linux-kernel - Re: -Os versus -O2

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Date:	Mon, 25 Jun 2007 09:35:33 +0200
From:	Segher Boessenkool <segher@...nel.crashing.org>
To:	david@...g.hm
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

>> 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.

It bases its optimisation decisions on the options you give
it, the profile feedback information you either or not gave
it, and a whole bunch of heuristics.

> 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.

For optimal performance, you need to fine-tune options yes,
per file (or per function even!)

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

I speak only for myself.  However this is the only way it _can_
be, the compiler isn't clairvoyant.  Some of the heuristics sure
could use some tuning, but they stay heuristics.


Segher

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