Re: [RFC] add ubuntu patch which adds atomic builtin support to gcc 4.3.[3 but also other microversions]

[Robert Schuster <thebohemian@gmx.net>]

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Hi,
first of all. As Stefan pointed out the change was needed while working
for Buglabs on OpenJDK support. So it was decided that the introduction
of the mentioned GCC change is handled by Stefan Schmidt now.

But I would like to elaborate on the side-effects of the patch.

Am 16.06.2010 19:12, schrieb Khem Raj:
>
> Those applications are working fine and they will keep working fine as
> long as you dont recompile them
> and you make sure that libgcc is updated then whenever you rebuild an
> application that uses the atomic
> builtins will start using them until then it should work without using them.
>   
Yes, already built and shipped applications are not affected at all.
It is only that when certain packages are built again with the new GCC,
the package's binary
will be different although PR (and PV) did not change. The reason is
that most configure scripts check
for the availability of the GCC intrinsics. If they are found they make
use of it, if not they either use a different implementation
(libatomic-ops for example)
or bring their own (software running on ARM Linux 2.6 can assume that
the neccessary kernel helpers exist and make use of them[0]) or silently
disable
multithread support (that is the case for llvm 2.7).

And already compiled software will also not magically pick up the
implementation found in libgcc because:
1) it was not there before
2) I am under the impression that at least some of the builtins are
actually macros and thus bits of machinecode are copied into each binary
using them.

However as said. Stefan is now dealing with the issue. He has gotten two
patches (one from Ubuntu, another one from the GCC 4.4 stable branch)
that we needed to get things going and is most likely checking the
effects they have.

Regards,
Robert

[0] - code like this can be found in many projects; search for
__kernel_dmb and __kernel_cmpxchg:
http://code.google.com/p/nativeclient/source/browse/trunk/src/native_client/src/include/linux/arm/atomic_ops.h?r=857
> Maybe I am missing something it would be helpful if Robert could
> explain potential scenarios
> where it would break precompiled binaries. I don't see it.
>
>   
>> Graeme
>>
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