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Re: Failure to dlopen libgomp due to static TLS data
- From: Rich Felker <dalias at libc dot org>
- To: "H.J. Lu" <hjl dot tools at gmail dot com>, Jakub Jelinek <jakub at redhat dot com>, Ulrich Weigand <uweigand at de dot ibm dot com>, GNU C Library <libc-alpha at sourceware dot org>, GCC Development <gcc at gcc dot gnu dot org>, Richard Henderson <rth at redhat dot com>
- Date: Thu, 12 Feb 2015 18:55:30 -0500
- Subject: Re: Failure to dlopen libgomp due to static TLS data
- Authentication-results: sourceware.org; auth=none
- References: <201502121519 dot t1CFJMAe018776 at d03av02 dot boulder dot ibm dot com> <20150212160959 dot GS23507 at brightrain dot aerifal dot cx> <20150212161145 dot GD1746 at tucnak dot redhat dot com> <CAMe9rOqk9quw-VQU_=+WwEaPuvdjtzbwdG3NQrA0EbJbwyZBnw at mail dot gmail dot com> <20150212170724 dot GW23507 at brightrain dot aerifal dot cx> <20150212234211 dot GQ4274 at bubble dot grove dot modra dot org>
On Fri, Feb 13, 2015 at 10:12:11AM +1030, Alan Modra wrote:
> 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 <firstname.lastname@example.org> 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.
Were you measuring static-allocated TLSDESC vs non-TLSDESC GD model?
That's the case where there should be a "big" difference, though I'm
still somewhat skeptical of the benefits in real-world usage cases.
I think Alexandre Oliva had a tool along with the original paper to
measure the performance, and I did some simple testing myself a while
back I could dig up the source for.