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

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Date:	Fri, 28 Feb 2014 22:29:23 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Arnaldo Carvalho de Melo <acme@...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>,
	Arnaldo Carvalho de Melo <acme@...hat.com>
Subject: Re: [PATCH 2/2] perf tools: Fix strict alias issue for find_first_bit

On Fri, Feb 28, 2014 at 06:25:51PM -0300, Arnaldo Carvalho de Melo wrote:
> From: Jiri Olsa <jolsa@...hat.com>
> 
> When compiling perf tool code with gcc 4.4.7 I'm getting
> following error:
> 
>     CC       util/session.o
>   cc1: warnings being treated as errors
>   util/session.c: In function ‘perf_session_deliver_event’:
>   tools/perf/util/include/linux/bitops.h:109: error: dereferencing pointer ‘p’ does break strict-aliasing rules
>   tools/perf/util/include/linux/bitops.h:101: error: dereferencing pointer ‘p’ does break strict-aliasing rules
>   util/session.c:697: note: initialized from here
>   tools/perf/util/include/linux/bitops.h:101: note: initialized from here
>   make[1]: *** [util/session.o] Error 1
>   make: *** [util/session.o] Error 2
> 
> The aliased types here are u64 and unsigned long pointers, which is safe
> for the find_first_bit processing.
> 
> This error shows up for me only for gcc 4.4 on 32bit x86, even for
> -Wstrict-aliasing=3, while newer gcc are quiet and scream here for
> -Wstrict-aliasing={2,1}. Looks like newer gcc changed the rules for
> strict alias warnings.
> 
> 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?
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