linux-kernel - Re: [PATCH 2/2] perf tools: Fix strict alias issue for find_first

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Date:	Thu, 6 Mar 2014 17:46:53 -0300
From:	Arnaldo Carvalho de Melo <acme@...stprotocols.net>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
	Jiri Olsa <jolsa@...hat.com>,
	Corey Ashford <cjashfor@...ux.vnet.ibm.com>,
	David Ahern <dsahern@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Namhyung Kim <namhyung@...nel.org>,
	Paul Mackerras <paulus@...ba.org>
Subject: Re: [PATCH 2/2] perf tools: Fix strict alias issue for find_first_bit

Em Fri, Feb 28, 2014 at 10:29:23PM +0100, Peter Zijlstra escreveu:
> On Fri, Feb 28, 2014 at 06:25:51PM -0300, Arnaldo Carvalho de Melo wrote:
> > From: Jiri Olsa <jolsa@...hat.com>

> > The gcc documentation offers workaround for valid aliasing by using
> > __may_alias__ attribute:

> >   http://gcc.gnu.org/onlinedocs/gcc-4.4.0/gcc/Type-Attributes.html

> > Using this workaround for the find_first_bit function.

> Hurm; didn't I suggest using -fno-strict-aliasing just like the kernel
> does? Because the C aliasing rules are bonghits heavy?

Either way would solve the problem at hand, I just used the smaller
hammer offered.

I thought that Ingo had something against this, but only reference I
could find now about it was this:

--------------------------------------------

commit 65014ab36196f6d86edc9ee23759d6930b9d89a8
Author: Ingo Molnar <mingo@...e.hu>
Date:   Wed Sep 2 14:55:55 2009 +0200

    perf tools: Work around strict aliasing related warnings
    
    Older versions of GCC are rather stupid about strict aliasing:

<SNIP>

    Make it clear to GCC that we intend with those pointers, by passing
    them through via an explicit (void *) cast.
    
    We might want to add -fno-strict-aliasing as well, like the kernel
    itself does.
    
    Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>
    Cc: Mike Galbraith <efault@....de>
    Cc: Paul Mackerras <paulus@...ba.org>
    Cc: Arnaldo Carvalho de Melo <acme@...hat.com>
    Cc: Frederic Weisbecker <fweisbec@...il.com>
    LKML-Reference: <new-submission>
    Signed-off-by: Ingo Molnar <mingo@...e.hu>
--------------------------------------------

Jiri's bandaid is for _newer_ compilers, so...

- Arnaldo
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