[kudu-CR] Disable -ftree-vectorize on gcc 4.8

["Todd Lipcon (Code Review)" <ge...@cloudera.org>]

Hello Dan Burkert,

I'd like you to do a code review.  Please visit

    http://gerrit.cloudera.org:8080/2274

to review the following change.

Change subject: Disable -ftree-vectorize on gcc 4.8
......................................................................

Disable -ftree-vectorize on gcc 4.8

After studying a failed precommit build for several hours, I
couldn't find anything wrong with the code that was segfaulting.
Additionally, it only failed on gcc 4.8 in -O3 mode. After a bit
more debugging, I found that adding no-ops compiler barriers like
'__asm__ __volatile__("");' made the segfault go away, which made
me suspect that there was some particular optimization gone bad.

Eventually, I tracked it down to the -ftree-vectorize option which
is enabled by default in -O3 mode. Looking online it isn't clear
exactly which bug we're hitting, but a GCC developer says the following
in one related bug report[1]:

> There are a few vectorizer wrong-code bugs on the 4.8 branch that I
> haven't managed to backport yet from 4.9.

Given this, I think it's prudent to disable -ftree-vectorize on
gcc 4.8 release builds. Entirely blacklisting gcc 4.8 would be a bit
heavy-handed, given it's the default compiler on Ubuntu 14.04.

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61068

Change-Id: I6ec6afea173e473a95bea9ca180ca2dedfc2c21d
---
M CMakeLists.txt
1 file changed, 10 insertions(+), 0 deletions(-)


  git pull ssh://gerrit.cloudera.org:29418/kudu refs/changes/74/2274/1
-- 
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Gerrit-MessageType: newchange
Gerrit-Change-Id: I6ec6afea173e473a95bea9ca180ca2dedfc2c21d
Gerrit-PatchSet: 1
Gerrit-Project: kudu
Gerrit-Branch: master
Gerrit-Owner: Todd Lipcon <to...@apache.org>
Gerrit-Reviewer: Dan Burkert <da...@cloudera.com>