[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [Xen-devel] [PATCH v2] x86/current: Provide additional information to optimise get_cpu_info()


  • To: xen-devel@xxxxxxxxxxxxxxxxxxx
  • From: Marcin Cieslak <saper@xxxxxxxxxx>
  • Date: Sat, 13 Sep 2014 16:10:59 +0000 (UTC)
  • Delivery-date: Sat, 13 Sep 2014 16:15:35 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xen.org>

>> Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
> Exactly as with c/s d55c5eefe "x86: use compiler visible "add" instead of
> inline assembly "or" in get_cpu_info()", this is achieved by providing more
> information to the compiler.
>
> This causes a net drop of almost 4K of .text
>
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> CC: Jan Beulich <JBeulich@xxxxxxxx>
>
> ---
> v2: Less speculation about generated code in the comment
> ---
>  xen/include/asm-x86/current.h |    6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/xen/include/asm-x86/current.h b/xen/include/asm-x86/current.h
> index 2081015..b95fd79 100644
> --- a/xen/include/asm-x86/current.h
> +++ b/xen/include/asm-x86/current.h
> @@ -25,9 +25,9 @@ struct cpu_info {
>  
>  static inline struct cpu_info *get_cpu_info(void)
>  {
> -    unsigned long tos;
> -    __asm__ ( "and %%rsp,%0" : "=r" (tos) : "0" (~(STACK_SIZE-1)) );
> -    return (struct cpu_info *)(tos + STACK_SIZE) - 1;
> +    register unsigned long sp asm("rsp");
> +
> +    return (struct cpu_info *)((sp & ~(STACK_SIZE-1)) + STACK_SIZE) - 1;
>  }

Hello, it seems to me that the above code fails on me with clang 3.4
on FreeBSD-CURRENT:

xen/include/asm/current.h:30:33:
        error: variable 'sp' is uninitialized when used here 
[-Werror,-Wuninitialized]

Reverting df0ae94fd56d5f9c64089364efecb1793442360b helps.

There is a workaround suggested in 
http://llvm.org/devmtg/2014-02/slides/moller-llvmlinux.pdf:

diff --git a/xen/include/asm-x86/current.h b/xen/include/asm-x86/current.h
index b95fd79..e133d9d 100644
--- a/xen/include/asm-x86/current.h
+++ b/xen/include/asm-x86/current.h
@@ -26,6 +26,7 @@ struct cpu_info {
 static inline struct cpu_info *get_cpu_info(void)
 {
     register unsigned long sp asm("rsp");
+    asm("" : "=r" (sp));
 
     return (struct cpu_info *)((sp & ~(STACK_SIZE-1)) + STACK_SIZE) - 1;
 }

That silences the warning (not sure it it works).

//Marcin
   



_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel


 


Rackspace

Lists.xenproject.org is hosted with RackSpace, monitoring our
servers 24x7x365 and backed by RackSpace's Fanatical Support®.