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

[Chris Bieneman]

> On Aug 3, 2015, at 10:30 AM, Mehdi Amini <mehdi.amini at apple.com> wrote:
> 
>> 
>> On Aug 3, 2015, at 10:24 AM, Chris Bieneman <beanz at apple.com <mailto: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? 

I think this would need a case-by-case determination that I haven’t done. There are quite a few occurrences of NDEBUG in headers:

> grep -r NDEBUG ./include/ | wc -l
      77

I imagine many of these aren’t actually ABI issues, but I know that ABI issues have existed in the past.

-Chris

> 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
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>> 
>> 
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