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

[Andreas Mohr]

Hi,

On Mon, Jun 12, 2006 at 01:14:34PM -0400, Chuck Ebbert 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*.
As such whatever optimization we can gain (e.g. the compiler merging
multiple current_thread_info() invocations) is very worthwhile.
I've come up with this C variant recently, too, but I didn't think
it would make too much of a difference, however it seems it's really
useful when going towards newer gcc versions. 

Andreas Mohr

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