WKdmCompress_16k.s [plain text]
/*
* Copyright (c) 2000-2014 Apple Inc. All rights reserved.
*
* @APPLE_OSREFERENCE_LICENSE_HEADER_START@
*
* This file contains Original Code and/or Modifications of Original Code
* as defined in and that are subject to the Apple Public Source License
* Version 2.0 (the 'License'). You may not use this file except in
* compliance with the License. The rights granted to you under the License
* may not be used to create, or enable the creation or redistribution of,
* unlawful or unlicensed copies of an Apple operating system, or to
* circumvent, violate, or enable the circumvention or violation of, any
* terms of an Apple operating system software license agreement.
*
* Please obtain a copy of the License at
* http://www.opensource.apple.com/apsl/ and read it before using this file.
*
* The Original Code and all software distributed under the License are
* distributed on an 'AS IS' basis, WITHOUT WARRANTY OF ANY KIND, EITHER
* EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES,
* INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY,
* FITNESS FOR A PARTICULAR PURPOSE, QUIET ENJOYMENT OR NON-INFRINGEMENT.
* Please see the License for the specific language governing rights and
* limitations under the License.
*
* @APPLE_OSREFERENCE_LICENSE_HEADER_END@
*/
/*
This file contains arm64 hand optimized implementation of WKdm memory page compressor.
int WKdm_compress (WK_word* src_buf, WK_word* dest_buf, WK_word* scratch, unsigned int bytes_budget) input :
src_buf : address of input page (length = 1024 words)
dest_buf : address of output buffer (may not be 16-byte aligned)
scratch : a 16-byte aligned 4k bytes scratch memory provided by the caller,
bytes_budget : a given byte target in compression
output :
if the input buffer can be compressed within the given byte budget, the dest_buf is written with compressed data and the function returns with number of bytes for the compressed data
o.w., the function returns -1 to signal that the input data can not be compressed with the given byte budget.
During the scan and tag process, each word that can not be compressed will be written to dest_buf, followed by a 12-bytes header + 256-bytes tag area.
When the functions returns -1, dest_buf is filled with all those words that can not be compressed and should be considered undefined.
The worst-case scenario is that all words can not be compressed. Hence, the minimum size requirement for dest_buf should be 12+256+4096 = 4364 bytes to prevent from memory fault.
The 4th argument bytes_budget is the target compress budget in bytes.
Should the input page can be compressed within the budget, the compressed data is written to *dest_buf, and the function returns the number of compressed bytes.
Otherwise, the function returns -1 (to signal to the caller that the page can not be compressed).
WKdm Compression algorithm is briefly stated as follows:
There is a dynamically updated dictionary consisting of 16 words. Each dictionary word is initialized to 1 at the point of entry to the function.
For a nonzero input word x, its 8-bits (10-bits scaled up) is used to determine a corresponding word from the dictionary, represented by dict_index (4-bits) and dict_word (32-bits).
a. k = (x>>10)&255 c. dict_word = dictionary[dict_index] Each input word x is classified/tagged into 4 classes :
0 : x = 0
1 : (x>>10) == (dict_word>>10), bits 10:31 of the input word match a dictionary word
2 : (x>>10) != (dict_word>>10), the above condition (22 higher bits matched) is not met, meaning a dictionary miss
3 : (x == dict_word), the exact input word is in the dictionary
For each class, different numbers of bits are needed for the decompressor to reproduce the original input word.
0 : 2-bits tag (32->2 compression)
1 : 2-bits tag + 4-bits dict_index + 10-bits lower bits (32->16 compression)
2 : 2-bits tag + 32-bits new word (32->34 expansion)
3 : 2-bits tag + 4-bits dict_index (32->6 compression)
It is obvious now that WKdm compress algorithm works well for pages where there are lots of zero words (32->2) and/or there are freqeunt repeats of some word patterns (32->6).
the output bit stream (*dest_buf) consists of
a. 12 bytes header
b. 256 bytes for 1024 packed tags
c. (varying number of) words for new words not matched to dictionary word.
d. (varying number of) 32-bit words for packed 4-bit dict_indices (for class 1 and 3)
e. (varying number of) 32-bit words for packed 10-bit low bits (for class 1)
the header is actually of 3 words that specify the ending offset (in 32-bit words) from the start of the bit stream of c,d,e, respectively.
