linux-kernel - Re: [patch 2/8] compiler-gcc.h: add more comments to RELOC

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Message-ID: <alpine.DEB.1.10.0901210848080.7422@qirst.com>
Date:	Wed, 21 Jan 2009 08:50:25 -0500 (EST)
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	Richard Henderson <rth@...ddle.net>
cc:	Rusty Russell <rusty@...tcorp.com.au>, Ingo Molnar <mingo@...e.hu>,
	akpm@...ux-foundation.org, torvalds@...ux-foundation.org,
	andi@...stfloor.org, ak@...ux.intel.com, sfr@...b.auug.org.au,
	travis@....com, linux-kernel@...r.kernel.org
Subject: Re: [patch 2/8] compiler-gcc.h: add more comments to RELOC_HIDE

On Mon, 19 Jan 2009, Richard Henderson wrote:

> > We are talking about
> >
> > (long)&foo + long_variable
> >
> > Are you saying that the compiler will be ignoring the high bits in
> > variable because of the size of foo?
>
> No, I'm saying that all those high bits will be passed along and won't
> fit in the 16-bit relocation that'll come out of the assembler, leading
> to a hard linker error.

Why would a 16 bit relocation be generated if the compiler knows that a 32
bit/64 bit entity is added to the address of a variable?

> > It looks like its useless and more an indication of either a broken
> > compiler or wrong assumptions about the compiler. Removing RELOC_HIDE
> > should allow the compiler to freely optimize the per cpu address
> > calculations.
>
> Something I'm pretty sure we don't want the compiler to be able to do.

Other compilers (like icc) seem to have no problem with it. Why not?
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