[cfe-dev] __thread keyword, LLVM 3.2 & Xcode 4.6

[Matthieu Monrocq]

On Wed, Nov 7, 2012 at 12:47 AM, Seth Cantrell <seth.cantrell at gmail.com>wrote:

> On Nov 6, 2012, at 4:04 AM, Jean-Daniel Dupas <devlists at shadowlab.org>
> wrote:
>
> >
> > Le 6 nov. 2012 à 01:37, James Gregurich <bayoubengal at me.com> a écrit :
> >
> >> hi.
> >>
> >> I just updated to Xcode 4.6.  I note the following:
> >>
> >>
> >> $
> /Applications/Xcode46-DP1.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang
> --version
> >> Apple clang version 4.2 (tags/Apple/clang-424.0.11) (based on LLVM
> 3.2svn)
> >> Target: x86_64-apple-darwin12.2.0
> >> Thread model: posix
> >>
> >>
> >> It is my understanding from the release notes, that LLVM 3.2 is support
> thread-local storage. I just re-ran my test using the '__thread' keyword
> from the last time I asked about this and I still just get one instance of
> the object rather than one-per-thread.
> >
> >
> > The __thread keyword is a C extension (it not part of the standard).
> Using it with C++ as is even less specified than using it with C.
> >
> > Moreover, it has already be specified in the previous discussion that
> supporting C++ TLS required OS support. Updating Xcode does not change that.
> >
>
> gcc 4.8 now implements thread_local with a performance penalty for global
> thread_local variables: http://gcc.gnu.org/gcc-4.8/changes.html#cxx
>
> I guess that function-local thread_local variables can use the same scheme
> for initialization as function-local static variables



I would be very interested to know what this "penalty" is. I have a couple
idea of what it *could* be, but no idea about what it really is.

-- Matthieu
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/cfe-dev/attachments/20121107/f2430b2e/attachment.html>