x86: Use inline assembler instead of global register variable to get sp

LTO in gcc 4.6/47. has trouble with global register variables. They were used
to read the stack pointer. Use a simple inline assembler statement with
a mov instead.

This also helps LLVM/clang, which does not support global register
variables.

[ hpa: Ideally this should become a builtin in both gcc and clang. ]

v2: More general asm constraint. Fix description (Jan Beulich)

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Link: http://lkml.kernel.org/r/1382458079-24450-6-git-send-email-andi@firstfloor.org
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

diff --git a/arch/x86/include/asm/thread_info.h b/arch/x86/include/asm/thread_info.h
index 3ba3de4..e1940c0 100644 (file)
--- a/arch/x86/include/asm/thread_info.h
+++ b/arch/x86/include/asm/thread_info.h
@@ -163,9 +163,11 @@ struct thread_info {
  */
 #ifndef __ASSEMBLY__
 
-
-/* how to get the current stack pointer from C */
-register unsigned long current_stack_pointer asm("esp") __used;
+#define current_stack_pointer ({               \
+       unsigned long sp;                       \
+       asm("mov %%esp,%0" : "=g" (sp));        \
+       sp;                                     \
+})
 
 /* how to get the thread information struct from C */
 static inline struct thread_info *current_thread_info(void)