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

[Martin J. O'Riordan via cfe-dev]

Hi Eric,

 

In the email I sent on the 27th April (“[LibC++] Bug in implementation of 'std::shared_ptr'”), the problem I was seeing was:

 

  o  LibC++ library built with -DLIBCXX_ENABLE_RTTI=ON and -D_LIBCPP_BUILD_STATIC.

  o  User code compiled with ‘-fno-rtti -D_LIBCPP_BUILD_STATIC’.

 

Having ‘_LIBCPP_BUILD_STATIC’ defined by both the library builder and the library user was where it broke.  I removed ‘-D_LIBCPP_BUILD_STATIC’ from the user and it worked.  But I wasn’t building the static library with RTTI disabled because it is stated as an unsupported configuration.

 

So are you advising that with the proposed fix, that I should revert to defining ‘_LIBCPP_BUILD_STATIC’ as the user?

 

Thanks,

 

            MartinO

 

From: Eric Fiselier [mailto:eric at efcs.ca] 
Sent: 04 May 2017 00:43
To: clang developer list <cfe-dev at lists.llvm.org>
Cc: Marshall Clow <mclow.lists at gmail.com>; Duncan P. N. Exon Smith <dexonsmith at apple.com>; Ed Maste <emaste at freebsd.org>; Martin J. O'Riordan <martin.oriordan at movidius.com>; Anton Korobeynikov <anton at korobeynikov.info>
Subject: [RFC] Possible ABI break for libc++ built with -fno-rtti and -D_LIBCPP_BUILD_STATIC

 

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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