Diagnostic change only, to confirm whether a theory is correct or

not when chasing an error.

Author: AndyJGraham

src/coreclr/jit/emit.cpp

@@ -9122,11 +9122,22 @@ void emitter::emitNxtIG(bool extend)
 // NOTE: It is expected that the GC effect of the removed instruction will be handled by the newly
 // generated replacement(s).
 //
-void emitter::emitRemoveLastInstruction()
+bool emitter::emitRemoveLastInstruction()
 {
     assert(emitLastIns != nullptr);
     assert(emitLastInsIG != nullptr);
 
+#ifdef TARGET_ARM64
+    // AJG - This is a temporary change, to prevent the first
+    //       instruction in a group to be optimised away.
+    //       This is for diagnostic and test purposes only.
+
+    if (emitCurIGinsCnt == 1)
+    {
+        return false;
+    }
+#endif
+
     JITDUMP("Removing saved instruction in %s:\n> ", emitLabelString(emitLastInsIG));
     JITDUMPEXEC(dispIns(emitLastIns))
 
@@ -9175,6 +9186,8 @@ void emitter::emitRemoveLastInstruction()
 
     emitLastIns   = nullptr;
     emitLastInsIG = nullptr;
+
+    return true;
 }
 
 /*****************************************************************************

src/coreclr/jit/emit.h

@@ -2181,7 +2181,7 @@ class emitter
     insGroup* emitSavIG(bool emitAdd = false);
     void emitNxtIG(bool extend = false);
 
-    void emitRemoveLastInstruction();
+    bool emitRemoveLastInstruction();
 
     bool emitCurIGnonEmpty()
     {

src/coreclr/jit/emitarm64.cpp

@@ -16219,24 +16219,32 @@ bool emitter::TryReplaceLdrStrWithPairInstr(
         }
 
         // Remove the last instruction written.
-        emitRemoveLastInstruction();
-
-        // We need to scale the immediate value by the operand size.
-        // This is either the "old" immediate (for ascending) or the
-        // "current" immediate (for descending) register order.
-        if (optimizationOrder == eRO_ascending)
-        {
-            // The FIRST register is at the lower offset
-            emitIns_R_R_R_I(optIns, oldReg1Attr, oldReg1, reg1, reg2, oldImm * size, INS_OPTS_NONE, reg1Attr);
-        }
-        else
+        // This has been temporarily changed to allow the function
+        // to "refuse" to remove an instruction for diagnostic purposes only.
+        if (emitRemoveLastInstruction())
         {
-            // The SECOND register is at the lower offset
-            emitIns_R_R_R_I(optIns, reg1Attr, reg1, oldReg1, reg2, imm * size, INS_OPTS_NONE, oldReg1Attr);
-        }
+            // The above function can refuse to remove an emitted
+            // instruction, for diagnostic purposes only.
 
-        // And now return true, to indicate that the second instruction descriptor is no longer to be emitted.
-        return true;
+            // It HAS removed an instruction this time.
+
+            // We need to scale the immediate value by the operand size.
+            // This is either the "old" immediate (for ascending) or the
+            // "current" immediate (for descending) register order.
+            if (optimizationOrder == eRO_ascending)
+            {
+                // The FIRST register is at the lower offset
+                emitIns_R_R_R_I(optIns, oldReg1Attr, oldReg1, reg1, reg2, oldImm * size, INS_OPTS_NONE, reg1Attr);
+            }
+            else
+            {
+                // The SECOND register is at the lower offset
+                emitIns_R_R_R_I(optIns, reg1Attr, reg1, oldReg1, reg2, imm * size, INS_OPTS_NONE, oldReg1Attr);
+            }
+
+            // And now return true, to indicate that the second instruction descriptor is no longer to be emitted.
+            return true;
+        }
     }
 
     return false;