linux-kernel - Re: Unnecessary Relocation Hiding?

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Date:	Fri, 25 Aug 2006 08:18:49 +0200
From:	Andi Kleen <ak@...e.de>
To:	"Dong Feng" <middle.fengdong@...il.com>
Cc:	"Paul Mackerras" <paulus@...ba.org>,
	"Christoph Lameter" <clameter@....com>,
	linux-kernel@...r.kernel.org
Subject: Re: Unnecessary Relocation Hiding?

On Friday 25 August 2006 03:30, Dong Feng wrote:
> Sorry for perhaps extending the specific question to a more generic
> one. In which cases shall we, in current or future development,
> prevent gcc from knowing a pointer-addition in the way RELOC_HIDE? And
> in what cases shall we just write pure C point addition?
> 
> After all, we are writing an OS in C not in pure assembly, so I am
> just trying to learn some generial rules to mimize the raw assembly in
> development.

In theory anything that is undefined in the C standard should be avoided
because gcc is free to make assumptions about it and generate unexpected 
code.

In practice Linux does a lot of not-quite-legal-in-portable-C things
already, but tries to avoid areas that are known to have miscompiled in
the past.

Best is to avoid undefined behaviour in new code.

-Andi
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