uskel/include/gc.s

.ifndef _gc_s_file
_gc_s_file:

.section .bss
_write_region_start:
	# begin of the active memory area
	cell 0
_write_region_end:
	# end of the active memory area (%rsp kinda starts here and goes down
	# towars the start)
	cell 0
_gc_trigger:
	# point in memory where the gc will trigger (we don't necessarily wait for the write region to fill up!)
	cell 0

_gc_last_size:
	# how much data we evacuated last time
	cell 0
_gc_min_alloc:
	# minimum possible allocation
	cell 0 # tunable constant
_gc_grow_ratio:
	# 256th's of the minimal amount of memory increment compared to the
	# last time. New minimal amount is compared as:
	# (ratio * last size) >> 8
	cell 0 # tunable constant
_gc_shrink_ratio:
	# 256th's of the ratio of post-gc still-free to-space that should be considered for discarding
	cell 0

_gc_region_start:
	# in GC, this region is being evacuated and will eventually disappear
	cell 0
_gc_region_end:
	# end of the disappear region
	cell 0

_gc_backup_thunk:
	# backup of %rsi so that we can use the register for other nonsense
	cell 0
_gc_backup_cont:
	# backup of %rbp for same reason
	cell 0

.section .text

.macro needs_alloc amount
	mov %rsp, %rax
	sub _write_region_start, %rax
	cmp \amount, %rax
	jb _uskel_gc
.endm

_uskel_alloc:
	mov %rsi, %r15 # %rsi is the return address; back it up

	# calculate the desired size to %r14
	mov _gc_min_alloc, %r14
	#add _gc_region_end, %r14
	#sub _gc_region_start, %r14

	# check if the desired size isn't greater because of the last gc use
	mov _gc_last_size, %rax
	mulq _gc_grow_ratio
	shr $8, %rax
	add _gc_min_alloc, %rax
	cmp %r14, %rax
	cmova %rax, %r14

	# check if we don't need even more space because we need to evacuate stuff
	mov _gc_region_end, %rax
	sub %rsp, %rax # trick -- if we counted from gc region start, allocated memory could never shrink
	cmp %r14, %rax
	cmova %rax, %r14

	and $0xfffffffffffffff8, %r14 #align

	alloc_goes_mmap:
	mov $9, %rax # mmap
	mov $0, %rdi # addr = NULL
	mov %r14, %rsi # len = %r14
	mov $0b11, %rdx # prot = PROT_READ 0b1 | PROT_WRITE 0b10
	mov $0x22, %r10 # flags = MAP_PRIVATE 0x2 | MAP_ANONYMOUS 0x20
	mov $-1, %r8 # fd = -1
	mov $0, %r9 # off = 0
	syscall

	# store the results
	mov %rax, _write_region_start
	add %r14, %rax
	mov %rax, _write_region_end
	mov %rax, %rsp # initialize writing into the new region

	jmp *%r15

_uskel_gc_init:
	mov %rsi, %r13
	movq $0x100, _gc_min_alloc # must be higher than 2x the biggest thunk possible
	movq $0x180, _gc_grow_ratio
	movq $0x40, _gc_shrink_ratio
	mov $0, %rsp # fake original rsp for first alloc run
	mov $_uskel_gc_init_cont, %rsi
	jmp _uskel_alloc
	_uskel_gc_init_cont:
	mov _write_region_start, %rax
	mov %rax, _gc_trigger
	jmp *%r13

_uskel_gc:
	# save what we did before ending up here
	mov %rbp, _gc_backup_thunk
	mov %rsi, _gc_backup_cont

	# first we need a new memory area
	mov _write_region_start, %rbx
	mov _write_region_end, %rcx
	mov %rbx, _gc_region_start
	mov %rcx, _gc_region_end
	mov $_uskel_gc_evacuate, %rsi
	jmp _uskel_alloc
	_uskel_gc_evacuate:

	# point the writer to the new memory area
	mov _write_region_end, %rsp
	mov %rsp, %r8 # % r8 is the "last thing that was scavenged"
	
	# start by evacuating the thunk and cont
	mov _gc_backup_thunk, %rbp
	mov $_uskel_gc_evacuate_cont_thunk, %rsi
	jmp _gc_evacuate
	_uskel_gc_evacuate_cont_thunk:
	mov %rbp, _gc_backup_thunk

	mov _gc_backup_cont, %rbp
	mov $_uskel_gc_evacuate_cont_cont, %rsi
	jmp _gc_evacuate
	_uskel_gc_evacuate_cont_cont:
	mov %rbp, _gc_backup_cont

	# scavenge everything
	_uskel_gc_scavenge:
	# start at what we wrote last
	mov %rsp, %rbp # rbp is the iterator (conveniently)
	mov %rsp, %r9 # % r9 stores where we started with this evacuate round

	# if the thing is already scavenged, we didn't write anything, mark done.
	cmp %rbp, %r8
	jbe _uskel_gc_scavenge_end

	_uskel_gc_scavenge1:
	# if all ok, scavenge one thing (moving %rbp) and recheck
	mov (%rbp), %rax
	jmp *-020(%rax) # scavenge position in infotable
	_gc_scavenge_ret:
	cmp %rbp, %r8
	ja _uskel_gc_scavenge1

	# everything above r9 is now scavenged, continue with next round
	mov %r9, %r8 # we started at r9, so that is now "done"
	jmp _uskel_gc_scavenge

	_uskel_gc_scavenge_end:
	# deallocate the old memory region
	mov $11, %rax # munmap
	mov _gc_region_end, %rsi
	mov _gc_region_start, %rdi # addr = gc start
	sub %rdi, %rsi # len = gc end - gc start
	syscall
	
	# recalculate the gc trigger point
	mov %rsp, %rax
	sub _write_region_start, %rax
	mulq _gc_shrink_ratio
	shr $8, %rax
	add _write_region_start, %rax
	mov %rax, _gc_trigger

	# save how much data we actually had at this point
	mov _write_region_end, %rax
	sub %rsp, %rax
	mov %rax, _gc_last_size

	# restore what we were doing
	mov _gc_backup_thunk, %rbp
	mov _gc_backup_cont, %rsi
	enter_rbp # for simplicity just restart the thunk

_gc_evacuate:
	# check if we are really out of the target region
	cmp _write_region_start, %rbp
	jb _gc_evacuate_go
	cmp _write_region_end, %rbp
	jae _gc_evacuate_go
	_gc_evacuate_skip:
	# if not, let's just jump to cont and leave %rbp as result
	jmp *%rsi
	_gc_evacuate_go:
	# if we should evacuate, jump to the evac routine
	mov %rbp, %r10
	mov (%rbp), %rax
	jmp *-030(%rax)
	_gc_evacuate_ret:
	# install the indirection
	movq $IND_code, 000(%r10)
	mov %rbp, 010(%r10)
	jmp *%rsi


.endif #_gc_s_file