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Tasklist

FS#54512 - [gcc] "bad codegen for __builtin_clz(unsigned short)"

Attached to Project: Arch Linux
Opened by Jeff Cook (jeffcookio) - Monday, 19 June 2017, 07:56 GMT
Last edited by Bartłomiej Piotrowski (Barthalion) - Sunday, 25 June 2017, 19:57 GMT
Task Type Bug Report
Category Upstream Bugs
Status Closed
Assigned To Bartłomiej Piotrowski (Barthalion)
Architecture All
Severity Medium
Priority Normal
Reported Version
Due in Version Undecided
Due Date Undecided
Percent Complete 100%
Votes 0
Private No

Details

GCC 7.1 contains a bug that results in invalid code generation when the `-mlzcnt` flag is set. This affects x64 and x86 at any optimization level. See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81015 for more details.

This bug has been characterized as "really scary" by Daniel Horn of Dropbox/Stanford. See https://github.com/dropbox/lepton/issues/95 and https://github.com/dropbox/lepton/commit/209463ae36042dbd29864526f739ad851a60757d.

The potential consequences of this issue seem far reaching enough to warrant proactive patching. I modified the gcc PKGBUILD locally and it seems to fix the problem. That diff is attached for convenience.
   0001-Fix-bad-codegen-in-gcc.p... (3.5 KiB)
This task depends upon

Closed by  Bartłomiej Piotrowski (Barthalion)
Sunday, 25 June 2017, 19:57 GMT
Reason for closing:  Fixed
Comment by Allan McRae (Allan) - Monday, 19 June 2017, 09:29 GMT
No need for a backported patch. It is on the gcc-7 branch, so the commit just needs bumped.

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