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Re: Default ISA/tuning flags for GCC, --enable-kernel= level for glibc

On Mon, Jan 26, 2009 at 11:15:01AM +0100, Jakub Jelinek wrote:
>Similarly, I wonder what CPUs we want to support in Fedora for PowerPC,
>if e.g. -march=power4 default wouldn't be possible.  On s390, is somebody

For ppc, the most common hardware would be the G4 PowerMacs.  Use of
-march=power4 would seem suboptimal there.  Which sort of sucks because
we run as much of the OS as we can at 32-bit, even on 64-bit hardware.
I'll look at this a bit more today and see what -march=power4 would really
do here in terms of compatibility.

For ppc64, using -march=power4 would be fine with me.  We don't really claim
to support the POWER3 or older 64-bit hardware, and POWER4 is fairly close
to the most common CPU (ppc970 in the G5 Mac/Powerstation).

>Another question is if we finally could bump the minimum supported kernel
>version in Fedora 11 glibc.  Currently Fedora 10 glibc requires 2.6.9 or
>later kernel, which means it can't assume a lot of stuff (e.g. private
>futexes, a lot of added syscalls etc.), which means it has to test for them
>at runtime of every program (e.g. every threaded program does a dummy futex
>syscall to check for private futexes, for futex realtime clock support and
>has to use variables for this, instead of oring constants in the mutexes
>etc., and glibc has to check for ENOSYS and have fallbacks compiled in).
>2.6.9 default comes I think still from the RHEL4 kernels.
>Could we bump this ideally to 2.6.29?  It would mean Fedora 11 userland
>only runs on 2.6.29 and later kernels, but we could gain speed and decrease
>size of glibc shared libraries.
>For the -march/-mcpu default changes, I think nothing is required from the
>koji build infrastructure (I don't think there are 31-bit mainframes
>around in koji/brew and I don't think there are 32-bit powerpcs or
>i386/i486s among the build boxes either), on the other side that decision
>has to be made almost immediately (before the world is rebuilt with gcc

I'm going to assmue that you are wanting a mass-rebuild of all of Fedora with
gcc 4.4.  Is that going to land this week?  From a scheduling perspective,
if a rebuild is going to happen it would be fairly important to have most of
it done by Beta.  Feature freeze is March 3rd.  Beta freeze is March 10.

>For glibc --enable-kernel= default this requires that all koji build boxes
>and anything else that needs to run Fedora 11 binaries runs at least 2.6.29
>kernels, but the change might be still delayed for a few weeks.

The koji build boxes all run RHEL 5.  Getting them upgraded to a not-yet-
released kernel seems unlikely.


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