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

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Date:	Tue, 25 Nov 2014 22:05:15 +0100
From:	Michal Marek <mmarek@...e.cz>
To:	Josh Stone <jistone@...hat.com>
CC:	linux-kernel@...r.kernel.org, "Frank Ch. Eigler" <fche@...hat.com>,
	Jakub Jelinek <jakub@...hat.com>,
	Josh Boyer <jwboyer@...oraproject.org>,
	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

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.

Michal
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