assembly

Demystifying the Red Zone: Optimizing Leaf Functions

2 months ago

The Red Zone is a 128-byte safe area below the stack pointer in x86-64 and ARM64 Mac systems, defined by the System V ABI.
It allows leaf functions to skip stack pointer adjustments, optimizing performance by reducing instruction count.
Kernel asynchronous writes, such as signal frames, avoid the Red Zone, ensuring data within 128 bytes of rsp remains untouched.
An example proof-of-work function demonstrates the optimization, saving 6% of instructions by avoiding stack bookkeeping.
Windows omits the Red Zone, using a 32-byte shadow space for register argument spilling instead.
Assembly code validates the Red Zone's protection by showing memory within it survives signal handling, while outside data is clobbered.