System V x86-64 Calling Convention

A calling convention is the contract between caller and callee: where do arguments live, where does the return value land, who saves which registers, how is the stack aligned. On Linux/macOS/BSD x86-64 we follow the System V AMD64 ABI (Windows uses a different one).

Argument passing — first six integer args in registers

Integer (and pointer) args 1..6 go in:

arg #   1     2     3     4     5    6
reg    RDI   RSI   RDX   RCX   R8   R9

Args 7+ are pushed on the stack in right-to-left order (so the leftmost stack arg is at the lowest address on entry).

Return value

Integer/pointer return value in RAX (high half of a 128-bit return uses RDX).

Caller-saved vs callee-saved

If you want a value to survive a call, either put it in a callee-saved reg, or spill it.

Stack alignment — the 16-byte rule

RSP must be a multiple of 16 at the moment the call instruction executes. The call then pushes an 8-byte return address, so the callee begins execution with RSP at 8 mod 16; the callee's own push rbp brings the frame back to 16-byte alignment for the body. Forgetting this manifests as crashes inside libc functions that use SSE.

Example: emit a call with N args

For f(1, 2, 3, 4, 5, 6, 7, 8):

mov rdi, 1
mov rsi, 2
mov rdx, 3
mov rcx, 4
mov r8,  5
mov r9,  6
push 8           ; stack args right-to-left
push 7
call f
add rsp, 16      ; clean up 2 stack args (2*8)

After the call, the caller is responsible for restoring RSP (the callee uses ret which only pops the return address).

What our exercise builds

Given lines <func> <argc>, emit the asm that loads args 1..argc (literal values) into the right registers/stack slots, calls func, then cleans up the stack if there were >6 args.

References: System V AMD64 ABI §3.2 (psABI document), chibicc codegen.c gen_args, Sandler Ch. 9 (functions).