Re: [patch] i386: use C code for current_thread_info()

[Chuck Ebbert <76306.1226@compuserve.com>]

In-Reply-To: <20060612184833.GA29177@rhlx01.fht-esslingen.de>

On Mon, 12 Jun 2006 20:48:33 +0200, Andreas Mohr wrote:

> > Kernel code starts out ~30K bytes smaller with gcc 4.1 and using C
> > for current_thread_info() helps even more than with 4.0.  Nice...
> 
> Especially since current_thread_info() often has an AGI stall (read:
> severe pipeline stall) since it often cannot properly intermingle
> with nearby opcodes due to lack of suitable ones, e.g. at a
> function prologue.
> mov    $0xffffe000,%eax
> and    %esp,%eax
> are fundamentally incompatible due to having to wait for the address
> generation before the "and" can be executed.
> This shows up during profiling quite noticeably (IIRC 8 hits vs. 1 to 2
> hits on other places), which really hurts since this function is used
> basically *everywhere*.

Hmmm.  The compiler does it this way:

  mov    %esp,%eax
  and    $0xffffe000,%eax

which could be faster because esp can be moved to eax while the mask
is being fetched.

-- 
Chuck