linux-kernel - Re: [PATCH RESEND] Kbuild: Add an option to enable GCC VTA

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Date:	Tue, 25 Nov 2014 16:24:38 -0500
From:	Josh Boyer <jwboyer@...oraproject.org>
To:	Michal Marek <mmarek@...e.cz>
Cc:	Josh Stone <jistone@...hat.com>,
	"Linux-Kernel@...r. Kernel. Org" <linux-kernel@...r.kernel.org>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	Jakub Jelinek <jakub@...hat.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Markus Trippelsdorf <markus@...ppelsdorf.de>,
	Michel Dänzer <michel@...nzer.net>
Subject: Re: [PATCH RESEND] Kbuild: Add an option to enable GCC VTA

On Tue, Nov 25, 2014 at 4:05 PM, Michal Marek <mmarek@...e.cz> wrote:
> Dne 25.11.2014 v 00:46 Josh Stone napsal(a):
>> On 11/24/2014 01:46 PM, Michal Marek wrote:
>>> Dne 21.11.2014 v 19:40 Josh Stone napsal(a):
>>>> Due to recent codegen issues, gcc -fvar-tracking-assignments was
>>>> unconditionally disabled in commit 2062afb4f804a ("Fix gcc-4.9.0
>>>> miscompilation of load_balance() in scheduler").  However, this reduces
>>> [...]
>>>> With gcc-4.9.2-1.fc22, I can now build v3.18-rc5 with Fedora's i686 and
>>>> x86_64 configs, and this is completely clean with GCC_COMPARE_DEBUG.
>>>
>>> According to gcc's bug#61801, this is really fixed in 4.9.2 (commit
>>> 556537c4 in the git mirror). So how about checking for this minimal
>>> version instead of a new Kconfig option?
>>
>> That's possible, if the new Kconfig option is really undesirable.  But
>> given that there's a similar DWARF4 option, which this pairs well with,
>> I thought a VTA option was a good choice.
>>
>> Besides 4.9.2 though, this particular fix has also been backported to
>> 4.8.4, and who knows what various distro maintainers may do.
>
> My idea was that if we switch -fvar-tracking-assignments on with gcc
> 4.9.2+, then yes, some users with patched compilers will not benefit
> from it, but it will eventually work out of the box for "everybody."
> With the kconfig option, an uninformed user has no idea whether or not
> it is a good idea to switch it on. It should at least mention that it is
> safe to enable with 4.9.2+. And BTW
>
>     make KCLFAGS=-fvar-tracking-assignments
>
> works as well, as the user-supplied flags are applied last.

I actually tried this after Kyle McMartin mentioned it, and thought
about doing it in the Fedora kernel spec file instead of taking the
patch.  It's a bit less noticeable when trying to reproduce build
environments though, and with it being a cc-option in the Makefile we
can rely on that to filter out compilers and arches that don't support
the flag.

josh
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