linux-kernel - Re: -Os versus -O2

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Message-Id: <513fca4651d60bae8fe6a567c665893c@kernel.crashing.org>
Date:	Mon, 25 Jun 2007 09:41:50 +0200
From:	Segher Boessenkool <segher@...nel.crashing.org>
To:	david@...g.hm
Cc:	Benjamin LaHaise <bcrl@...ck.org>, linux-kernel@...r.kernel.org,
	Willy Tarreau <w@....eu>,
	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

>> -Os is "as fast as you can without bloating the code size",
>> so that is the expected result for CPUs that don't need
>> special hand-holding around certain performance pitfalls.
>
> this sounds like you are saying that people wanting performance should 
> pick -Os.

That is true on most CPUs.  Some CPUs really really need
some of things that -Os disables (compared to -O2) for
decent performance though (branch target alignment...)

> what should people pick who care more about code size then anything 
> else? (examples being embedded development where you may be willing to 
> sacrafice speed to avoid having to add additional chips to the design)

-Os and tune some options.  There is extensive work being
done over the last few years to make GCC more suitable for
embedded targets btw.  But the -O1/-O2/-O3/-Os gives you
four choices only, it's really not so hard to understand
I hope that for more specific goals you need to add more
specific options?


Segher

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