[LLVMdev] Explicit template instantiations in libc++

[Bob Wilson]

> On Aug 12, 2014, at 2:17 PM, Justin Bogner <mail at justinbogner.com> wrote:
> 
> Most of libc++ doesn't have explicit template instantiations, which
> leads to a pretty significant build time and code size cost when using
> libc++, since a large number of common templates will be emitted by the
> compiler and coalesced by the linker. Notably, in include/__config, we
> have:
> 
>    #ifndef _LIBCPP_EXTERN_TEMPLATE
>    #define _LIBCPP_EXTERN_TEMPLATE(...)
>    #endif
> 
> whereas before r189601 this was:

I think you mean r189610

> 
>    #define _LIBCPP_EXTERN_TEMPLATE(...) extern template __VA_ARGS__;
> 
> This was apparently done to fix http://llvm.org/bugs/show_bug.cgi?id=17027,
> but disabling explicit instantiations seems like a pretty big hammer
> considering the drawbacks.
> 
> I'd like to restore these instantiations. Any thoughts on how to handle
> things like pr17027 in a less heavy handed way?

As far as I can tell, the issue in pr17027 was that we had a bug in libc++ and the libc++ dylib on some versions of OS X shipped with that bug. Disabling extern templates “fixed” that by causing the newer code from the headers to be used instead of the older code in the dylib on the system. The real solution here is to fix the bug in the dylib and ship a newer version. There may still be a lag in the time it takes us to release fixes in the dylib, but that is just the way it goes.