Note that there might be padding bits in d (if the number of dict_indices does not divide by 8), and there are 2/12/22 padding bits for packing 3/2/1 low 10-bits in a 32-bit word.
The WKdm compress algorithm 1st runs a scan and classification pass, tagging and write unpacked data into temporary buffers. It follows by packing those data into the output buffer.
The temp buffers are
uint8_t tempTagsArray[1024] uint16_t tempLowBitsArray[1024] Since the new words (that can not matched fully or partially to the dictionary) are stored right after the header and the tags section and need no packing, we directly write them to
the destination buffer.
uint32_t *new_word = dest_buf+3+64 Now since we are given a byte budget for this compressor, we can monitor the byte (or bit) usage on the fly in the scanning and tagging pass.
byte_count -= 12 + 256 whenever an input word is classified as class
2 : byte_count -= 4 the compress function can early exit (return -1) should the page can not be compressed with the given byte budget (i.e., byte_count <= 0).
without showing the bit budget management, the pseudo code is given as follows:
uint8_t *tags=tempTagsArray uint8_t *partial=tempLowBitsArray for (i=0 if (x == 0) { // zero, 2-bits tag
*tags++ = 0
// find dict_index and dict_word from x
k = (x>>10)&255 dict_word = dictionary[dict_index] if (dict_word == x) { // exactly match
// 2-bits tag + 4-bits table index
*tags++ = 3 } else if (((x^dict_word)>>10)==0) { // 22 higher bits matched
// 2-bits tag + 4-bits table index + 10-bits lower partial
*tags++ = 1 *partial++ = x &0x3ff } else { // not matched
// 2-bits tag + 32-bits new word
*tags++ = 2 dictionary[dict_index] = x }
}
after this classification/tagging pass is completed, the 3 temp buffers are packed into the output *dest_buf:
1. 1024 tags are packed into 256 bytes right after the 12-bytes header
2. dictionary indices (4-bits each) are packed into are right after the new words section
3. 3 low 10-bits are packed into a 32-bit word, this is after the dictionary indices section.
cclee, 11/9/12
Added zero page, single value page, sparse page, early abort optimizations
rsrini, 09/14/14
*/
#define PAGES_SIZE_IN_KBYTES 16
#ifndef PAGES_SIZE_IN_KBYTES
#define PAGES_SIZE_IN_KBYTES 4
#endif
#if !((PAGES_SIZE_IN_KBYTES==4) || (PAGES_SIZE_IN_KBYTES==16))
#error "Only PAGES_SIZE_IN_KBYTES = 4 or 16 is supported"
#endif
.text
.align 4
/*
int WKdm_compress (WK_word* src_buf, WK_word* dest_buf, WK_word* scratch, unsigned int bytes_budget)
.globl _WKdm_compress_16k
_WKdm_compress_16k:
/*
------------------------- symbolizing register use -----------------------------------
*/
#define src_buf x0
#define next_input_word x0
#define dest_buf x1
#define scratch x2
#define byte_count x3
#define next_tag x4
#define tempTagsArray x2 // scratch
#define dictionary x5
#define remaining x6
#define next_full_patt x7
#define dict_location x8
#define wdict_location w8
#define next_qp x9
#define hashTable x10
#define tempQPosArray x11
#define next_low_bits x12
/*
this arm64 assembly code is ported from x86_64 assembly code,
therefore need such symbolization to quickly reuse the x86_64 assembly code
for these intermediate/temporary register use
*/
#define rax x13
#define eax w13
#define rcx x14
#define ecx w14
#define rdx x15
#define edx w15
#define rdi x0 /* after some point, x0/rdi becomes free other usage */
/*
------------------------- scratch memory --------------------------------------
need 16*4 (dictionary) + 256*4 (tempTagsArray) + 256*4 (tempQPosArray) + 1024*4 (tempLowBitsArray)
total 6208 bytes
[sp,#0] : dictionary
[scratch,#0] : tempTagsArray
[scratch,#1024] : tempQPosArray
[scratch,#2048] : tempLowBitsArray
*/
#define scale (PAGES_SIZE_IN_KBYTES/4)
#define SV_RETURN 0 // return value when SV, ZV page is found
#define MZV_MAGIC 17185 // magic value used to identify MZV page encoding
#define CHKPT_BYTES 416 // for early aborts: checkpoint after processing this many bytes. Must be in range [4..4096]
#define CHKPT_WORDS (CHKPT_BYTES/4) // checkpoint bytes in words
#define CHKPT_TAG_BYTES (CHKPT_BYTES/16) // size of the tags for CHKPT_BYTES of data
#define CHKPT_SHRUNK_BYTES 426 // for early aborts: max size of compressed stream to allow further processing ..
