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 <ja...@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.
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. Rich
