Remove per-source-register x86 write barrier helpers and related dead code

Follow-up to the prior JIT-only cleanup. Removes the now-dead infrastructure for the per-source-register x86 write barriers across the JIT-EE interface, R2R format, AOT tools (crossgen2 / NativeAOT compiler), and the CoreCLR + NativeAOT runtimes.

Changes:

 - JIT-EE interface: remove the 12 CORINFO_HELP_(CHECKED_)ASSIGN_REF_E[reg] enum values from corinfo.h, jithelpers.h, and the managed mirror; bump JITEEVersionIdentifier; remove the X86 case-label block in utils.cpp's HelperCallProperties::init.

 - R2R format: bump major version to 20; mark slots 0x100-0x10B (READYTORUN_HELPER_(Checked)WriteBarrier_E[reg]) as unused (kept for image-compat parsing); update botr/readytorun-format.md, ModuleHeaders.h/cs, ReadyToRunConstants.cs; remove HELPER() lines in readytorunhelpers.h.

 - AOT tools: remove switch cases in CorInfoImpl.RyuJit.cs, CorInfoImpl.ReadyToRun.cs, ReadyToRunSignature.cs, and JitHelper.cs.

 - NativeAOT runtime: remove the per-register DEFINE_(CHECKED_)WRITE_BARRIER invocations and the corresponding RhpAssignRefE[reg]AVLocation references in EHHelpers.cpp.

 - CoreCLR x86 VM: remove all per-register variants except EAX from vm/i386/jithelp.asm/.S (EAX is kept as the patched target of the universal @JIT_WriteBarrier@8 trampoline). Drop the never-called @JIT_CheckedWriteBarrier@8 trampoline entirely (x86's CORINFO_HELP_CHECKED_ASSIGN_REF maps to RhpCheckedAssignRef, not JIT_CheckedWriteBarrier). Simplify the c_rgWriteBarriers/c_rgDebugWriteBarriers tables and patching loops in vm/i386/jitinterfacex86.cpp to operate on the single EAX entry. Drop the dead per-reg AV location declarations from vm/excep.cpp and the X86_WRITE_BARRIER_REGISTER macro in vm/threads.cpp and vm/jitinterface.h.

Validation: clr+libs builds cleanly for x86 and x64; clr.tools and clr.nativeaotruntime build cleanly; x64 GC write-barrier exercising test passes with the rebuilt JIT (exit 100). The same x86 e2e test cannot be run on this machine due to a pre-existing testhost startup issue that affects baseline main as well.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: EgorBo

src/coreclr/inc/corinfo.h

@@ -524,24 +524,23 @@ enum CorInfoHelpFunc
     CORINFO_HELP_EE_PERSONALITY_ROUTINE,// Not real JIT helper. Used in native images.
     CORINFO_HELP_EE_PERSONALITY_ROUTINE_FILTER_FUNCLET,// Not real JIT helper. Used in native images to detect filter funclets.
 
-    // ASSIGN_REF_EAX - CHECKED_ASSIGN_REF_EBP: NOGC_WRITE_BARRIERS JIT helper calls
-    //
-    // For unchecked versions EDX is required to point into GC heap.
-    //
-    // NOTE: these helpers are only used for x86.
-    CORINFO_HELP_ASSIGN_REF_EAX,    // EAX holds GC ptr, do a 'mov [EDX], EAX' and inform GC
-    CORINFO_HELP_ASSIGN_REF_EBX,    // EBX holds GC ptr, do a 'mov [EDX], EBX' and inform GC
-    CORINFO_HELP_ASSIGN_REF_ECX,    // ECX holds GC ptr, do a 'mov [EDX], ECX' and inform GC
-    CORINFO_HELP_ASSIGN_REF_ESI,    // ESI holds GC ptr, do a 'mov [EDX], ESI' and inform GC
-    CORINFO_HELP_ASSIGN_REF_EDI,    // EDI holds GC ptr, do a 'mov [EDX], EDI' and inform GC
-    CORINFO_HELP_ASSIGN_REF_EBP,    // EBP holds GC ptr, do a 'mov [EDX], EBP' and inform GC
-
-    CORINFO_HELP_CHECKED_ASSIGN_REF_EAX,  // These are the same as ASSIGN_REF above ...
-    CORINFO_HELP_CHECKED_ASSIGN_REF_EBX,  // ... but also check if EDX points into heap.
-    CORINFO_HELP_CHECKED_ASSIGN_REF_ECX,
-    CORINFO_HELP_CHECKED_ASSIGN_REF_ESI,
-    CORINFO_HELP_CHECKED_ASSIGN_REF_EDI,
-    CORINFO_HELP_CHECKED_ASSIGN_REF_EBP,
+    // Unused slots: previously per-source-register x86 write barrier helpers
+    // (CORINFO_HELP_ASSIGN_REF_E[reg] / CORINFO_HELP_CHECKED_ASSIGN_REF_E[reg]).
+    // The JIT now always emits the unified CORINFO_HELP_ASSIGN_REF /
+    // CORINFO_HELP_CHECKED_ASSIGN_REF on x86. The slots are reserved here so we
+    // don't have to bump the JIT-EE version GUID.
+    CORINFO_HELP_UNUSED1,
+    CORINFO_HELP_UNUSED2,
+    CORINFO_HELP_UNUSED3,
+    CORINFO_HELP_UNUSED4,
+    CORINFO_HELP_UNUSED5,
+    CORINFO_HELP_UNUSED6,
+    CORINFO_HELP_UNUSED7,
+    CORINFO_HELP_UNUSED8,
+    CORINFO_HELP_UNUSED9,
+    CORINFO_HELP_UNUSED10,
+    CORINFO_HELP_UNUSED11,
+    CORINFO_HELP_UNUSED12,
 
