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Video Overview

  • Previously in 19-stack-frame

    • How does the C++ function stack frame work?
    • %rbp - register base pointer (starting point of stack for current fn)
    • %rsp - register stack pointer (end point of stack for current fn)

  • What is Red Zone?

    • Low-level compiler optimization to save a few CPU cycles
      • Avoids using subq $0x14 %rsp
    • Red Zone is 128 bytes (0x80) below $rsp that is auto accessible by leaf node functions
      • Fns that don't call other fns
    • Available for x86-64 on MacOS and Linux(?), but is absent on Windows

  • So how do you disable the compiler optimization?

    • -mno-red-zone flag to prevent %rsp compiler optimization
    • By default, %rsp is already moved by 0x80 bytes!

Helpful LLDB Commands

(lldb) p/x $rbp
# (unsigned long) 0x00007ff7bfefec40

(lldb) x/x $rbp-0x8
# 0x7ff7bfefec38: 0x00000001

(lldb) x/4xd $rbp-0x14
# 0x7ff7bfefec2c: 4
# 0x7ff7bfefec30: 3
# 0x7ff7bfefec34: 2
# 0x7ff7bfefec38: 1

Titles

  • x86 Assembly Red Zone Explained | C++ Tutorial
  • The "Hidden" 128 Bytes: x86 Assembly Red Zone Explained
  • How Compilers Optimize the Stack: The x86 Red Zone | C++ Tutorial
  • What is the x86 Assembly Red Zone? Leaf Function Optimization | C++ Tutorial

References

  • ChatGPT and Gemini
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