// .. to disable early aborts, set CHKPT_SHRUNK_BYTES to 4096
#if CHKPT_BYTES > 4096
#error CHKPT_BYTES must be <= 4096
#endif
#if CHKPT_BYTES < 4
#error CHKPT_BYTES must be >= 4
#endif
#if KERNEL
sub sp, sp, #64
st1.4s {v0,v1,v2,v3},[sp]
#endif
sub sp, sp, #64 // allocate for dictionary
mov dictionary, sp // use x5 to point to sp, so we can use sub xd, xn, sp
sub sp, sp, #64 // allocate space for saving callee-saved registers
mov x15, sp
stp x20, x21, [x15, #0] // save x20, x21
stp x22, x23, [x15, #16] // save x22, x23
stp x24, x25, [x15, #32] // save x24, x25
stp x26, x27, [x15, #48] // save x26, x27
/*
------- entwined statck space allocation, registers set up, and PRELOAD_DICTIONARY -------------------
*/
// NOTE: ALL THE DICTIONARY VALUES MUST BE INITIALIZED TO ZERO
// THIS IS NEEDED TO EFFICIENTLY DETECT SINGLE VALUE PAGES
mov next_tag, tempTagsArray // &tempTagsArray[0]
add next_qp, scratch, #(1024*scale) // next_qp
mov remaining, #(CHKPT_WORDS*scale) // remaining input words .. initially set to checkpoint
add next_full_patt, dest_buf, #(12+256*scale) // dest_buf + [TAGS_AREA_OFFSET + (num_input_words / 16)]*4
sub byte_count, byte_count, #(12+256*scale) // bit_count - header - tags
add next_low_bits, scratch, #(2048*scale) // &tempLowBitsArray[0]
stp xzr, xzr, [dictionary, #0] // initialize dictionary
adrp hashTable, _hashLookupTable@GOTPAGE
stp xzr, xzr, [dictionary, #16] // initialize dictionary
stp xzr, xzr, [dictionary, #32] // initialize dictionary
ldr hashTable, [hashTable, _hashLookupTable@GOTPAGEOFF]
stp xzr, xzr, [dictionary, #48] // initialize dictionary
#define EARLYCHECK 0
#define NORMAL 1
#define mode w20
#define start_next_full_patt x21
#define start_next_input_word x22
#define start_next_low_bits x23
#define r11 x24
#define r13 x25
#define byte_budget x26
#define start_next_qp tempQPosArray
add tempQPosArray, scratch, #(1024*scale) // &tempQPosArray[0]
mov mode, EARLYCHECK // indicate we are yet to evaluate the early aborts
mov start_next_full_patt, next_full_patt // remember the start of next_full_patt
mov start_next_input_word, next_input_word // remember the start of next_input_word
mov start_next_low_bits, next_low_bits // remember the start of next_low_bit
add byte_budget, byte_count, #(12+256*scale) // remember the byte budget
b L_loop
.align 4, 0x90
/* we've just detected a zero input word in edx */
L_RECORD_ZERO:
strb edx, [next_tag], #1 // *next_tag++ = ZERO b.le CHECKPOINT // if remaining = 0, break
/* -------------- scan/tag pass loop ------------------------- */
L_loop:
/* load new input word to edx */
ldr edx, [next_input_word], #4
cbz edx, L_RECORD_ZERO // if (input_word==0) RECORD_ZERO
/*
now the input word edx is nonzero, we next find the corresponding dictionary word (eax) and dict_location
*/
ubfm eax, edx, #10, #17
ldrb wdict_location, [hashTable, rax] // HASH_TO_DICT_BYTE_OFFSET(input_word)
ldr eax, [dictionary, dict_location] // dict_word = *dict_location /* detect whether we match input to its corresponding dictionary word */
eor eax, eax, edx // dict_word vs input_word
cbz eax, L_RECORD_EXACT // if identical, RECORD_EXACT
lsr eax, eax, #10 // HIGH_BITS(dict_word^input_word)
cbz eax, L_RECORD_PARTIAL // if identical, RECORD_PARTIAL
L_RECORD_MISS:
/*
