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From: "brian at soulspark dot org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/64941] New: -O3 breaks tar
Date: Thu, 05 Feb 2015 05:22:00 -0000
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X-Bugzilla-Product: gcc
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X-Bugzilla-Version: 4.8.3
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64941

            Bug ID: 64941
           Summary: -O3 breaks tar
           Product: gcc
           Version: 4.8.3
            Status: UNCONFIRMED
          Severity: major
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: brian at soulspark dot org

While emerging my gentoo I ended up with a version of tar-1.27.1 that wouldn't
extract one particular package: sys-apps/kbd-1.15.5 (although it worked on
other packages)

The compiled executable produced an error message when attempting to decompress
and untar the file:

tar xf /usr/portage/distfiles/kbd-1.15.5.tar.gz 
tar: Skipping to next header 
tar: Exiting with failure status due to previous errors

After a bit of trial and error I found if I changed the CFLAGS from:
1) CFLAGS = "-fomit-frame-pointer -pipe -march=native -mtune=native
-fexcess-precision=fast -O3"
to:
2) CFLAGS="-march=native -mtune=native -O2 -pipe -fomit-frame-pointer
-fexcess-precision=fast"
then the code would work as designed.

This is a 64 bit system with abi_32 enabled, but I don't think that makes any
difference in this case.

Steps to reproduce: using gcc 4.8.3 compile the attached tar, the run the
executable to extract the attached kbd. It will fail with the error message
above.

Change to -O2 and it will work fine.