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

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Date:	Mon, 12 Jan 2009 11:33:49 -0600 (CST)
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	Ingo Molnar <mingo@...e.hu>
cc:	Rusty Russell <rusty@...tcorp.com.au>, akpm@...ux-foundation.org,
	torvalds@...ux-foundation.org, andi@...stfloor.org,
	ak@...ux.intel.com, rth@...ddle.net, 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 Sat, 10 Jan 2009, Ingo Molnar wrote:

> 2) Special structure sizes
>
>  For example we want to load a value from "&var + LARGE_OFFSET" (common
>  percpu construct). If 'var' is a C structure that happens to have a size
>  of 255 bytes, then GCC could legitimately assume that 'LARGE_OFFSET' is
>  0..255 [inclusive], and could optimize LARGE_OFFSET to be truncated to a
>  single byte.


But that is not what is occurring. We do not do &var + LARGE_OFFSET.
Instead one does

	((void *)&var) + LARGE_OFFSET

or
	((unsigned long)&var) + LARGE_OFFSET


The cast should cause the C compiler to drop all assumptions about size.
And the definition of RELOC_HIDE for icc and generic compilers in the tree
show that this is working.

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