Hi,

A while back I filed a feature request for a build option to use the libc++ runtime, with a mostly PoC patch attached.
Such a feature would increase GCC's usability on platforms like Apple's OS for Mac, but possibly on FreeBSD too. (And if it's technically possible to use Microsoft's runtime that could become another option down the road.)

I've since had the time to test this feature more thoroughly, and have begun bringing it into a form where it might be considerable for inclusion. I'm attaching a first draft, including only the changes to the driver but none of the required changes to the build system yet. I'm doing this in my local 7.2.0 build, so the patch is against that source, for now. I'm hoping the modified files evolve little enough that the patch also applies to the current development branch, and that it's thus useful enough to get some constructive feedback telling me I'm on the right path here.

I'd also really like to rally some "coworkers" on this, esp. if they're familiar with the (rather daunting) configure-and-build system and the procedure of "getting a new feature in"!

About the attached patch and approach:

I've copied the clang option syntax for selecting the C++ runtime: `-stdlib=libstdc++` or `-stdlib=libc++`, this is to maximise compiler compatibility. I think the compiler should be configured to use the platform runtime by default, i.e. libc++ on Mac OS X 10.9 and later. Currently this is hardwired with #ifdefs but that should probably become a configure setting.

The C++ runtime headerfiles for libc++ will have to found in the appropriate location, which will need to be configurable as on Mac they typically live inside the Xcode app bundle which is freely relocatable.

There is a catch with using libc++: older versions don't contain the sized C++14 new/delete operators (those were provided implicitly by clang). The operators are there in the libc++ from LLVM 3.9 and newer, but it appears that means that the system libc++ only contains them in Mac OS 10.12 and newer. It is of course possible to install a newer version, link to it and run it via DYLD_INSERT_LIBRARIES . The other option that appears to work is to use libsupc++ as the provider for those operators by including it as the last link library, in line with the procedure on llvm.org how to use libc++ on Linux. My patch currently adds libsupc++ on 10.11 and earlier; this should also be done more properly (though I presume libsupc++ will only be used if it provides symbols not already provided by the libraries linked before it).

Thanks,
René Bertin