Function

Function Call

• Stack vs register calling convention
  • Wether the arguments are stored on the stack or in the registers.
  • Refer to the ABI convention.
  • May be alternated in the program, e.g. x86 offers cdecl and fastcall instruction
  • Register calling is faster for leaf functions (don’t call function and don’t access memory)
  • Reg Calling is likely to be used in architectures that have abundant registers.
• Stack Calling Convention
  • Save caller saved registers.
  • Push args in reverse order
  • Call the function = PUSH PC, and JUMP f
  • Leave room for local variables
  • Save callee saved registers registers in the stack.
  • Such “preamble” is common across languages, and can be used to identify function calls even without debugging symbols.
• Register Calling Convention
  • Save registers
  • Move n args to R{0..n}
  • Additional args are pushed to stack
  • Call f
  • Before pushing local variables to the stack, save space for args, as they need to be saved when calling a new function before return!
• What happens on stack
  • CALL instruction pushes return address to stack
  • PUSH %rbp
  • MOV %rsp %rbp - new stack frame
  • MOV %rbp %rsp - clean up frame
  • POP %rbp - go back to previous stack frame1
  • RET