[libcxx] r215740 - Revert "Turn off extern templates for most uses."

Eric Fiselier eric at efcs.ca
Sun Aug 17 19:30:18 PDT 2014

> FWIW, I don't think that MSan was *ever* intended to not have false
positives with an uninstrumented standard library. So I really don't
understand why this is an interesting thing to dig into.

That is new information to me so I'll have to take that into consideration.
What I was trying to avoid was breaking MSAN usability for end users of
Since its unlikely that they have a instrumented standard library it would
be nice if their system libc++ didn't always cause the first MSAN failure.

Since __attribute__((__always_inline__)) seems to cause a lot of these
failures I imagine it is possible to reduce the FP's without removing the
extern template declarations.
In that case it might still be work putting time into.


On Sun, Aug 17, 2014 at 8:24 PM, Howard Hinnant <howard.hinnant at gmail.com>

> On Aug 17, 2014, at 9:26 PM, Justin Bogner <mail at justinbogner.com> wrote:
> > Howard Hinnant <howard.hinnant at gmail.com> writes:
> >> On Aug 17, 2014, at 9:06 PM, Justin Bogner <mail at justinbogner.com>
> wrote:
> >>
> >>> I really don't think it's worth the cost of insantiating these very
> >>> fundamental templates in *every single user* to work around a
> limitation
> >>> in the memory sanitizer. This is an unreasonable amount of overhead for
> >>> standard library types.
> >>
> >> Always measure.  I’m not saying you’re wrong.  I’m saying you’re
> >> stating a performance conclusion without measurements (which should
> >> never be acceptable).
> >
> > I did measure :) Though, I sent it to llvm-dev and it probably should've
> > been cfe-dev. Sorry about that.
> >
> > http://lists.cs.uiuc.edu/pipermail/llvmdev/2014-August/075793.html
> Ah, I have not been monitoring llvm-dev.  Thank you for the link.
> Howard
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