if we are here, the input word can not be derived from the dictionary,
we write the input word as a new word,
and update the dictionary with this new word
*/
subs byte_count, byte_count, #4 // byte_count -= 4
b.le L_budgetExhausted // return -1 to signal this page is not compressable
str edx, [next_full_patt], #4 // *next_full_patt++ = input_word subs remaining, remaining, #1 // remaing-- strb eax, [next_tag], #1 // *next_tag++ = 2 for miss
b.gt L_loop // // if remaining > 0, repeat
b CHECKPOINT
L_done_search:
// SET_QPOS_AREA_START(dest_buf,next_full_patt)
sub rax, next_full_patt, dest_buf // next_full_patt - dest_buf
lsr eax, eax, #2 // offset in 4-bytes
str eax, [dest_buf] // dest_buf[0] = next_full_patt - dest_buf
/* -------------------------- packing 1024 tags into 256 bytes ----------------------------------------*/
// boundary_tmp = WK_pack_2bits(tempTagsArray, (WK_word *) next_tag, dest_buf + HEADER_SIZE_IN_WORDS) add rdi, dest_buf, #12 // dest_buf
mov rcx, tempTagsArray // &tempTagsArray[0]
L_pack_2bits:
ld1.2s {v0,v1,v2,v3},[rcx],#32
shl.2d v1,v1,#4
shl.2d v3,v3,#4
orr.8b v0, v0, v1
orr.8b v2, v2, v3
ushr.2d v1, v0, #30
ushr.2d v3, v2, #30
orr.8b v0, v0, v1
orr.8b v2, v2, v3
zip1.2s v0, v0, v2
st1.2s {v0},[rdi],#8
cmp next_tag, rcx
b.hi L_pack_2bits
/* --------------------------------- packing 4-bits dict indices into dest_buf ---------------------------------- */
/* 1st, round up number of 4-bits dict_indices to a multiple of 8 and fill in 0 if needed */
sub rax, next_qp, tempQPosArray // eax = num_bytes_to_pack = next_qp - (char *) tempQPosArray lsr eax, eax, #3 // num_packed_words = (num_bytes_to_pack + 7) >> 3
add rcx, tempQPosArray, rax, lsl #3 // endQPosArray = tempQPosArray + 2*num_source_words
lsl rax, rax, #2
subs byte_count, byte_count, rax
b.lt L_budgetExhausted
cmp rcx, next_qp // endQPosArray vs next_qp
b.ls 2f // if (next_qp >= endQPosArray) skip the following zero paddings
sub rax, rcx, next_qp
mov edx, #0
tst eax, #4
b.eq 1f
str edx, [next_qp], #4
1: tst eax, #2
b.eq 1f
strh edx, [next_qp], #2
1: tst eax, #1
b.eq 2f
strb edx, [next_qp], #1
2:
mov rdi, next_full_patt // next_full_patt
cmp rcx, tempQPosArray // endQPosArray vs tempQPosArray
ldr eax, [dest_buf]
b.ls L20 // if (endQPosArray <= tempQPosArray) skip the following
mov rdx, tempQPosArray // tempQPosArray
/* packing 4-bits dict indices into dest_buf */
L_pack_4bits:
ldr rax, [rdx], #8 // src_next[1]:src_next[0]
orr rax, rax, rax, lsr #28 // eax = src_next[0] | (src_next[1] << 4)
cmp rcx, rdx // source_end vs src_next
str eax, [rdi], #4 // *dest_next++ = temp
// SET_LOW_BITS_AREA_START(dest_buf,boundary_tmp) lsr eax, eax, #2 // boundary_tmp - dest_buf in words
L20:
str eax, [dest_buf,#4] // dest_buf[1] = boundary_tmp - dest_buf
/* --------------------------- packing 3 10-bits low bits into a 32-bit word in dest_buf[] ----------------------------------------- */
add rcx, scratch, #(2048*scale) // tempLowBitsArray
sub rdx, next_low_bits, rcx // next_low_bits - tempLowBitsArray (in bytes)
lsr rdx, rdx, #1 // num_tenbits_to_pack (in half-words)
subs edx, edx, #3 // pre-decrement num_tenbits_to_pack by 3
b.lt 1f // if num_tenbits_to_pack < 3, skip the following loop
0:
subs byte_count, byte_count, #4 // byte_count -= 4
b.le L_budgetExhausted // return -1 to signal this page is not compressable
