Push instruction at the start of main

I am trying to write a C code with just inline functions so that there would be no access to stack. But I see the compiler is inserting a push instruction at the start of the main. Also, this behaviour is not seen every time. With a slight change in the code sometimes I don't see the push instruction. I would like to know the what is the reason behind this

Target CPU: Cortex M23

Compiler: GCC

Code structure:

#include "interface.h"

int main(void)

{

unsigned int operation;

operation = <SFR read>;
status = check_status(operation);
if(status)
{
return(status);
}

Disassembly:

int main(void)
{
c00069c: b510 push {r4, lr}

<rest of the inline functions>

}

Header file: interface.h

inline unsigned int check_status(unsigned int oper) __attribute__((always_inline));

inline unsigned int check_status(unsigned int oper)

{

 <function body>

}