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

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Date:	Sat, 1 Mar 2014 10:12:58 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	David Ahern <dsahern@...il.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	linux-kernel@...r.kernel.org, Jiri Olsa <jolsa@...hat.com>,
	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@...hat.com>
Subject: Re: [PATCH 2/2] perf tools: Fix strict alias issue for find_first_bit

* David Ahern <dsahern@...il.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
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