x86_64 asm quicksort

quicksort.s

#!/bin/bash
# (gdb) r
# -> SIGTRAP
# (gdb) x/12g $rax
# -> stuff is sorted
// 2>/dev/null; as -g $0 -o $0.o && ld $0.o -o $0.bin && gdb $0.bin; exit

.data
stuff: # numbers to sort
	.quad 74, 3, 53, 11, 52, 94, 16, 12, 48, 88, 19, 82
	len = (. - stuff) / 8

.text
.global _start

quicksort:
	dec %rdi                    # base case
	jle end

	push %rbx
	mov %rdi, %rax              # swap index   -> rax
	lea (%rsi, %rdi, 8), %rbx   # pivot addr   -> rbx
	mov %rdi, %rcx              # loop counter -> rcx
	mov (%rbx), %r8             # pivot value  -> r8

compare:
	mov -8(%rsi, %rcx, 8), %rdx
	cmp %r8, %rdx               # compare array[rcx-1] against pivot
	jl noswap
	xchg %rdx, -8(%rsi, %rax, 8)# swap array[rcx-1] and index if pivot bigger
	mov %rdx, -8(%rsi, %rcx, 8)
	dec %rax                    # and decrease the index
noswap:
	loop compare                # keep comparing until %rcx == 0

	xchg %r8, (%rsi, %rax, 8)   # done comparing, swap pivot into position
	mov %r8, (%rbx)

	sub %rax, %rdi              # calc size for 2nd recursion
	push %rdi                   #  and store in stack
	mov %rax, %rdi              # set new size argument for 1st recursion
	lea 8(%rsi, %rax, 8), %rax  # calc array for 2nd recursion
	push %rax                   #  and store in stack
	call quicksort              # do 1st/lower recursion

	pop %rsi                    # get stuff back off the stack
	pop %rdi
	pop %rbx
	jmp quicksort               # tail recursion for 2nd/upper section
	                            # quicksort(array + index + 1, size - index - 1)
end:
	ret

_start:
	sub $8, %rsp
	mov $stuff, %rsi
	mov $len, %rdi
	call quicksort
	mov $stuff, %rax           # put &stuff in %rax for gdb
	int $3                     # breakpoint