[gentoo-amd64] Meaning of gcc64 USE flag

[gentoo-amd64] Meaning of gcc64 USE flag

[Robert Cernansky]

Hi all,

sys-kernel/linux-headers have gcc64 USE flag. Its desctiption says:
"Use 64-bit kernel compiler". It's not quite clear for me what it
means.

Can anybody explain more? What are the pros and cons to switch it on/off?

Robert


-- 
Robert Cernansky
E-mail: hslists2@zoznam.sk
Jabber: HS@jabber.sk

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gentoo-amd64@gentoo.org mailing list

Re: [gentoo-amd64] Meaning of gcc64 USE flag

[Simon Stelling]

Robert Cernansky wrote:
> Can anybody explain more? What are the pros and cons to switch it on/off?

The USE flag is only effective on ppc64 and sparc where the kernel 
sometimes is the only code that is compiled 64bit and therefore a 
seperate, much smaller version of gcc is used to build the kernel. The 
flag should in fact be masked everywhere except for those profiles, 
which is what bug 144535 [1] deals with.

[1] http://bugs.gentoo.org/show_bug.cgi?id=144535

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Kind Regards,

Simon Stelling
Gentoo/AMD64 Developer
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gentoo-amd64@gentoo.org mailing list

Re: [gentoo-amd64] Meaning of gcc64 USE flag

[Peter Davoust]

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I may be stating the obvious, depending on what you know, but as far as I
understand it goes something like this:

Info goes to the cpu of a 32 bit machine in chunks of, you guessed it: 32
bits. Obviously with 64 bit machines it's 64 bit chunks, but the interesting
thing is that many if not all 64 bit chips can recognize when they get
chunks of 32 bit code, and emulate. I think that the gcc64 use flag means
that you will compile your kernel in 64 bit code. That may mean that you
compiled your kernel with a 32 bit compiler before. Now considering you're
on the gentoo-amd64 forum, I assume you have x86_64 gentoo and that your
arch is set to x86_64, so the use flag seems a bit redundant to me. I'm
probably wrong, I have no experience with this, it's late, I'm tired.
Please, someone correct me if I'm just being an idiot, or if that's not the
question you asked.

In the unlikely event that I'm right, here would be some pro's and cons. You
may not be fully unlocking the potential of your shiny new 64 bit machine by
using a 32 bit kernel. The pro would be that you may (not sure, depends on
what your kernel is before you recompile) be able to use 32 bit software.
Many closed source programs and some programs that emulate the closed source
stuff are in 32 bit, and have trouble in 64. I think you'd only run into
trouble with java and flash using 64 bit kernel. Adobe still doesn't offer
64 bit flash to my knowledge, and java is a bit of a pain in any flavor.

Again, I may be just babbling, and if I spent this e-mail regurgitating
things that you already know, I apologize. That was my interpretation, hope
it helped.

-Peter


On 12/27/06, Simon Stelling <blubb@gentoo.org> wrote:
>
> Robert Cernansky wrote:
> > Can anybody explain more? What are the pros and cons to switch it
> on/off?
>
> The USE flag is only effective on ppc64 and sparc where the kernel
> sometimes is the only code that is compiled 64bit and therefore a
> seperate, much smaller version of gcc is used to build the kernel. The
> flag should in fact be masked everywhere except for those profiles,
> which is what bug 144535 [1] deals with.
>
> [1] http://bugs.gentoo.org/show_bug.cgi?id=144535
>
> --
> Kind Regards,
>
> Simon Stelling
> Gentoo/AMD64 Developer
> --
> gentoo-amd64@gentoo.org mailing list
>
>

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