[PATCH] Work around ppc64 compiler bug

In the process of optimising our per cpu data code, I found a ppc64
compiler bug that has been around forever. Basically the current
RELOC_HIDE can end up trashing r30. Details of the bug can be found at

  http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25572

This bug is present in all compilers before 4.1. It is masked by the
fact that our current per cpu data code is inefficient and causes
other loads that end up marking r30 as used.

A workaround identified by Alan Modra is to use the =r asm constraint
instead of =g.

Signed-off-by: Anton Blanchard <anton@samba.org>
[ Verified that this makes no real difference on x86[-64] */
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 7 insertions, 1 deletions

diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
index 2e05e1e6b0e..6e1c44a935d 100644
--- a/include/linux/compiler-gcc.h
+++ b/include/linux/compiler-gcc.h
@@ -11,9 +11,15 @@
 
 /* This macro obfuscates arithmetic on a variable address so that gcc
    shouldn't recognize the original var, and make assumptions about it */
+/*
+ * 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
+ * case either is valid.
+ */
 #define RELOC_HIDE(ptr, off)                                    \
   ({ unsigned long __ptr;                                       \
-    __asm__ ("" : "=g"(__ptr) : "0"(ptr));              \
+    __asm__ ("" : "=r"(__ptr) : "0"(ptr));              \
     (typeof(ptr)) (__ptr + (off)); })