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

[Fāng-ruì Sòng via llvm-dev]

On Fri, Apr 10, 2020 at 12:26 PM David Blaikie <dblaikie at gmail.com> wrote:

>
>
> On Fri, Apr 10, 2020 at 10:15 AM Fangrui Song via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> On 2020-04-10, Michael Kruse via llvm-dev wrote:
>> >#ifndef NDEBUG in the LLVM source and assert() are at least somewhat
>> >linked. For instance, consider
>> >
>> https://github.com/llvm/llvm-project/blob/8423a6f36386aabced2a367c0ea53487901d31ca/llvm/lib/Transforms/Scalar/IndVarSimplify.cpp#L2668
>> >
>> >The #ifndef NDEBUG is used to guard the value that checked in an
>> >assert() later. Only enabling assert() will cause the build to fail
>> >because the value is not defined. Another example is a loop only for
>> >the assert, eg.
>> >
>> >    #ifndef NDEBUG
>> >    for (auto &Val : Container)
>> >      assert(Val && "all");
>> >    #endif
>> >
>> >where the loop would loop over nothing with assertions disable. We
>> >cannot have a 'change all 'NDEBUG to LLVM_NO_DEBUG_CHECKS'. Even if we
>> >manage to leave all NDEBUG that are linked to some use of assert(), it
>> >doubles the number of configurations that might break in some
>> >configuration such as IndVarSimplify. I understand NDEBUG as `remove
>> >all the code only useful for developers`, independent of whether we
>> >also want debug symbols.
>> >
>> >I'd find it more useful to discuss what should NOT be covered under
>> >the blanket term LLVM_ENABLE_ASSERTIONS. Example from the past are
>> >LLVM_ENABLE_STATS and LLVM_ENABLE_DUMP that once was also using
>> >NDEBUG.
>>
>> +1 for bring up this topic.
>>
>>
>> The code base sometimes uses #ifdef NDEBUG to guard the definitions of
>> some struct/class members and functions.
>>
>> This means there are inherent ABI incompatibility between non-NDEBUG and
>> NDEBUG object files. I hoped that mixing object files could work(at
>> least in some configurations.  Also note that having different
>> definitions of inline functions leads to ODR violation) but I think this
>> cannot be changed any more.
>>
>
> Not sure I'm following - what "cannot be changed anymore"? If there are
> ABI changing uses of NDEBUG they can and should be changed to
> LLVM_ENABLE_ABI_BREAKING_CHECKS.
>

I agree that many NDEBUG uses are actually LLVM_ENABLE_ABI_BREAKING_CHECKS.
By "cannot be changed anymore" I mean there are so many NDEBUG (700+ files),
I am not sure we can still make mixing non-NDEBUG and NDEBUG object files
work...


>
>

>> >As was mentioned, Support/Debug.h might be another candidate.
>> >But IMHO the compile-time/execution-time/code-size impact of these are
>> >too small to justify the increase combinatorial complexity of
>> >configuration. At the last LLVM DevMtg we actually discussed how to
>> >*reduce* the number of independent configuration parameters.
>> >
>> >Michael
>>
>>
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>>
>
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