[LLVMdev] Ideas for making llvm-config --cxxflags more useful

[Mehdi Amini]

> On Aug 3, 2015, at 10:24 AM, Chris Bieneman <beanz at apple.com> wrote:
> 
> Hey Tom,
> 
> I’m not a regular user of llvm-config, but this sounds completely right to me, and it would be a significant improvement over what we have now.
> 
> The only question I want to raise is, what about NDEBUG? There are headers that conditionalize on NDEBUG, which could lead to ABI incompatibility in the C++ API.

Is it something that can be fixed or would it be too complicated to handle? 
It would be nice in general to be able to link a “Non assert” build of Clang with an “Assert" version of  LLVM (and vice-versa). 

And on the original topic: +1 for Tom proposal/goal, it makes sense to me.

—
Mehdi



> 
> Thanks for doing this,
> -Chris
> 
>> On Jul 30, 2015, at 1:04 PM, tom at stellard.net wrote:
>> 
>> Hi,
>> 
>> My understanding of llvm-config --cxxflags is that it is supposed to report
>> which flags are necessary to compile a program that will include LLVM's
>> headers and link against its libraries.  What it currently reports is
>> all of the flags which were used to compile LLVM.  This is not very useful,
>> because users are required in most cases to filter out flags they don't
>> want.
>> 
>> I would like to try to fix this so that it reports only the bare minimum
>> of required flags.  As an example here all the flags that it reports in
>> my autoconf build of llvm:
>> 
>> -I/usr/local/llvm/3.8/include
>> -D_DEBUG
>> -D_GNU_SOURCE
>> -D__STDC_CONSTANT_MACROS
>> -D__STDC_FORMAT_MACROS
>> -D__STDC_LIMIT_MACROS
>> -O3
>> -fomit-frame-pointer
>> -std=c++11
>> -fvisibility-inlines-hidden
>> -fno-exceptions
>> -fno-rtti
>> -fPIC
>> -ffunction-sections
>> -fdata-sections
>> -Wcast-qual
>> 
>> Of these flags the only ones that are really required are (c++ experts
>> please correct me if I'm wrong here):
>> 
>> -I/usr/local/llvm/3.8/include
>> -D__STDC_CONSTANT_MACROS
>> -D__STDC_FORMAT_MACROS
>> -D__STDC_LIMIT_MACROS
>> -std=c++11
>> 
>> Technically the -D__STDC* macros are only required if you include
>> Support/DataTypes.h, but I think that is hard to avoid.
>> 
>> As I understand, The rest of the flags are not required in 100% of the
>> use cases.
>> 
>> My proposal for fixing this is to remove everything but the 5 options listed
>> above.
>> 
>> For flags like -fno-rtti (are there others?) that are required in some cases
>> (I think -fno-rtti is required only if you sub-class LLVM objects), I would propose
>> adding a separate flag like --uses-rtti.  This would give users more fine-grained
>> control over which flags they use, and also would let them choose the correct
>> flag since, for example, -fno-rtti is not understood by MSVC.
>> 
>> How do people feel about this proposal?
>> 
>> Thanks,
>> Tom
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> 
> 
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