[cfe-dev] A proposal for ABI migrations in libc++

[Peter Collingbourne]

Hi,

These thoughts are motivated by my rejected std::deque patch.

In general I don't think we have a good mechanism for making
ABI-breaking changes in libc++.  The existing mechanism is to
increment _LIBCPP_ABI_VERSION and make all ABI-breaking changes in
the same commit along with the increment.  This makes it difficult for
ABI-breaking changes to be developed incrementally, as there is no good
place to keep the changes until the ABI-breaking megapatch; complicates
debugging, as it would be difficult to bisect an ABI-breaking megapatch;
and fails to accommodate implementors who are not constrained by ABI
compatibility requirements.

The proposal I have in mind is to introduce a macro, _LIBCPP_ABI_NEXT,
which all ABI-breaking changes are made conditional on.  Defining this
macro will cause _LIBCPP_ABI_VERSION to be defined to some arbitrary
non-numeric value, such as "next" (without quotes).  The lib/buildit
and test/testit scripts are modified so that the library can be built
and tested with or without _LIBCPP_ABI_NEXT defined.  Any implementor
who does not have ABI compatibility requirements for libc++ may compile
the library with _LIBCPP_ABI_NEXT defined, but the implementor must
ensure that the compiler defines this macro in every C++ translation
unit.

When we are ready to make an ABI transition, the value of
_LIBCPP_ABI_VERSION is incremented and every place in the source code
which is conditional on the definition of _LIBCPP_ABI_NEXT is updated
as if the macro were defined.

Thoughts?

Thanks,
-- 
Peter