subs edx, edx, #3 // num_tenbits_to_pack-=3
ldr rax, [rcx], #6
bfm rax, rax, #58, #9 // pack 1st toward 2nd
bfm rax, rax, #58, #25 // pack 1st/2nd toward 3rd
lsr rax, rax, #12
str eax, [rdi], #4 // pack w0,w1,w2 into 1 dest_buf word
b.ge 0b // if no less than 3 elements, back to loop head
1: adds edx, edx, #3 // post-increment num_tenbits_to_pack by 3
b.eq 3f // if num_tenbits_to_pack is a multiple of 3, skip the following
subs byte_count, byte_count, #4 // byte_count -= 4
b.le L_budgetExhausted // return -1 to signal this page is not compressable
ldrh eax,[rcx] // w0
subs edx, edx, #1 // num_tenbits_to_pack--
b.eq 2f //
ldrh edx, [rcx, #2] // w1
orr eax, eax, edx, lsl #10 // w0 | (w1<<10)
2: str eax, [rdi], #4 // write the final dest_buf word
3: sub rax, rdi, dest_buf // boundary_tmp - dest_buf
lsr eax, eax, #2 // boundary_tmp - dest_buf in terms of words
str eax, [dest_buf, #8] // SET_LOW_BITS_AREA_END(dest_buf,boundary_tmp)
lsl w0, eax, #2 // boundary_tmp - dest_buf in terms of bytes
L_done:
// restore registers and return
mov x15, sp
ldp x20, x21, [x15, #0] // restore x20, x21
ldp x22, x23, [x15, #16] // restore x22, x23
ldp x24, x25, [x15, #32] // restore x24, x25
ldp x26, x27, [x15, #48] // restore x26, x27
add sp, sp, #128 // deallocate for dictionary + saved register space
#if KERNEL
ld1.4s {v0,v1,v2,v3},[sp],#64
#endif
ret lr
.align 4
L_budgetExhausted:
mov x0, #-1
b L_done
.align 4,0x90
L_RECORD_EXACT:
/*
we have an exact match of the input word to its corresponding dictionary word
write tag/dict_index to the temorary buffers
*/
mov eax, #3
lsr w14, wdict_location, #2 // divide by 4 for word offset
subs remaining, remaining, #1 // remaing-- strb w14, [next_qp], #1 // *next_qp = word offset (4-bit)
b.gt L_loop
b CHECKPOINT // if remaining = 0, break
.align 4,0x90
L_RECORD_PARTIAL:
/*
we have a partial (high 22-bits) match of the input word to its corresponding dictionary word
write tag/dict_index/low 10 bits to the temorary buffers
*/
mov ecx, #1
strb ecx, [next_tag], #1 // *next_tag++ = 1 for partial matched
str edx, [dictionary, dict_location] // *dict_location = input_word lsr eax, wdict_location, #2 // offset in 32-bit word
and edx, edx, #1023 // lower 10 bits
strb eax, [next_qp], #1 // update *next_qp++
strh edx, [next_low_bits], #2 // save next_low_bits++
b.gt L_loop
CHECKPOINT:
cbz mode, L_check_compression_ratio // if this this an early abort check..
L_check_zero_page:
cmp start_next_full_patt, next_full_patt // check if any dictionary misses in page
b.ne L_check_single_value_page
cmp start_next_qp, next_qp // check if any partial or exact dictionary matches
b.ne L_check_single_value_page
mov x0, #SV_RETURN // Magic return value
b L_done
L_check_single_value_page:
sub rax, next_full_patt, start_next_full_patt // get # dictionary misses
lsr rax, rax, #2
sub r11, next_qp, start_next_qp // get # dictionary hits (exact + partial)
sub r13, next_low_bits, start_next_low_bits // get # dictionary partial hits
lsr r13, r13, #1
// Single value page if one of the follwoing is true:
// partial == 0 AND hits == 1023(for 4K page) AND miss == 1 AND tag[0] == 2 (i.e. miss)
// partial == 1 AND hits == 1024(for 4K page) AND tag[0] == 1 (i.e. partial)
//
cbnz r13, 1f // were there 0 partial hits?
