linux-kernel - Re: -Os versus -O2

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Date:	Mon, 25 Jun 2007 00:13:46 -0700 (PDT)
From:	david@...g.hm
To:	Segher Boessenkool <segher@...nel.crashing.org>
cc:	Benjamin LaHaise <bcrl@...ck.org>, linux-kernel@...r.kernel.org,
	Arjan van de Ven <arjan@...radead.org>,
	Adrian Bunk <bunk@...sta.de>,
	Oleg Verych <olecom@...wer.upol.cz>, rae l <crquan@...il.com>
Subject: Re: -Os versus -O2

On Mon, 25 Jun 2007, Segher Boessenkool wrote:

>>  then do we need a new option 'optimize for best overall performance' that
>>  goes for size (and the corresponding wins there) most of the time, but is
>>  ignored where it makes a huge difference?
>
> That's -Os mostly.  Some awful CPUs really need higher
> loop/label/function alignment though to get any
> performance; you could add -falign-xxx options for those.
>
>>  in reality this was a flaw in gcc that on modern CPU's with the larger
>>  difference between CPU speed and memory speed it still preferred to unroll
>>  loops (eating more memory and blowing out the cpu cache) when it shouldn't
>>  have.
>
> You told it to unroll loops, so it did.  No flaw.  If you
> feel the optimisations enabled by -O2 should depend on the
> CPU tuning selected, please file a PR.
>
> Also note that whether or not it is profitable to unroll
> a particular loop depends largely on how "hot" that loop
> is, and GCC doesn't know much about that if you don't feed
> it profiling information (it can guess a bit, sure, but it
> can guess wrong too).

actually, what you are saying is that the compiler can't know enough to 
figure out how to optimize for speed. it will just do what you tell it to, 
either unroll loops or not.

this argues that both O2 and Os are incorrect for a project to use and 
instead the project needs to make it's own decisions on this.

if this is the true feeling of the gcc team I'm very disappointed, it 
feels like a huge step backwards.

David Lang
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