You might not be considering the big picture. If the extra register is used in a callee that wasn't inlined, that slot on the stack will be free to be reused by

You might not be considering the big picture. If the extra register is used in a callee that wasn't inlined, that slot on the stack will be free to be reused by other callees that aren't inlined. So inlining into a function with multiple callees can increase the peak stack usage for the entire callgraph.