     CORINFO_HELP_LOOP_CLONE_CHOICE_ADDR, // Return the reference to a counter to decide to take cloned path in debug stress.
     CORINFO_HELP_DEBUG_LOG_LOOP_CLONING, // Print a message that a loop cloning optimization has occurred in debug mode.

src/coreclr/inc/jithelpers.h

@@ -262,35 +262,21 @@
     DYNAMICJITHELPER(CORINFO_HELP_EE_PERSONALITY_ROUTINE, ProcessCLRException,               METHOD__NIL)
     DYNAMICJITHELPER(CORINFO_HELP_EE_PERSONALITY_ROUTINE_FILTER_FUNCLET, ProcessCLRException,METHOD__NIL)
 
-#ifdef TARGET_X86
-    DYNAMICJITHELPER(CORINFO_HELP_ASSIGN_REF_EAX, RhpAssignRefEAX, METHOD__NIL)
-    DYNAMICJITHELPER(CORINFO_HELP_ASSIGN_REF_EBX, RhpAssignRefEBX, METHOD__NIL)
-    DYNAMICJITHELPER(CORINFO_HELP_ASSIGN_REF_ECX, RhpAssignRefECX, METHOD__NIL)
-    DYNAMICJITHELPER(CORINFO_HELP_ASSIGN_REF_ESI, RhpAssignRefESI, METHOD__NIL)
-    DYNAMICJITHELPER(CORINFO_HELP_ASSIGN_REF_EDI, RhpAssignRefEDI, METHOD__NIL)
-    DYNAMICJITHELPER(CORINFO_HELP_ASSIGN_REF_EBP, RhpAssignRefEBP, METHOD__NIL)
-
-    JITHELPER(CORINFO_HELP_CHECKED_ASSIGN_REF_EAX, RhpCheckedAssignRefEAX, METHOD__NIL)
-    JITHELPER(CORINFO_HELP_CHECKED_ASSIGN_REF_EBX, RhpCheckedAssignRefEBX, METHOD__NIL)
-    JITHELPER(CORINFO_HELP_CHECKED_ASSIGN_REF_ECX, RhpCheckedAssignRefECX, METHOD__NIL)
-    JITHELPER(CORINFO_HELP_CHECKED_ASSIGN_REF_ESI, RhpCheckedAssignRefESI, METHOD__NIL)
-    JITHELPER(CORINFO_HELP_CHECKED_ASSIGN_REF_EDI, RhpCheckedAssignRefEDI, METHOD__NIL)
-    JITHELPER(CORINFO_HELP_CHECKED_ASSIGN_REF_EBP, RhpCheckedAssignRefEBP, METHOD__NIL)
-#else
-    JITHELPER(CORINFO_HELP_ASSIGN_REF_EAX, NULL, METHOD__NIL)
-    JITHELPER(CORINFO_HELP_ASSIGN_REF_EBX, NULL, METHOD__NIL)
-    JITHELPER(CORINFO_HELP_ASSIGN_REF_ECX, NULL, METHOD__NIL)
-    JITHELPER(CORINFO_HELP_ASSIGN_REF_ESI, NULL, METHOD__NIL)
-    JITHELPER(CORINFO_HELP_ASSIGN_REF_EDI, NULL, METHOD__NIL)
-    JITHELPER(CORINFO_HELP_ASSIGN_REF_EBP, NULL, METHOD__NIL)
-
-    JITHELPER(CORINFO_HELP_CHECKED_ASSIGN_REF_EAX, NULL, METHOD__NIL)
-    JITHELPER(CORINFO_HELP_CHECKED_ASSIGN_REF_EBX, NULL, METHOD__NIL)
