[cfe-dev] Better support for statically linking libc++/libc++abi

[Nico Weber via cfe-dev]

Hi,

Chrome for Android switched to libc++ in version 45, which launched a few
weeks ago. This went well, thanks for everyone working on libc++ for the
great work :-) (If anyone is curious about the issues we had during the
transition I'm happy to expand on that, but most of it was fairly boring –
tests relying on the iteration order of hash_set etc.)

One feature request of sorts: Chrome for Android statically links libc++
(except for ASan builds, we link dynamically in that case). One side effect
of the switch is that the chrome binary grew quite a few public symbols
from the libc++ headers. Chrome is built with -fvisibility=hidden, but a
bunch of places in libc++ contain explicit visiblity("default")
annotations. These make a lot of sense for shared-library builds of libc++,
but for static library builds it makes less sense.

It looks like it might be possible to work around this by
defining _LIBCPP_FUNC_VIS, _LIBCPP_TYPE_VIS, _LIBCPP_EXCEPTION_ABI to
nothing, but that seems a bit hacky. (I also don't know if the prebuilt
libc++.a in the Android NDK was built like that, but that's probably
off-topic for this list :-) Also, those symbols can be suppressed via
-Wl,--exclude-libs=libc++_static.a anyhow).

In the same vein: Symbols in libc++abi have the same issue, but there's not
even a workaround for it there as far as I can tell – there are a bunch of
literal `__visibility__("default")` and `#pragma GCC visibility
push(default)`s scattered around. (We don't currently use libc++abi in
chrome/android – we use libgcc and libatomic for now, and we still build
with gcc.)

My requests:
1. Would it be possible to put the visibility annotations in libcxxabi
behind some macro, so that projects can choose to get private visibility
for everything?
2. Is defining the three macros I mentioned above a good way to force
symbols to not have public visibility in libc++, or should projects not
rely on that? In the latter case, could there be an explicit, supported
toggle for this too?

Thanks,
Nico
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