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Bug 1619354 - wrong result when compiled with -funroll-loops
Summary: wrong result when compiled with -funroll-loops
Keywords:
Status: CLOSED CURRENTRELEASE
Alias: None
Product: Fedora
Classification: Fedora
Component: gcc
Version: 31
Hardware: s390x
OS: Linux
unspecified
unspecified
Target Milestone: ---
Assignee: Jakub Jelinek
QA Contact: Fedora Extras Quality Assurance
URL:
Whiteboard:
Depends On:
Blocks: ZedoraTracker
TreeView+ depends on / blocked
 
Reported: 2018-08-20 15:10 UTC by Dan Horák
Modified: 2020-08-10 14:38 UTC (History)
12 users (show)

Fixed In Version:
Doc Type: If docs needed, set a value
Doc Text:
Clone Of:
Environment:
Last Closed: 2020-08-10 14:38:20 UTC
Type: Bug
Embargoed:

Attachments (Terms of Use)
reduced standalone test case (1.29 KB, text/plain)
2018-08-20 15:10 UTC, Dan Horák 		no flags Details
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System ID Private Priority Status Summary Last Updated
IBM Linux Technology Center 173351 0 None None None 2019-08-07 11:14:33 UTC

Description Dan Horák 2018-08-20 15:10:30 UTC 
Created attachment 1477282 [details]
reduced standalone test case

Description of problem:
After an introduction of __build_clz() based pow2_ceil() implementation [1] in jemalloc, a unit test [2] started to fail in the test-suite on s390x. It was reported as [3], but it turned into a compiler issue (IMO) caused by the -funroll-loops option.

[1] https://github.com/jemalloc/jemalloc/commit/4c548a61c89b0472b9952fcc4090eb00c2a88870
[2] https://github.com/jemalloc/jemalloc/blob/dev/test/unit/bit_util.c
[3] https://github.com/jemalloc/jemalloc/issues/1307

Version-Release number of selected component (if applicable):
gcc-8.2.1-2.fc29.s390x
Comment 1 Dan Horák 2018-08-20 15:13:40 UTC 
Seems it's affecting only s390x, this came from our CI for jemalloc that checks also armv7, aarch64, ppc64, ppc64le and x86_64.
Comment 2 Dan Horák 2018-08-20 15:14:36 UTC 
The error message from running the test case is

Failed assertion: (pow2_ceil_u32(((uint32_t)1) << i)) == (((uint32_t)1) << i) --> 1 != 2: Unexpected result, i=1
Comment 3 Marek Polacek 2018-08-20 15:18:20 UTC 
Have you tried -fsanitize=undefined?
Comment 4 Dan Horák 2018-08-20 15:24:02 UTC 
I've tried now and nothing is reported

[sharkcz@devel10 ~]$ gcc -std=gnu11 -Wall -pipe -g3 -O2 -funroll-loops -fsanitize=undefined -o t t.c
[sharkcz@devel10 ~]$ ./t
Failed assertion: (pow2_ceil_u32(((uint32_t)1) << i)) == (((uint32_t)1) << i) --> 1 != 2: Unexpected result, i=1
Comment 5 Dan Horák 2018-08-20 15:26:03 UTC 
for the record, the generic pow2_ceil() produces correct results in the test
Comment 6 IBM Bug Proxy 2018-11-20 16:31:52 UTC 
------- Comment From Andreas.Krebbel.com 2018-11-20 11:23 EDT-------
This is a bug in the clztidi2 pattern in the S/390 backend of GCC. I've just committed a patch to mainline.

https://gcc.gnu.org/ml/gcc-patches/2018-11/msg01736.html

Although it appears to be quite difficult to trigger this probably should be backported to basically all distro levels currently in use :(
Comment 7 Dan Horák 2018-11-20 17:24:43 UTC 
Hanns, I guess it makes sense to clone this bug for RHEL gcc too.
Comment 8 Hanns-Joachim Uhl 2018-11-21 08:19:05 UTC 
(In reply to Dan Horák from comment #7)
> Hanns, I guess it makes sense to clone this bug for RHEL gcc too.
.
Hello Dan,
... agree ... 
... please go ahead cloning this bugzilla to the related RHEL releases ...
Thanks for your attention and support.
Comment 9 Ben Cotton 2019-08-13 16:56:25 UTC 
This bug appears to have been reported against 'rawhide' during the Fedora 31 development cycle.
Changing version to '31'.
Comment 10 Ben Cotton 2019-08-13 19:14:11 UTC 
This bug appears to have been reported against 'rawhide' during the Fedora 31 development cycle.
Changing version to 31.
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