linux-kernel - Re: [PATCH 3.15 33/37] Fix gcc-4.9.0 miscompilation of load

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Message-ID: <20140730072120.GB1652@laptop.redhat.com>
Date:	Wed, 30 Jul 2014 09:21:20 +0200
From:	Jakub Jelinek <jakub@...hat.com>
To:	Markus Trippelsdorf <markus@...ppelsdorf.de>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	Michel Dänzer <michel@...nzer.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 3.15 33/37] Fix gcc-4.9.0 miscompilation of
 load_balance()  in scheduler

On Wed, Jul 30, 2014 at 09:13:08AM +0200, Markus Trippelsdorf wrote:
> On 2014.07.30 at 08:53 +0200, Jakub Jelinek wrote:
> > On Tue, Jul 29, 2014 at 06:49:09PM -0700, Greg Kroah-Hartman wrote:
> > > 3.15-stable review patch.  If anyone has any objections, please let me know.
> > 
> > IMNSHO this is a too big hammer approach.  The bug happened on a single
> > file only (right?), so if anything, IMHO it could be disabled for that
> > single file only, and better do it only for compilers with the bug.
> 
> No. There are many more files affected. It just happens that Linus
> analyzed the assembly of this single file (fair.c) and found a bug. 
> Just build your redhat distro kernel with GCC_COMPARE_DEBUG=1 and you'll
> see. So unless someone analyzes the assembly output of all other
> affected files by hand and finds no issues, one has to assume the worst.

I'm talking about wrong-code issues.  For -fcompare-debug, we indeed check
it primarily during gcc bootstrap (through bootstrap-debug) and some
testcases, and we'll certainly try to build some more code with
-fcompare-debug and fix the issues.

	Jakub
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