[cfe-dev] [RFC] Possible ABI break for libc++ built with -fno-rtti and -D_LIBCPP_BUILD_STATIC

[Eric Fiselier via cfe-dev]

Hi All,

Libc++ currently has an ABI incompatibility when built as a static library
with no RTTI. Specifically some std::shared_ptr internals have different
vtable layouts in this configuration.

I would like to fix this by making the vtable definition consistent across
all configurations, but that requires breaking the ABI of a very rare
configuration of libc++. I suspect all users of this configuration can take
the ABI break immediately, but I would like to confirm this before
proceeding.

The configuration in question is a static libc++ library built with
-DLIBCXX_ENABLE_RTTI=OFF and -D_LIBCPP_BUILD_STATIC,

Is anybody using this configuration and can't take an ABI break?

Note that `_LIBCPP_BUILD_STATIC` must be manually defined by the user at
both when building and using libc++, and it is not supported by the CMake
build system. This should mean that almost nobody is defining it.

The proposed changes can be found here: https://reviews.llvm.org/D32838

/Eric
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