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

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Date:	Fri, 16 Jan 2009 14:37:32 -0600 (CST)
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 Thu, 15 Jan 2009, Richard Henderson wrote:

> I didn't explore the space of possible solutions, merely gave Rusty a solution
> that I knew would work, and would never fail because the compiler would never
> look through the asm.
>
> I wouldn't be surprised if the compiler thought "(long)&foo - large_constant"
> could be put back together into a small-data relocation, simply because at the
> level at which that optimization is performed, we've thrown away type data
> like long and void*; we only have modes.

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?

> Why are you wanting to change this at all?  It works as it is.

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.
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