Re: gcc inlining heuristics was Re: [PATCH -v7][RFC]: mutex: implementadaptive spinning

[Bernd Schmidt]

Andi Kleen wrote:
> On Sun, Jan 11, 2009 at 04:21:03PM -0800, Linus Torvalds wrote:
>>
>> On Mon, 12 Jan 2009, Andi Kleen wrote:
>>> so at least least for this case it works. Your case also doesn't work 
>>> for me. So it looks like gcc didn't like something you did in your test 
>>> program.
>> I very intentionally used _different_ types.
>>
>> If you use the same type, gcc will apparenrly happily say "hey, I can 
>> combine two variables of the same type with different liveness into the 
>> same variable".
> 
> Confirmed.
> 
>> But that's not the interesting case.
> 
> Weird. I wonder where this strange restriction comes from.

Something at the back of my mind said "aliasing".

$ gcc linus.c -O2 -S ; grep subl linus.s
        subl    $1624, %esp
$ gcc linus.c -O2 -S -fno-strict-aliasing; grep subl linus.s
        subl    $824, %esp

That's with 4.3.2.


Bernd
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