linux-kernel - Re: [PATCH RFC 0/6] Implement per-processor data areas for i386.

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Date:	Sun, 27 Aug 2006 10:34:46 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Andreas Mohr <andi@...x01.fht-esslingen.de>
CC:	linux-kernel@...r.kernel.org,
	Chuck Ebbert <76306.1226@...puserve.com>,
	Zachary Amsden <zach@...are.com>,
	Jan Beulich <jbeulich@...ell.com>, Andi Kleen <ak@...e.de>,
	Andrew Morton <akpm@...l.org>
Subject: Re: [PATCH RFC 0/6] Implement per-processor data areas for i386.

Andreas Mohr wrote:
> This is interesting, since even by doing a non-elegant
> current->... --> struct task_struct *tsk = current;
> replacement for excessive uses of current, I was able to gain almost 10kB
> within a single file already!
> I guess it's due to having tried that on an older installation with gcc 3.2,
> which probably does less efficient opcode merging of current_thread_info()
> requests compared to a current gcc version.
> IOW, .text segment reduction could be quite a bit higher for older gcc:s.
>   

That doesn't sound likely.  current_thread_info() is only about 2 
instructions.  Are you looking at the .text segment size, or the actual 
file size?  The latter can be very misleading in the presence of debug info.

>> This uses the x86 segmentation stuff in a way similar to NPTL's way of
>> implementing Thread-Local Storage.  It relies on the fact that each CPU
>> has its own Global Descriptor Table (GDT), which is basically an array
>> of base-length pairs (with some extra stuff).  When a segment register
>> is loaded with a descriptor (approximately, an index in the GDT), and
>> you use that segment register for memory access, the address has the
>> base added to it, and the resulting address is used.
>>     
>
> Not a problem for more daring user-space apps (i.e. Wine), I hope?
>   

No.  The LDT is still available for userspace use.

    J
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