[llvm-dev] NDEBUG in Header Files

Philip Pfaffe via llvm-dev llvm-dev at lists.llvm.org
Mon Jul 4 04:24:09 PDT 2016


I only checked the dev mailing list for a duplicate, didn't think of
looking for it in commits.

There's some valid criticism in that thread, so I'll think about this some
more and if I come up with any more ideas I'll reply on phab.

- Philip

2016-07-01 20:16 GMT+02:00 Matthias Braun <mbraun at apple.com>:

> This has also come up with llvm before: 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
> 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> 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
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