llvm dev - Jul 2015 - [LLVMdev] Ideas for making llvm-config --cxxflags more useful

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

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

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