x86-64

Guidelines that apply only to x86-64 code under ostd/src/arch/x86/.

Align the stack to 16 bytes before a call (16b-align-rsp-before-call)

The System V AMD64 ABI requires %rsp to be 16-byte aligned at the point of a call, so that the callee sees a 16-byte-aligned frame once the return address is pushed. In hand-written assembly that calls into Rust or C, keep %rsp 16-byte aligned immediately before each call: when an entry or trap path has pushed an odd number of 8-byte values, push one more (a scratch register, or a dummy) to restore the alignment.

A misaligned stack is undefined behavior — the Rust or C callee may use SSE instructions such as movaps that fault on an unaligned operand.

See also: the System V AMD64 ABI (§3.2.2, “The Stack Frame”); ostd/src/arch/x86/trap/syscall.S, which pushes an extra register to keep %rsp aligned before calling into Rust.