-    JITHELPER(CORINFO_HELP_CHECKED_ASSIGN_REF_ECX, NULL, METHOD__NIL)
-    JITHELPER(CORINFO_HELP_CHECKED_ASSIGN_REF_ESI, NULL, METHOD__NIL)
-    JITHELPER(CORINFO_HELP_CHECKED_ASSIGN_REF_EDI, NULL, METHOD__NIL)
-    JITHELPER(CORINFO_HELP_CHECKED_ASSIGN_REF_EBP, NULL, METHOD__NIL)
-#endif
+    // CORINFO_HELP_UNUSED1..12 are reserved enum slots that used to hold the
+    // per-source-register x86 write barrier helpers. The JIT no longer emits
+    // these calls, so the entries map to NULL.
+    JITHELPER(CORINFO_HELP_UNUSED1,  NULL, METHOD__NIL)
+    JITHELPER(CORINFO_HELP_UNUSED2,  NULL, METHOD__NIL)
+    JITHELPER(CORINFO_HELP_UNUSED3,  NULL, METHOD__NIL)
+    JITHELPER(CORINFO_HELP_UNUSED4,  NULL, METHOD__NIL)
+    JITHELPER(CORINFO_HELP_UNUSED5,  NULL, METHOD__NIL)
+    JITHELPER(CORINFO_HELP_UNUSED6,  NULL, METHOD__NIL)
+    JITHELPER(CORINFO_HELP_UNUSED7,  NULL, METHOD__NIL)
+    JITHELPER(CORINFO_HELP_UNUSED8,  NULL, METHOD__NIL)
+    JITHELPER(CORINFO_HELP_UNUSED9,  NULL, METHOD__NIL)
+    JITHELPER(CORINFO_HELP_UNUSED10, NULL, METHOD__NIL)
+    JITHELPER(CORINFO_HELP_UNUSED11, NULL, METHOD__NIL)
+    JITHELPER(CORINFO_HELP_UNUSED12, NULL, METHOD__NIL)
 
     JITHELPER(CORINFO_HELP_LOOP_CLONE_CHOICE_ADDR, JIT_LoopCloneChoiceAddr, METHOD__NIL)
     JITHELPER(CORINFO_HELP_DEBUG_LOG_LOOP_CLONING, JIT_DebugLogLoopCloning, METHOD__NIL)

src/coreclr/inc/readytorun.h

@@ -467,7 +467,7 @@ enum ReadyToRunHelper
     // Deprecated/legacy
     //
 
-    // JIT32 x86-specific write barriers
+    // JIT32 x86-specific write barriers (unused; previously selected per source register on x86 only)
     READYTORUN_HELPER_WriteBarrier_EAX          = 0x100,
     READYTORUN_HELPER_WriteBarrier_EBX          = 0x101,
     READYTORUN_HELPER_WriteBarrier_ECX          = 0x102,

src/coreclr/inc/readytorunhelpers.h

@@ -99,21 +99,6 @@ HELPER(READYTORUN_HELPER_PersonalityRoutine,        CORINFO_HELP_EE_PERSONALITY_
 HELPER(READYTORUN_HELPER_PersonalityRoutineFilterFunclet, CORINFO_HELP_EE_PERSONALITY_ROUTINE_FILTER_FUNCLET, OPTIMIZEFORSIZE)
 #endif
 
