linux-kernel - Re: Why not build kernel with -O3

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Date:	Wed, 08 Apr 2015 08:06:07 -0400
From:	Austin S Hemmelgarn <ahferroin7@...il.com>
To:	Pengfei Yuan <0xcoolypf@...il.com>
CC:	Mike Galbraith <umgwanakikbuti@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: Why not build kernel with -O3

On 2015-04-07 21:00, Pengfei Yuan wrote:
> Could you please provide some examples that I can investigate?
> Thanks!
>
> 2015-04-08 2:05 GMT+08:00 Austin S Hemmelgarn <ahferroin7@...il.com>:
>> On 2015-04-07 06:09, Mike Galbraith wrote:
>>> On Tue, 2015-04-07 at 15:56 +0800, Pengfei Yuan wrote:
>>>> I am trying legacy GCC versions.
>>>> But I am not able to try different architectures.
>>>
>>> The point of my reply wasn't to get you to actually test the world ;-)
>>>
>>> I was indirectly pointing out that "works for me" is not good enough
>>> justification.  Much checking for safety/benefit required.
>>>
>> Safety especially, -O3 is known to cause perfectly standards-compliant
>> code to break in weird ways in user-space.
>>
>>
I can't remember any off the top of my head, but it does say explicitly 
in the GCC manual to be careful with -O3.  IIRC, most of the issues 
relate to -O3 enabling -ffast-math (which tends to really mess with code 
that expects strict IEEE 754 compliance), so it may not be as much of an 
issue for kernel code.  You might look into some of the projects that 
use -O3 by default (I think most of the Mozilla so0ftware does these 
days, and I know that there are others, I just can't remember what right 
now).


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