[PATCH 48/74] x86, lto: Use inline assembler instead of global register variable to get sp
From: Andi Kleen 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 instead. I verified this generates the same binary (on 64bit) as the original register variable. Signed-off-by: Andi Kleen --- arch/x86/include/asm/thread_info.h |8 +--- 1 files changed, 5 insertions(+), 3 deletions(-) diff --git a/arch/x86/include/asm/thread_info.h b/arch/x86/include/asm/thread_info.h index 89f794f..d9fbfa1 100644 --- a/arch/x86/include/asm/thread_info.h +++ b/arch/x86/include/asm/thread_info.h @@ -167,9 +167,11 @@ 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" : "=r" (sp));\ + sp; \ +}) /* how to get the thread information struct from C */ static inline struct thread_info *current_thread_info(void) -- 1.7.7.6 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: [PATCH 48/74] x86, lto: Use inline assembler instead of global register variable to get sp
>>> Andi Kleen 08/19/12 4:59 AM >>> >I verified this generates the same binary (on 64bit) as the original >register variable. This isn't very surprising given that the modified code is inside a CONFIG_X86_32 conditional (as ought to be obvious from the code using %%esp). Given that it's being used as operand to a binary &, the resulting code - if the compiler handles this only half way sensibly - can hardly be expected to be identical. >-register unsigned long current_stack_pointer asm("esp") __used; >+#define current_stack_pointer ({ \ >+unsigned long sp;\ >+asm("mov %%esp,%0" : "=r" (sp));\ >+sp;\ >+}) It would get closer to the original if you used "=g" (I noticed in a few earlier patches already that you like to use "=r" in places where a register is not strictly required, thus reducing the flexibility the compiler has). Also, given that this is more a workaround for a compiler deficiency, shouldn't this be conditional upon use of LTO? Jan -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: [PATCH 48/74] x86, lto: Use inline assembler instead of global register variable to get sp
On Sun, Aug 19, 2012 at 09:37:27AM +0100, Jan Beulich wrote: > >>> Andi Kleen 08/19/12 4:59 AM >>> > >I verified this generates the same binary (on 64bit) as the original > >register variable. > > This isn't very surprising given that the modified code is inside a > CONFIG_X86_32 conditional (as ought to be obvious from the code using > %%esp). Given that it's being used as operand to a binary &, the resulting > code - if the compiler handles this only half way sensibly - can hardly be > expected to be identical. Doh! Thanks. I'll double check. You're right it'll likely change code. But it shouldn't be common. > > >-register unsigned long current_stack_pointer asm("esp") __used; > >+#define current_stack_pointer ({ \ > >+unsigned long sp;\ > >+asm("mov %%esp,%0" : "=r" (sp));\ > >+sp;\ > >+}) > > It would get closer to the original if you used "=g" (I noticed in a few > earlier patches already that you like to use "=r" in places where a register > is not strictly required, thus reducing the flexibility the compiler has). My fingers have =r hardcoded. Will fix. > > Also, given that this is more a workaround for a compiler deficiency, > shouldn't this be conditional upon use of LTO? I think it's cleaner than the global reg var, so unconditional should be fine. It wouldn't surprise me if global reg causes trouble even without LTO, i probably just triggered some latent bug. -Andi -- a...@linux.intel.com -- Speaking for myself only. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/