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

[Chris Bieneman]

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.

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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