CMU Computer Systems: Machine-Level Programming (Procedures)

Mechanisms in Procedures

• Passing control

  • To beggining of procedure code
  • Back to return point

• Passing data

  • Procedure arguments
  • Return value

• Memory management

  • Allocate during procedure execution
  • Deallocate upon return

• Mechanisms all implemented with machine instructions

• x86-64 implemention of a procedure uses only those mechanisms required

Procedures

• Stack Structure

• Calling Conventions

  • Passing control
  • Passing data
  • Managing local data

• Illustration of Recursion

x86-64 Stack

• Region of memory managed with stack discipline

• Grows toward lower address

• Register %rsp contains lowest stack address

  • address of “top” element

• Operation: Push and Pop

Procedure Control Flow

• Use stack to support procedure call and return

• Procedure call: call label

  • Push return address on stack
  • Jump to label

• Return address:

  • Address of the next instruction right after call
  • Example from disassembly

• Procedure return: ret

  • Pop address from stack
  • Jump to address

Procedure Data Flow (ABI rule)

• Registers

  • First 6 arguments: %rdi %rsi %rdx %rcx %r8 %r9

• Return value: %rax

• Only allocate stack space when needed

Stack-Based Languages

• Languages that support recursion

  • C, Pascal, Java

  • Code must be “Reentrant”

    • Multiple simultaneous instantiations of single procedure

  • Need some place to store state of each instantiation

    • Arguments
    • Local variables
    • Return pointer

• Stack discipline

  • State for given procedure needed for limited time

    • From when called to when return

  • Callee returns before caller does

• Stack allocated in Frames

  • state for single procedure instantiation

Stack Frames

• Contents

  • Return information
  • Local storage (if needed)
  • Temporary space (if needed)

• Management

  • Space allocated when enter procedure

    • “Set-up” code
    • Includes push by call instruction

  • Deallocated when return

    • “Finish” code
    • Includes pop by ret instruction

Register Saving Conventions

• “Caller Saved”

  • Caller saves temporary values in its frame before the call

• “Callee Saved”

  • Callee saves temporary values in its frame before using
  • Callee restores them before returning to caller

x86-64 Linux Register Usage

• %rax

  • Return value
  • Also caller-saved
  • Can be modified by procedure

• %rdi, …, %r9

  • Arguments
  • Also caller-saved
  • Can be modified by procedure

• %r10, %r11

  • Caller-saved
  • Can be modified by procedure

• %rbx, %r12, %r13, %r14

  • Callee-saved
  • Callee must save & restore

• %rbp

  • Callee-saved
  • Callee must save & restore
  • May be used as frame pointer
  • Can mix & match

• %rsp

  • Special form of callee save
  • Restored to original value upon exit from procedure

Observations About Recursion

• Handled Without Special Consideration

  • Stack frames mean that each function call has private storage

    • Saved registers & local variables
    • Saved return pointer

  • Register saving conventions prevent one function call from corrupting another’s call

    • Unless the C code explicitly does so

  • Stack discipline follows call/ return pattern

• Also works for mutual recursion

  • P calls Q; Q calls P

x86-64 Procedure Summary

• Important Points

  • Stack is the right data structure for procedure call/ return

    • If P calls Q, then Q returns before P

• Recursion (& mutual recursion) handled by normal calling conventions

  • Can safely store values in local stack frame and in callee-saved registers
  • Put function arguments at top of stack
  • Result return in %rax

• Pointers are addresses of values

  • On stack or global