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

[Marcin Cieslak <saper@xxxxxxxxxx>]

>> 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
   



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