From: Andi Kleen <andi@firstfloor.org>
Date: Sat, 10 Jan 2009 00:40:53 +0000 (-0800)
Subject: compiler-gcc.h: add more comments to RELOC_HIDE
X-Git-Tag: v2.6.29-rc1~14
X-Git-Url: http://pilppa.com/gitweb/?a=commitdiff_plain;h=85c210edc46d602a1562aeea0fc74919349c8cf0;p=linux-2.6-omap-h63xx.git

compiler-gcc.h: add more comments to RELOC_HIDE

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>
---

diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
index af40f8eb86f..1514d534dee 100644
--- a/include/linux/compiler-gcc.h
+++ b/include/linux/compiler-gcc.h
@@ -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