[llvm-dev] Question: Should we consider valid a variable defined #ifndef NDEBUG scope and used in assert?

[Matthias Braun via llvm-dev]

On the topic of NDEBUG usage in public headers; this may be interesting: https://reviews.llvm.org/D38511

> On Oct 4, 2017, at 3:02 PM, Quentin Colombet via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> Yep they are messing up with the DEBUG macros…
> 
> Sorry for the noise!
> 
>> On Oct 4, 2017, at 2:58 PM, Quentin Colombet via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>> 
>> 
>> Good point, I forgot to check the standard.
>> Given the clang was failing I assumed the code was wrong x).
>> 
>> I am guessing there is something weird with the project, because indeed, paragraph 7.2 of the standard says:
>> The assert macro is redefined according to the current state of NDEBUG each time that
>> 
>> <assert.h> is included. 
>> 
>> 
>>> On Oct 4, 2017, at 2:53 PM, Craig Topper <craig.topper at gmail.com <mailto:craig.topper at gmail.com>> wrote:
>>> 
>>> I thought NDEBUG was what the assert macro also used to decide whether to be an assert or not.
>>> 
>>> ~Craig
>>> 
>>> On Wed, Oct 4, 2017 at 2:41 PM, Quentin Colombet via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>>> Hi,
>>> 
>>> While audit our code, we found out a strange pattern in one of the LLVM headers and we were wondering if that was expected or if it should be fixed.
>>> 
>>> Namely the problem looks like this:
>>> #ifndef NDEBUG
>>>   // Define some variable.
>>> #endif
>>> 
>>> assert(/*use this variable, i.e., outside of the ifndef NDEBUG scope*/);
>>> 
>>> This happens in include/llvm/IR/ValueHandle.h for the variable Poisoned line 494
>>> 
>>> This works because when we build LLVM with assert we explicitly disable NDEBUG:
>>> if( LLVM_ENABLE_ASSERTIONS )
>>>   […]
>>>   # On non-Debug builds cmake automatically defines NDEBUG, so we
>>>   # explicitly undefine it:
>>>   if( NOT uppercase_CMAKE_BUILD_TYPE STREQUAL "DEBUG" )
>>>     add_definitions( -UNDEBUG )
>>> 
>>> If we use this header in a different project and thus, with potentially different rules for macro definitions, the compiler complains with
>>> error:   use of undeclared identifier ‘Poisoned'
>>> 
>>> I think the right thing to do is to fix the code to work without the special way of setting the macros, but I’d like people opinions first.
>>> 
>>> Thanks,
>>> -Quentin
>>> 
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>>> 
>> 
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