linux-kernel - Re: -Os versus -O2

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Message-Id: <d66e01c9d04540d058a93d5b70421985@kernel.crashing.org>
Date:	Mon, 25 Jun 2007 09:03:35 +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

> 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).

Segher

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