From 85c210edc46d602a1562aeea0fc74919349c8cf0 Mon Sep 17 00:00:00 2001 From: Andi Kleen Date: Fri, 9 Jan 2009 16:40:53 -0800 Subject: [PATCH] compiler-gcc.h: add more comments to RELOC_HIDE Requested by C. Lameter Signed-off-by: Andi Kleen Cc: Christoph Lameter Cc: Andi Kleen Cc: Rusty Russell Cc: Stephen Rothwell Cc: Mike Travis Cc: Ingo Molnar Cc: Richard Henderson Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- include/linux/compiler-gcc.h | 14 ++++++++++++-- 1 file changed, 12 insertions(+), 2 deletions(-) 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 -- 2.41.0