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From: Ingo Molnar <mingo@kernel.org>
To: David Ahern <dsahern@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
	Arnaldo Carvalho de Melo <acme@infradead.org>,
	linux-kernel@vger.kernel.org, Jiri Olsa <jolsa@redhat.com>,
	Corey Ashford <cjashfor@linux.vnet.ibm.com>,
	Frederic Weisbecker <fweisbec@gmail.com>,
	Ingo Molnar <mingo@elte.hu>, Namhyung Kim <namhyung@kernel.org>,
	Paul Mackerras <paulus@samba.org>,
	Arnaldo Carvalho de Melo <acme@redhat.com>
Subject: Re: [PATCH 2/2] perf tools: Fix strict alias issue for find_first_bit
Date: Sat, 1 Mar 2014 10:12:58 +0100	[thread overview]
Message-ID: <20140301091258.GB5885@gmail.com> (raw)
In-Reply-To: <531123D8.6000201@gmail.com>


* David Ahern <dsahern@gmail.com> wrote:

> On 2/28/14, 2:29 PM, Peter Zijlstra wrote:
> >Hurm; didn't I suggest using -fno-strict-aliasing just like the kernel
> >does? Because the C aliasing rules are bonghits heavy?
> 
> you, and Ingo in 2009 -- 65014ab3

Yeah, so that's certainly true for the kernel, but for user-space the 
aggressive optimizations that come with the aliasing rules were pretty 
good, last I checked.

So it would be nice to check the code generation and performance 
impact of -fno-strict-aliasing on perf (if any). If the impact is 
restricted to an odd few annotations for weird, low-level methods
like find_bit(), then we might be able to live with it.

The aliasing warnings can also find real bugs and uncleanlinesses.

So I'm really of two minds regarding this.

Thanks,

	Ingo

  reply	other threads:[~2014-03-01  9:13 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2014-02-28 21:25 [GIT PULL 0/2] perf/urgent fixes Arnaldo Carvalho de Melo
2014-02-28 21:25 ` [PATCH 1/2] perf tools: fix BFD detection on opensuse Arnaldo Carvalho de Melo
2014-02-28 21:25 ` [PATCH 2/2] perf tools: Fix strict alias issue for find_first_bit Arnaldo Carvalho de Melo
2014-02-28 21:29   ` Peter Zijlstra
2014-03-01  0:03     ` David Ahern
2014-03-01  9:12       ` Ingo Molnar [this message]
2014-03-06 20:46     ` Arnaldo Carvalho de Melo
2014-03-01  9:14 ` [GIT PULL 0/2] perf/urgent fixes Ingo Molnar

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