[PATCH] zlib: Fix big performance regression

[Christophe Leroy]

Le 27/06/2024 à 21:40, Tom Rini a écrit :
> On Thu, Jun 27, 2024 at 01:34:55PM -0600, Tom Rini wrote:
>> On Thu, Jun 27, 2024 at 10:25:21AM +0200, Christophe Leroy wrote:
>>
>>> Commit 340fdf1303dc ("zlib: Port fix for CVE-2016-9841 to U-Boot")
>>> brings a big performance regression in inflate_fast(), which leads
>>> to watchdog timer reset on powerpc 8xx.
>>>
>>> It looks like that commit does more than what it describe, it
>>> especially removed an important optimisation that was doing copies
>>> using halfwords instead of bytes. That unexpected change multiplied
>>> by almost 4 the time spent in inflate_fast() and increased by 40%
>>> the overall time needed to uncompress linux kernel image.
>>>
>>> So partially revert that commit but keep post incrementation as it
>>> is the initial purpose of said commit.
>>>
>>> Fixes: 340fdf1303dc ("zlib: Port fix for CVE-2016-9841 to U-Boot")
>>> Signed-off-by: Christophe Leroy <christophe.leroy at csgroup.eu>
>>> ---
>>>   lib/zlib/inffast.c | 51 ++++++++++++++++++++++++++++++++++++----------
>>>   1 file changed, 40 insertions(+), 11 deletions(-)
>>
>> Both this, and my mostly revert lead to CI failures around compression
>> tests:
>> https://source.denx.de/u-boot/u-boot/-/jobs/859329
> 
> A full revert however, is fine.
> 

Is it that ? :

FAILED 
test/py/tests/test_ut.py::test_ut[ut_compression_compression_test_gzip]

It seems like when the optimisation was added by commit cd514aeb996e 
("zlib: Optimize decompression"), only the pre-increment implementation 
was available.

When POSTINC was added by commit e89516f031db ("zlib: split up to match 
original source tree"), I guess it was not verified because POSTINC is 
#undef by zlib.h.

With the following change I don't get the FAILED 
ut_compression_compression_test_gzip CI anymore 
(https://source.denx.de/u-boot/custodians/u-boot-mpc8xx/-/jobs/859937)

diff --git a/lib/zlib/inffast.c b/lib/zlib/inffast.c
index c271d85ea1..5dc0574202 100644
--- a/lib/zlib/inffast.c
+++ b/lib/zlib/inffast.c
@@ -257,7 +257,7 @@ void inflate_fast(z_streamp strm, unsigned start)
                         sfrom = (unsigned short *)(from - OFF);
                         loops = len >> 1;
                         do
-                           PUP(sout) = get_unaligned(++sfrom);
+                           PUP(sout) = get_unaligned(sfrom++);
                         while (--loops);
                         out = (unsigned char *)sout + OFF;
                         from = (unsigned char *)sfrom + OFF;


Christophe