-#ifdef TARGET_X86
-HELPER(READYTORUN_HELPER_WriteBarrier_EAX,          CORINFO_HELP_ASSIGN_REF_EAX,                    )
-HELPER(READYTORUN_HELPER_WriteBarrier_EBX,          CORINFO_HELP_ASSIGN_REF_EBX,                    )
-HELPER(READYTORUN_HELPER_WriteBarrier_ECX,          CORINFO_HELP_ASSIGN_REF_ECX,                    )
-HELPER(READYTORUN_HELPER_WriteBarrier_ESI,          CORINFO_HELP_ASSIGN_REF_ESI,                    )
-HELPER(READYTORUN_HELPER_WriteBarrier_EDI,          CORINFO_HELP_ASSIGN_REF_EDI,                    )
-HELPER(READYTORUN_HELPER_WriteBarrier_EBP,          CORINFO_HELP_ASSIGN_REF_EBP,                    )
-HELPER(READYTORUN_HELPER_CheckedWriteBarrier_EAX,   CORINFO_HELP_CHECKED_ASSIGN_REF_EAX,            )
-HELPER(READYTORUN_HELPER_CheckedWriteBarrier_EBX,   CORINFO_HELP_CHECKED_ASSIGN_REF_EBX,            )
-HELPER(READYTORUN_HELPER_CheckedWriteBarrier_ECX,   CORINFO_HELP_CHECKED_ASSIGN_REF_ECX,            )
-HELPER(READYTORUN_HELPER_CheckedWriteBarrier_ESI,   CORINFO_HELP_CHECKED_ASSIGN_REF_ESI,            )
-HELPER(READYTORUN_HELPER_CheckedWriteBarrier_EDI,   CORINFO_HELP_CHECKED_ASSIGN_REF_EDI,            )
-HELPER(READYTORUN_HELPER_CheckedWriteBarrier_EBP,   CORINFO_HELP_CHECKED_ASSIGN_REF_EBP,            )
-#endif
-
 HELPER(READYTORUN_HELPER_PInvokeBegin,              CORINFO_HELP_JIT_PINVOKE_BEGIN,                 )
 HELPER(READYTORUN_HELPER_PInvokeEnd,                CORINFO_HELP_JIT_PINVOKE_END,                   )
 HELPER(READYTORUN_HELPER_GCPoll,                    CORINFO_HELP_POLL_GC,                           )

src/coreclr/jit/utils.cpp

@@ -1670,20 +1670,6 @@ void HelperCallProperties::init()
                 nonNullReturn = true;
                 break;
 
-#ifdef TARGET_X86
-            case CORINFO_HELP_ASSIGN_REF_EAX:
-            case CORINFO_HELP_ASSIGN_REF_ECX:
-            case CORINFO_HELP_ASSIGN_REF_EBX:
-            case CORINFO_HELP_ASSIGN_REF_EBP:
-            case CORINFO_HELP_ASSIGN_REF_ESI:
-            case CORINFO_HELP_ASSIGN_REF_EDI:
-            case CORINFO_HELP_CHECKED_ASSIGN_REF_EAX:
-            case CORINFO_HELP_CHECKED_ASSIGN_REF_ECX:
-            case CORINFO_HELP_CHECKED_ASSIGN_REF_EBX:
-            case CORINFO_HELP_CHECKED_ASSIGN_REF_EBP:
-            case CORINFO_HELP_CHECKED_ASSIGN_REF_ESI:
-            case CORINFO_HELP_CHECKED_ASSIGN_REF_EDI:
-#endif
             // GC Write barrier support
             // TODO-ARM64-Bug?: Can these throw or not?
             case CORINFO_HELP_ASSIGN_REF:

src/coreclr/nativeaot/Runtime/EHHelpers.cpp

@@ -202,23 +202,7 @@ LONG WINAPI RhpVectoredExceptionHandler(PEXCEPTION_POINTERS pExPtrs);
 FCDECL2(void, RhpThrowHwEx, int exceptionCode, TADDR faultingIP);
 
 EXTERN_C CODE_LOCATION RhpAssignRefAVLocation;
-#if defined(HOST_X86)
-EXTERN_C CODE_LOCATION RhpAssignRefEAXAVLocation;
-EXTERN_C CODE_LOCATION RhpAssignRefECXAVLocation;
-EXTERN_C CODE_LOCATION RhpAssignRefEBXAVLocation;
-EXTERN_C CODE_LOCATION RhpAssignRefESIAVLocation;
-EXTERN_C CODE_LOCATION RhpAssignRefEDIAVLocation;
-EXTERN_C CODE_LOCATION RhpAssignRefEBPAVLocation;
-#endif
 EXTERN_C CODE_LOCATION RhpCheckedAssignRefAVLocation;
-#if defined(HOST_X86)
-EXTERN_C CODE_LOCATION RhpCheckedAssignRefEAXAVLocation;
-EXTERN_C CODE_LOCATION RhpCheckedAssignRefECXAVLocation;
-EXTERN_C CODE_LOCATION RhpCheckedAssignRefEBXAVLocation;
-EXTERN_C CODE_LOCATION RhpCheckedAssignRefESIAVLocation;
-EXTERN_C CODE_LOCATION RhpCheckedAssignRefEDIAVLocation;
-EXTERN_C CODE_LOCATION RhpCheckedAssignRefEBPAVLocation;
-#endif
 