cmp r11, #(256*PAGES_SIZE_IN_KBYTES - 1) // were there 1023 dictionary hits
b.ne 1f
cmp rax, #1 // was there exacly 1 dictionary miss?
b.ne 1f
ldrb edx, [tempTagsArray] // read the very 1st tag
cmp edx, #2 // was the very 1st tag a miss?
b.eq L_is_single_value_page
1:
cmp r13, #1 // was there 1 partial hit?
b.ne L_check_mostly_zero
cmp r11, #(256*PAGES_SIZE_IN_KBYTES) // were there 1024 dictionary hits
b.ne L_check_mostly_zero
ldrb edx, [tempTagsArray] // read the very 1st tag
cmp edx, #1 // was the very 1st tag a partial?
b.ne L_check_mostly_zero
L_is_single_value_page:
mov x0, #SV_RETURN // Magic return value
b L_done
L_check_mostly_zero:
// how much space will the sparse packer take?
add rax, rax, r11 // rax += (next_qp - start_next_qp)
mov rdx, #6
mov rcx, #4
madd r11, rax, rdx, rcx // r11 = rax * 6 (i.e. 4 byte word + 2 byte offset) + 4 byte for header
sub rax, next_low_bits, start_next_low_bits // get bytes consumed by lower-10 bits
mov rdx, #1365
mul rax, rax, rdx
sub rdx, next_full_patt, start_next_full_patt // get bytes consumed by dictionary misses
add rax, rdx, rax, lsr #11 // rax = 2/3*(next_low_bits - start_next_low_bits) + (next_full_patt - start_next_full_patt)
sub rdx, next_qp, start_next_qp
add rax, rax, rdx, lsr #1 // rax += (next_qp - start_next_qp)/2
add rax, rax, #(12+256*scale) // rax += bytes taken by the header + tags
cmp rax, r11 // is the default packer the better option?
b.lt L_done_search
cmp r11, byte_budget // can the sparse packer fit into the given budget?
b.gt L_budgetExhausted
L_sparse_packer:
mov edx, #MZV_MAGIC
str edx, [dest_buf], #4 // header to indicate a sparse packer
mov rdx, #0 // rdx = byte offset in src of non-0 word
1:
ldr rax, [start_next_input_word, rdx] // rax = read dword
cbnz rax, 5f // is dword != 0
3:
add rdx, rdx, #8 // 8 more bytes have been processed
4:
cmp rdx, #(4096*scale) // has the entire page been processed
b.ne 1b
mov x0, r11 // store the size of the compressed stream
b L_done
5:
cbz eax, 6f // is lower word == 0
str eax, [dest_buf], #4 // store the non-0 word in the dest buffer
strh edx, [dest_buf], #2 // store the byte index
6:
lsr rax, rax, 32 // get the upper word into position
cbz eax, 3b // is dword == 0
add rdx, rdx, #4
str eax, [dest_buf], #4 // store the non-0 word in the dest buffer
strh edx, [dest_buf], #2 // store the byte index
add rdx, rdx, #4
b 4b
L_check_compression_ratio:
mov mode, NORMAL
mov remaining, #((1024 - CHKPT_WORDS)*scale) // remaining input words to process
cbz remaining, CHECKPOINT // if there are no remaining words to process
sub rax, next_low_bits, start_next_low_bits // get bytes consumed by lower-10 bits
mov rdx, #1365
mul rax, rax, rdx
sub rdx, next_full_patt, start_next_full_patt // get bytes consumed by dictionary misses
add rax, rdx, rax, lsr #11 // rax = 2/3*(next_low_bits - start_next_low_bits) + (next_full_patt - start_next_full_patt)
sub rdx, next_qp, start_next_qp
add rax, rax, rdx, lsr #1 // rax += (next_qp - start_next_qp)/2
subs rax, rax, #((CHKPT_SHRUNK_BYTES - CHKPT_TAG_BYTES)*scale)
// rax += CHKPT_TAG_BYTES b.gt L_budgetExhausted // if rax is > 0, we need to early abort
b L_loop // we are done