[PATCH v2 10/16] xz: Optimize for-loop conditions in the BCJ decoders

Lasse Collin
2024-07-21T13:37:16Z

Compilers cannot optimize the addition "i + 4" away since theoretically
it could overflow.

Reviewed-by: Sam James <sam@gentoo.org>
Signed-off-by: Lasse Collin <lasse.collin@tukaani.org>
---
lib/xz/xz_dec_bcj.c | 23 ++++++++++++++++++-----
1 file changed, 18 insertions(+), 5 deletions(-)

diff --git a/lib/xz/xz_dec_bcj.c b/lib/xz/xz_dec_bcj.c
index ab9237ed6db8..e0b4bf4999c0 100644
--- a/lib/xz/xz_dec_bcj.c
+++ b/lib/xz/xz_dec_bcj.c
@@ -161,7 +161,9 @@ static size_t bcj_powerpc(struct xz_dec_bcj *s, uint8_t *buf, size_t size)
size_t i;
uint32_t instr;

- for (i = 0; i + 4 <= size; i += 4) {
+ size &= ~(size_t)3;
+
+ for (i = 0; i < size; i += 4) {
instr = get_unaligned_be32(buf + i);
if ((instr & 0xFC000003) == 0x48000001) {
instr &= 0x03FFFFFC;
@@ -218,7 +220,9 @@ static size_t bcj_ia64(struct xz_dec_bcj *s, uint8_t *buf, size_t size)
/* Instruction normalized with bit_res for easier manipulation */
uint64_t norm;

- for (i = 0; i + 16 <= size; i += 16) {
+ size &= ~(size_t)15;
+
+ for (i = 0; i < size; i += 16) {
mask = branch_table[buf[i] & 0x1F];
for (slot = 0, bit_pos = 5; slot < 3; ++slot, bit_pos += 41) {
if (((mask >> slot) & 1) == 0)
@@ -266,7 +270,9 @@ static size_t bcj_arm(struct xz_dec_bcj *s, uint8_t *buf, size_t size)
size_t i;
uint32_t addr;

- for (i = 0; i + 4 <= size; i += 4) {
+ size &= ~(size_t)3;
+
+ for (i = 0; i < size; i += 4) {
if (buf[i + 3] == 0xEB) {
addr = (uint32_t)buf[i] | ((uint32_t)buf[i + 1] << 8)
| ((uint32_t)buf[i + 2] << 16);
@@ -289,7 +295,12 @@ static size_t bcj_armthumb(struct xz_dec_bcj *s, uint8_t *buf, size_t size)
size_t i;
uint32_t addr;

- for (i = 0; i + 4 <= size; i += 2) {
+ if (size < 4)
+ return 0;
+
+ size -= 4;
+
+ for (i = 0; i <= size; i += 2) {
if ((buf[i + 1] & 0xF8) == 0xF0
&& (buf[i + 3] & 0xF8) == 0xF8) {
addr = (((uint32_t)buf[i + 1] & 0x07) << 19)
@@ -317,7 +328,9 @@ static size_t bcj_sparc(struct xz_dec_bcj *s, uint8_t *buf, size_t size)
size_t i;
uint32_t instr;

- for (i = 0; i + 4 <= size; i += 4) {
+ size &= ~(size_t)3;
+
+ for (i = 0; i < size; i += 4) {
instr = get_unaligned_be32(buf + i);
if ((instr >> 22) == 0x100 || (instr >> 22) == 0x1FF) {
instr <<= 2;
--
2.45.2