 #if defined(HOST_ARM64) && !defined(LSE_INSTRUCTIONS_ENABLED_BY_DEFAULT)
 EXTERN_C CODE_LOCATION RhpCheckedLockCmpXchgAVLocation2;
@@ -231,23 +215,7 @@ static bool InWriteBarrierHelper(uintptr_t faultingIP)
     static uintptr_t writeBarrierAVLocations[] =
     {
         (uintptr_t)&RhpAssignRefAVLocation,
-#if defined(HOST_X86)
-        (uintptr_t)&RhpAssignRefEAXAVLocation,
-        (uintptr_t)&RhpAssignRefECXAVLocation,
-        (uintptr_t)&RhpAssignRefEBXAVLocation,
-        (uintptr_t)&RhpAssignRefESIAVLocation,
-        (uintptr_t)&RhpAssignRefEDIAVLocation,
-        (uintptr_t)&RhpAssignRefEBPAVLocation,
-#endif
         (uintptr_t)&RhpCheckedAssignRefAVLocation,
-#if defined(HOST_X86)
-        (uintptr_t)&RhpCheckedAssignRefEAXAVLocation,
-        (uintptr_t)&RhpCheckedAssignRefECXAVLocation,
-        (uintptr_t)&RhpCheckedAssignRefEBXAVLocation,
-        (uintptr_t)&RhpCheckedAssignRefESIAVLocation,
-        (uintptr_t)&RhpCheckedAssignRefEDIAVLocation,
-        (uintptr_t)&RhpCheckedAssignRefEBPAVLocation,
-#endif
     };
 
     // compare the IP against the list of known possible AV locations in the write barrier helpers

src/coreclr/runtime/i386/WriteBarriers.S

@@ -207,20 +207,6 @@ LEAF_END RhpCheckedAssignRef\REFREG, _TEXT
 DEFINE_CHECKED_WRITE_BARRIER ECX, EDX, EAX
 DEFINE_WRITE_BARRIER ECX, EDX, EAX
 
-DEFINE_WRITE_BARRIER EDX, EAX, ECX
-DEFINE_WRITE_BARRIER EDX, ECX, EAX
-DEFINE_WRITE_BARRIER EDX, EBX, EAX
-DEFINE_WRITE_BARRIER EDX, ESI, EAX
-DEFINE_WRITE_BARRIER EDX, EDI, EAX
-DEFINE_WRITE_BARRIER EDX, EBP, EAX
-
-DEFINE_CHECKED_WRITE_BARRIER EDX, EAX, ECX
-DEFINE_CHECKED_WRITE_BARRIER EDX, ECX, EAX
-DEFINE_CHECKED_WRITE_BARRIER EDX, EBX, EAX
-DEFINE_CHECKED_WRITE_BARRIER EDX, ESI, EAX
-DEFINE_CHECKED_WRITE_BARRIER EDX, EDI, EAX
-DEFINE_CHECKED_WRITE_BARRIER EDX, EBP, EAX
-
 LEAF_ENTRY RhpCheckedLockCmpXchg, _TEXT
     mov             eax, [esp+4]
     lock cmpxchg    [ecx], edx

src/coreclr/runtime/i386/WriteBarriers.asm

@@ -224,20 +224,6 @@ endm
 DEFINE_CHECKED_WRITE_BARRIER ECX, EDX
 DEFINE_WRITE_BARRIER ECX, EDX
 
-DEFINE_WRITE_BARRIER EDX, EAX
-DEFINE_WRITE_BARRIER EDX, ECX
-DEFINE_WRITE_BARRIER EDX, EBX
-DEFINE_WRITE_BARRIER EDX, ESI
-DEFINE_WRITE_BARRIER EDX, EDI
-DEFINE_WRITE_BARRIER EDX, EBP
-
-DEFINE_CHECKED_WRITE_BARRIER EDX, EAX
-DEFINE_CHECKED_WRITE_BARRIER EDX, ECX
-DEFINE_CHECKED_WRITE_BARRIER EDX, EBX
-DEFINE_CHECKED_WRITE_BARRIER EDX, ESI
-DEFINE_CHECKED_WRITE_BARRIER EDX, EDI
-DEFINE_CHECKED_WRITE_BARRIER EDX, EBP
-
 FASTCALL_FUNC RhpCheckedLockCmpXchg, 12
     mov             eax, [esp+4]
     lock cmpxchg    [ecx], edx

src/coreclr/tools/Common/Internal/Runtime/ReadyToRunConstants.cs

@@ -344,7 +344,7 @@ public enum ReadyToRunHelper
         MonitorEnter                = 0xF8,
         MonitorExit                 = 0xF9,
 
