Re: Why not build kernel with -O3

From: Austin S Hemmelgarn
Date: Wed Apr 08 2015 - 08:06:16 EST

On 2015-04-07 21:00, Pengfei Yuan wrote:
Could you please provide some examples that I can investigate?

2015-04-08 2:05 GMT+08:00 Austin S Hemmelgarn <ahferroin7@xxxxxxxxx>:
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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