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compiler-gcc.h: add more comments to RELOC_HIDE

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Requested by C. Lameter

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Cc: Christoph Lameter <cl@linux-foundation.org>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Mike Travis <travis@sgi.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Richard Henderson <rth@twiddle.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Andi Kleen 2009-01-09 16:40:53 -08:00 committed by Linus Torvalds
parent 2d96d1053d
commit 85c210edc4
1 changed files with 12 additions and 2 deletions
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14
include/linux/compiler-gcc.h
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@ -11,9 +11,19 @@
/* The "volatile" is due to gcc bugs */
#define barrier() __asm__ __volatile__("": : :"memory")
/* This macro obfuscates arithmetic on a variable address so that gcc
shouldn't recognize the original var, and make assumptions about it */
/*
* This macro obfuscates arithmetic on a variable address so that gcc
* shouldn't recognize the original var, and make assumptions about it.
*
* This is needed because the C standard makes it undefined to do
* pointer arithmetic on "objects" outside their boundaries and the
* gcc optimizers assume this is the case. In particular they
* assume such arithmetic does not wrap.
*
* A miscompilation has been observed because of this on PPC.
* To work around it we hide the relationship of the pointer and the object
* using this macro.
*
* Versions of the ppc64 compiler before 4.1 had a bug where use of
* RELOC_HIDE could trash r30. The bug can be worked around by changing
* the inline assembly constraint from =g to =r, in this particular