linux-kernel - Re: [RFC] perf tools: Fix strict alias issue for find_first

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Date:	Wed, 26 Feb 2014 18:18:05 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jiri Olsa <jolsa@...hat.com>
Cc:	linux-kernel@...r.kernel.org,
	Corey Ashford <cjashfor@...ux.vnet.ibm.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@...stprotocols.net>,
	David Ahern <dsahern@...il.com>
Subject: Re: [RFC] perf tools: Fix strict alias issue for find_first_bit

On Wed, Feb 26, 2014 at 06:14:26PM +0100, Jiri Olsa wrote:
> hi,
> got issue below when compiling perf tool on i686 with gcc 4.4,
> but not sure the patch is correct workaround here.
> 
> thanks for comments,
> jirka
> 
> 
> ---
> 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’:
> /root/linux/tools/perf/util/include/linux/bitops.h:109: error: dereferencing pointer ‘p’ does break strict-aliasing rules
> /root/linux/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
> /root/linux/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
> 

The kernel builds with -fno-strict-aliasing because the C aliasing rules
are a bunch of monkey poo. Perf tool should probably do the same.
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