[llvm-dev] [RFC] Usage of NDEBUG as a guard for non-assert debug code

[David Truby via llvm-dev]

Hi all,

During discussions about assertions in the Flang project, we noticed that there are a lot of cases in LLVM that #ifndef NDEBUG is used as a guard for non-assert code that we want enabled in debug builds.
This works fine on its own, however it affects the behaviour of LLVM_ENABLE_ASSERTIONS;  since NDEBUG controls whether assertions are enabled or not, a lot of debug code gets enabled in addition to asserts if you specify this flag. This goes contrary to the name of the flag I believe also its intention. Specifically in Flang we have a case where someone wants to ship a build with assertions enabled, but doesn't want to drag in all the extra things that are controlled by NDEBUG in LLVM.

In my opinion we ideally want LLVM_ENABLE_ASSERTIONS to _only_ enable assertions and do nothing else. I don't think this is possible without changing the use of NDEBUG elsewhere as NDEBUG controls whether assert is enabled.
I propose we should be using another macro (something like LLVM_DEBUG_CHECKS ?) that is enabled in Debug builds, and possibly controlled by another cmake flag (LLVM_ENABLE_DEBUG_CHECKS ?) for code that we want enabled for debugging but not in releases. This would allow LLVM_ENABLE_ASSERTIONS to do what it says on the tin and actually enable assertions only.

Does anyone else have any thoughts on this?

Thanks
David Truby
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