Re: gcc and the kernel

["Steven Bosscher" <stevenb dot gcc at gmail dot com>]

On Mon, Dec 29, 2008 at 9:22 PM, Brian O'Mahoney <omb@bluewin.ch> wrote:
> OK, there are a few more things to say about gcc, and the kernel, and
> distributions (also the way patches to both work, to both).
>
> Stipulations: (a) I always try to run the current stable kernel, released by
> the 'stable-team' led by GKH. But he is getting too good, stable is
> 2.6.27.10 but I am running 2.6.27.7 and 2.6.28 is too bleeding edge.
> (b) I am running a very heavily patched SuSE 11.0 but I have the 'SuSE
> build factory' in my Yast list so for lots of things I am actually executing
> SuSE 11.2 alpha code. My system gcc is:
>
> gcc -v
> Using built-in specs.
> Target: x86_64-suse-linux
> Configured
> with: ../configure --prefix=/usr --with-local-prefix=/usr/local --infodir=/usr/share/info --mandir=/usr/share/man --libdir=/usr/lib64 --libexecdir=/usr/lib64 --enable-languages=c,c++,objc,fortran,obj-c++,java,ada --enable-checking=release --with-gxx-include-dir=/usr/include/c++/4.3 --enable-ssp --disable-libssp --with-bugurl=http://bugs.opensuse.org/ --with-pkgversion='SUSE
> Linux' --disable-libgcj --with-slibdir=/lib64 --with-system-zlib --enable-__cxa_atexit --enable-libstdcxx-allocator=new --disable-libstdcxx-pch --program-suffix=-4.3 --enable-version-specific-runtime-libs --enable-linux-futex --without-system-libunwind --with-cpu=generic --build=x86_64-suse-linux
> Thread model: posix
> gcc version 4.3.2 20080715 (prerelease) [gcc-4_3-branch revision 137837]
>  (SUSE Linux)
>
> ie not to shabby, but __NOT__ too new either, and note it is a SuSE special:
> 4.3.2 20080715 (prerelease) [gcc-4_3-branch revision 137837], and you would
> need to look at the (s)rpm in the 11.0 source repository to see what they
> changed, ... but also see below.
>
> But I happen, right now, to need the VERY latest gcc,
> so /opt/gcc-4.4-20081219/bin/gcc is a recent snapshot and I can select it
> either in my PATH or with 'export GCC=/opt/gcc-4.4-20081219/bin/gcc' etc
>
> SO
>
> (1) The kernel dosn't use the latest gcc, and even quite old gcc's compile the
> kernel
>
> (2) Kernel space code is not the same, ie the same (binary)API as user space
> and gets interrupts, so some user space optimizations are always turned off
> in the kernel, grep lkml/google for Tovolds+alias to read an old flame-war.
>
> (3) if a gcc compiles 2.6.x it will normally compile 2.6.x.y ie the patch
> releases.
>
> (4) I think there is someone on the Debian integration team who tries to build
> the latest kernels with the latest gcc, he finds lots of problems and submits
> bugs to both camps. I dont envy him.
>
> Last, a word to the wise, compiler developers, are by nature fairly agressive
> but, unless you want to work on gcc itself, it is wise to stay a bit behind
> the bleeding edge, and, unless your systems are excellently backed up,
> ___DONT_BUILD_THE_KERNEL___ with and untested gcc. I mean a kernel you are
> going to try to run. For an example of why not see the 'ix68 string
> direction' fiasco noting that an interrupt can happen anywhere and that the
> CPU x flag is a non-deterministic (to the interrupt code) part of the
> interrupted context. Oh what fun!
>
> [snip from LWN.NET]
>
> "Since version 4.3, gcc changed its behaviour concerning the x86/x86-64
> ABI and the direction flag, that is it now assumes that the direction
> flag is cleared at the entry of a function and it doesn't clear once
> more if needed.
>
> This causes some problems with the Linux kernel which does not clear
> the direction flag when entering a signal handler. The small code below
> (for x86-64) demonstrates that. ..." If you dont know see www.lwn.net.
>
> --
> Greetings, Brian
>

So, after all these words (congrats on your nice system), what did you
want to say?

Gr.
Steven