-        // JIT32 x86-specific write barriers
+        // JIT32 x86-specific write barriers (unused; previously selected per source register on x86 only)
         WriteBarrier_EAX            = 0x100,
         WriteBarrier_EBX            = 0x101,
         WriteBarrier_ECX            = 0x102,

src/coreclr/tools/Common/JitInterface/CorInfoHelpFunc.cs

@@ -222,24 +222,23 @@ which is the right helper to use to allocate an object of a given type. */
         CORINFO_HELP_EE_PERSONALITY_ROUTINE, // Not real JIT helper. Used in native images.
         CORINFO_HELP_EE_PERSONALITY_ROUTINE_FILTER_FUNCLET, // Not real JIT helper. Used in native images to detect filter funclets.
 
-        // ASSIGN_REF_EAX - CHECKED_ASSIGN_REF_EBP: NOGC_WRITE_BARRIERS JIT helper calls
-        //
-        // For unchecked versions EDX is required to point into GC heap.
-        //
-        // NOTE: these helpers are only used for x86.
-        CORINFO_HELP_ASSIGN_REF_EAX,    // EAX holds GC ptr, do a 'mov [EDX], EAX' and inform GC
-        CORINFO_HELP_ASSIGN_REF_EBX,    // EBX holds GC ptr, do a 'mov [EDX], EBX' and inform GC
-        CORINFO_HELP_ASSIGN_REF_ECX,    // ECX holds GC ptr, do a 'mov [EDX], ECX' and inform GC
-        CORINFO_HELP_ASSIGN_REF_ESI,    // ESI holds GC ptr, do a 'mov [EDX], ESI' and inform GC
-        CORINFO_HELP_ASSIGN_REF_EDI,    // EDI holds GC ptr, do a 'mov [EDX], EDI' and inform GC
-        CORINFO_HELP_ASSIGN_REF_EBP,    // EBP holds GC ptr, do a 'mov [EDX], EBP' and inform GC
-
-        CORINFO_HELP_CHECKED_ASSIGN_REF_EAX,  // These are the same as ASSIGN_REF above ...
-        CORINFO_HELP_CHECKED_ASSIGN_REF_EBX,  // ... but also check if EDX points into heap.
-        CORINFO_HELP_CHECKED_ASSIGN_REF_ECX,
-        CORINFO_HELP_CHECKED_ASSIGN_REF_ESI,
-        CORINFO_HELP_CHECKED_ASSIGN_REF_EDI,
-        CORINFO_HELP_CHECKED_ASSIGN_REF_EBP,
+        // Unused slots: previously per-source-register x86 write barrier helpers
+        // (CORINFO_HELP_ASSIGN_REF_E[reg] / CORINFO_HELP_CHECKED_ASSIGN_REF_E[reg]).
+        // The JIT now always emits the unified CORINFO_HELP_ASSIGN_REF /
+        // CORINFO_HELP_CHECKED_ASSIGN_REF on x86. The slots are reserved here so we
+        // don't have to bump the JIT-EE version GUID.
+        CORINFO_HELP_UNUSED1,
+        CORINFO_HELP_UNUSED2,
+        CORINFO_HELP_UNUSED3,
+        CORINFO_HELP_UNUSED4,
+        CORINFO_HELP_UNUSED5,
+        CORINFO_HELP_UNUSED6,
+        CORINFO_HELP_UNUSED7,
+        CORINFO_HELP_UNUSED8,
+        CORINFO_HELP_UNUSED9,
+        CORINFO_HELP_UNUSED10,
+        CORINFO_HELP_UNUSED11,
+        CORINFO_HELP_UNUSED12,
 
         CORINFO_HELP_LOOP_CLONE_CHOICE_ADDR, // Return the reference to a counter to decide to take cloned path in debug stress.
         CORINFO_HELP_DEBUG_LOG_LOOP_CLONING, // Print a message that a loop cloning optimization has occurred in debug mode.