may need to update exactness.
-                                impInlineInfo->regExprClassHndIsExact &= isExact;
+                                impInlineInfo->retExprClassHndIsExact &= isExact;
                             }
                         }
                     }