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Re: Failure to dlopen libgomp due to static TLS data


On Thu, Feb 12, 2015 at 12:07:24PM -0500, Rich Felker wrote:
> On Thu, Feb 12, 2015 at 08:56:26AM -0800, H.J. Lu wrote:
> > On Thu, Feb 12, 2015 at 8:11 AM, Jakub Jelinek <jakub@redhat.com> wrote:
> > > On Thu, Feb 12, 2015 at 11:09:59AM -0500, Rich Felker wrote:
> > >> On Thu, Feb 12, 2015 at 04:18:57PM +0100, Ulrich Weigand wrote:
> > >> > Hello,
> > >> >
> > >> > we're running into a problem related to use of initial-exec access to
> > >> > TLS variables in dynamically-loaded libraries.  Now, in general, this
> > >> > is actually not supported.  However, there seems to an "inofficial"
> > >> > extension that allows selected system libraries to use small amounts
> > >> > of static TLS space to allow critical variables to be defined to use
> > >> > the initial-exec model even in dynamically-loaded libraries.
> > >>
> > >> This usage is supposed to be deprecated. Why isn't libgomp using
> > >> TLSDESC/gnu2 model?
> > >
> > > Because it is significantly slower.
> > 
> > And TLSDESC/gnu2 model isn't implemented for x32.
> > There are no tests for TLSDESC/gnu2 model in glibc.
> > I have no ideas if it works in glibc master on x86-32 or
> > x86-64 today.
> 
> Then fixing this should be a priority, IMO. Broken libraries using IE
> model "for performance" are a problem that's not going to go away
> until TLSDESC gets properly adopted.

I posted support for TLSDESC on powerpc back in 2009 (search for
powerpc _tls_get_addr call optimization).  The patch wasn't reviewed,
and I didn't push it because my benchmark tests didn't show a much of
a gain.  Quite possibly I wasn't using the right benchmark.

-- 
Alan Modra
Australia Development Lab, IBM


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