[JIT] Fix re-use val zero on GC tracking (#84051)

* Initial work

* Added genCodeForReuseVal

* Only create a label if the current IG is not empty

src/coreclr/jit/codegen.h

@@ -1120,6 +1120,7 @@ protected:
     void genCodeForPhysReg(GenTreePhysReg* tree);
     void genCodeForNullCheck(GenTreeIndir* tree);
     void genCodeForCmpXchg(GenTreeCmpXchg* tree);
+    void genCodeForReuseVal(GenTree* treeNode);
 
     void genAlignStackBeforeCall(GenTreePutArgStk* putArgStk);
     void genAlignStackBeforeCall(GenTreeCall* call);

src/coreclr/jit/codegenarmarch.cpp

@@ -143,9 +143,7 @@ void CodeGen::genCodeForTreeNode(GenTree* treeNode)
     // setting the GTF_REUSE_REG_VAL flag.
     if (treeNode->IsReuseRegVal())
     {
-        // For now, this is only used for constant nodes.
-        assert(treeNode->OperIs(GT_CNS_INT, GT_CNS_DBL, GT_CNS_VEC));
-        JITDUMP("  TreeNode is marked ReuseReg\n");
+        genCodeForReuseVal(treeNode);
         return;
     }
 

src/coreclr/jit/codegencommon.cpp

@@ -9442,3 +9442,30 @@ bool CodeGen::genCanOmitNormalizationForBswap16(GenTree* tree)
 
     return (cast->gtCastType == TYP_USHORT) || (cast->gtCastType == TYP_SHORT);
 }
+
+//----------------------------------------------------------------------
+// genCodeForReuseVal: Generate code for a node marked with re-using a register.
+//
+// Arguments:
+//   tree - The node marked with re-using a register
+//
+// Remarks:
+//   Generates nothing, except for when the node is a CNS_INT(0) where
+//   we will define a new label to propagate GC info. We want to do this
+//   because if the node is a CNS_INT(0) and is re-using a register,
+//   that register could have been used for a CNS_INT(ref null) that is GC
+//   tracked.
+//
+void CodeGen::genCodeForReuseVal(GenTree* treeNode)
+{
+    assert(treeNode->IsReuseRegVal());
+
+    // For now, this is only used for constant nodes.
+    assert(treeNode->OperIs(GT_CNS_INT, GT_CNS_DBL, GT_CNS_VEC));
+    JITDUMP("  TreeNode is marked ReuseReg\n");
+
+    if (treeNode->IsIntegralConst(0) && GetEmitter()->emitCurIGnonEmpty())
+    {
+        genDefineTempLabel(genCreateTempLabel());
+    }
+}

src/coreclr/jit/codegenxarch.cpp

@@ -1704,9 +1704,7 @@ void CodeGen::genCodeForTreeNode(GenTree* treeNode)
     // setting the GTF_REUSE_REG_VAL flag.
     if (treeNode->IsReuseRegVal())
     {
-        // For now, this is only used for constant nodes.
-        assert((treeNode->OperIsConst()));
-        JITDUMP("  TreeNode is marked ReuseReg\n");
+        genCodeForReuseVal(treeNode);
         return;
     }