[cfe-dev] [RFC] Proposing libcxx-config.py: a configuration tool for building and using libc++

[Mehdi Amini via cfe-dev]

> On Feb 9, 2017, at 4:18 PM, Eric Fiselier via cfe-dev <cfe-dev at lists.llvm.org> wrote:
> 
> Hi All,
> 
> Configuring a compiler to target or build libc++ is tricky. It often requires suppressing and rebuilding large parts of the CC1 compiler invocation. Doing this manually can be prohibitive, even for experienced users. Even doing it programmatically is hard and requires a lot of code because of the sheer number of configurations libc++ support.
> 
> These problems affects both libc++ and it's users. For libc++ CMake's statefulness makes it hard to generate the two different sets of flags needed to initially build the library and then to target it afterwards.

Can you elaborate on this issue? Without looking deeply at the problem, it seems difficult to me to figure exactly what the problem is.


> 
> Therefore I'm proposing a utility called `libcxx-config.py` which, similar to llvm-config, when queried will output the flags required to compile and link libc++. The tool will have two main purposes:
> 
> (1) Generating the compile and link flags required to use the just-built libc++ library.
> (2) Generating the compile and link flags required to build the libc++ dylib.
> 
> The goals for `libcxx-config.py` are:
> 
> (1) Reduce duplicate configuration logic between the CMake and test-suite.
> (2) Allow non-CMake users to correctly generate the flags required to build libc++. (Ex buildit)
> (3) Make it easier for projects, including libc++, to correctly configure to target the just-built libc++.
> (4) Allow the creation of a `test-libc++` tool which can compile and link test programs against libc++ (Often needed when debugging test suite failures).
> 
> Would anybody else have a need for this? Are there use cases anybody would like to see supported?
> 
> Some example usages I imagine for `libcxx-config.py` are:
> 
>   # Get the compile flags needed to build libc++.dylib
>   $ libcxx-config.py --building-library --compile-flags --compiler=clang++ --cxx-abi-library=libcxxabi
>   
>   # output: -nostdinc++ -I <libcxx-headers> -D_LIBCPP_BUILDING_LIBRARY -DLIBCXX_BUILDING_LIBCXXABI -std=c++11 -fvisibility-inlines-hidden

I’m not sure why it is not the role of CMake?

>   # Get the link flags needed to use libc++ with the specified compiler
>   $ libcxx-config.py --link-flags --compiler=g++
>   
>   # Output: -nodefaultlibs -L <path> -Wl,-rpath <path> -lc++experimental -lc++ -lm -lgcc_s -lgcc -lc -lgcc_s -lgcc

Isn’t it the role of the clang driver when given -stdlib=libc++ ?

— 
Mehdi


> 
> Any feedback is appreciated.
> 
> /Eric
> 
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