[llvm-dev] NDEBUG in Header Files

[Matthias Braun via llvm-dev]

This has also come up with llvm before: http://reviews.llvm.org/D11833 <http://reviews.llvm.org/D11833>

- Matthias

> On Jul 1, 2016, at 10:11 AM, Reid Kleckner via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> When we added the ABI breaking EpochTracker to DenseMap, we added a macro called LLVM_ENABLE_ABI_BREAKING_CHECKS, which is controlled by the CMake option LLVM_ABI_BREAKING_CHECKS. I think we should make all ABI-impacting uses of NDEBUG in headers use this macro instead.
> 
> I don't know what the current behavior is, but I would like this macro to correlate with NDEBUG by default. People packaging LLVM as a library for redistribution can turn off the ABI breaking checks manually.
> 
> Also, this has come up before, although Alp was concerned with Clang, not LLVM:
> http://lists.llvm.org/pipermail/llvm-dev/2013-November/067410.html <http://lists.llvm.org/pipermail/llvm-dev/2013-November/067410.html>
> 
> On Fri, Jul 1, 2016 at 5:37 AM, Philip Pfaffe via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
> Hi everyone,
> 
> we have several header files in which the NDEBUG macro is being used. In some of these, the NDEBUG macro changes the size, the interface, or the layout of a type. One example is AssertingVH, which turns into a flat, transparent Value* wrapper in Release build. Now everywhere you use these headers, the state of the NDEBUG flag must be the same, or else bad things could happen.
> 
> This turns out to be quite problematic for out-of-tree targets and projects, because it effectively forces them to use the same build type with which LLVM was compiled. Thus, a project maintainer needs to keep a Debug and a Release build of LLVM around, and is mostly barred from using binary distributions.
> 
> If the community agrees that this is a problem, I'd like to discuss two possible solutions.
> 
> Solution #1:
> Don't use NDEBUG (or any similar or depending macro) inside public-facing header files. I think this is the cleanest solution, but it requires a great amount of discipline on the side of developers and reviewers to stick with this pattern, and it will probably have a performance impact (e.g., for AssertingVH).
> 
> Solution #2:
> Replace NDEBUG with a custom flag (e.g. LLVM_NDEBUG), and have CMake take care of synchronizing the two. I.e. ensure that LLVM_NDEBUG is set iff NDEBUG is set. Introducing this is probably a lot more work, because it touches all subprojects.
> 
> What do you think?
> 
> Best,
> - Philip
> 
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev <http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev>
> 
> 
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20160701/7b2995e9/attachment.html>