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SubjectRe: [2.6 patch] kernel/sched*: optimize inlining
On Sun, May 11, 2008 at 12:21:32PM +0300, Adrian Bunk wrote:
> kernel/sched* contained tons of inline's, and the result of removing
> them all is impressing (with x86_64_defconfig)
> text data bss dec hex filename
> 39557 8234 280 48071 bbc7 kernel/sched.o
> 41792 8234 280 50306 c482 kernel/sched.o.old
>
> That's a 5.3% text size reduction (!), which is more than twice as much
> as the 2.3% the "optimized inlining" achieves on average for the whole
> kernel.

If we compare the size of sched.o in the three cases we see a clear effect:

text data bss dec hex filename
forced inline: 31257 2702 200 34159 856f kernel/sched.o
inline hint: 31105 2702 200 34007 84d7 kernel/sched.o
no inline (hint): 30704 2702 200 33606 8346 kernel/sched.o

The last line "no inline(hint)" is with Adrians patch applied.
So what is obvious from the above is that with the arch/gcc combination
I use here the inline hint has a clear effect and gcc inlines more
when we have given it a hint to do so than without the hint.
I conclude this solely on the cide size change between the line
"inline hint" and "no inline(hint)".

With adrians patch there were no difference in size with or
without the OPTIMIZE_INLINING enabled.

Or in other words the config option "OPTIMIZE_INLINING" is NOT
equal to removing all the inline annotations.

>
> Note that any remarks regarding whether this patch might affect the
> performance are invalid since noone cared about the performance when
> the "x86: add optimized inlining" commit that does the same for the
> whole kernel entered the tree.

In one case it was an option it was easy to turn off/on so we could
compare and modulus bugs it was a noop on gcc < 4.0.
With the patch below we revet back to the broken gcc inline algorithm on
gcc < 4.0 and it cannot as easy be turned of (have to revert this patch).
Both issues are worth to consider before applying this.

Sam


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