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Date:	Thu, 6 Mar 2014 17:46:53 -0300
From:	Arnaldo Carvalho de Melo <>
To:	Peter Zijlstra <>
Cc:	Ingo Molnar <>,,
	Jiri Olsa <>,
	Corey Ashford <>,
	David Ahern <>,
	Frederic Weisbecker <>,
	Ingo Molnar <>,
	Namhyung Kim <>,
	Paul Mackerras <>
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 <>

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

> >

> > 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 <>
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:


    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 <>
    Cc: Mike Galbraith <>
    Cc: Paul Mackerras <>
    Cc: Arnaldo Carvalho de Melo <>
    Cc: Frederic Weisbecker <>
    LKML-Reference: <new-submission>
    Signed-off-by: Ingo Molnar <>

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

- Arnaldo
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