[llvm-dev] NDEBUG in Header Files

Reid Kleckner via llvm-dev llvm-dev at lists.llvm.org
Fri Jul 1 10:11:49 PDT 2016

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

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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