[question] AutotoolsToolchain uses MSVC flags with clang

[jijjijj]

What is your question?

So I have a Win64 build with clang set as a compiler and I try to build mpg123 which uses Autotools to configure itself. The build fails because Autotools receive -MDd flag inside CFLAGS which is not supported by clang/clang++ only by clang-cl (which I don't want to use).

So I went to code of AutotoolsToolchain and I found this

Where _get_msvc_runtime_flag() returns the runtime flag appropriate for MS cl.exe or clang-cl but not for clang. And then in cflags() it just returns it without any regard to the actual compiler in use.

At the same time, when choosing flags relevant for mac/apple compilers it does filter the flags out in case the current system isn't appropriate.

What's up with that?

Have you read the CONTRIBUTING guide?

• I've read the CONTRIBUTING guide

[memsharded]

Hi @jijjijj

Thanks for your question.

Autotools receive -MDd flag inside CFLAGS which is not supported by clang/clang++ only by clang-cl (which I don't want to use).

Could you please clarify which clang are you using in Windows, the msvc-backed clang (irrespective of clang or clang-cl front), or the msys2-backed clang?

In case you haven't read this, this blog post is relevant: https://blog.conan.io/2022/10/13/Different-flavors-Clang-compiler-Windows.html

I think the MD flag should be supported by both clang/clang-cl msvc-backed compilers, as they use the msvc runtime.

[jijjijj]

@memsharded
Thank you for the response.

I use CLang 19.1.3 provided by LLVM. I downloaded it right from their GitHub page. According to the article it should fall under the "MSVC-backed" clangs

[memsharded]

I am having a look.
These are the lines for clang-cl produced by CMake (for release and Debug respectively):

[1/2] C:\ws\LLVM\clang18\bin\clang-cl.exe  /nologo -TP  
-IC:\Users\conanws\kk\include /DWIN32 /D_WINDOWS /GR /EHsc /O2 /Ob2 /DNDEBUG
-MD -std:c++14
/showIncludes /FoCMakeFiles\mypkg.dir\src\mypkg.cpp.obj /FdCMakeFiles\mypkg.dir\mypkg.pdb 
-c -- C:\Users\conanws\kk\src\mypkg.cpp

[1/2] C:\ws\LLVM\clang18\bin\clang-cl.exe  /nologo -TP 
-IC:\Users\conanws\kk\include /DWIN32 /D_WINDOWS /GR /EHsc /Zi /Ob0 /Od /RTC1 
-MDd -std:c++14 
/showIncludes /FoCMakeFiles\mypkg.dir\src\mypkg.cpp.obj /FdCMakeFiles\mypkg.dir\mypkg.pdb
-c -- C:\Users\conanws\kk\src\mypkg.cpp

And with clang:

[1/2] C:\ws\LLVM\clang18\bin\clang++.exe  -IC:/Users/conanws/kk/include -O3 -DNDEBUG 
-D_DLL -D_MT -Xclang --dependent-lib=msvcrt -std=c++14 -MD -MT
CMakeFiles/mypkg.dir/src/mypkg.cpp.obj -MF CMakeFiles\mypkg.dir\src\mypkg.cpp.obj.d 
-o CMakeFiles/mypkg.dir/src/mypkg.cpp.obj -c C:/Users/conanws/kk/src/mypkg.cpp

[1/2] C:\ws\LLVM\clang18\bin\clang++.exe  -IC:/Users/conanws/kk/include -O0 -g -Xclang -gcodeview
-D_DEBUG -D_DLL -D_MT -Xclang --dependent-lib=msvcrtd -std=c++14 -MD -MT
CMakeFiles/mypkg.dir/src/mypkg.cpp.obj -MF CMakeFiles\mypkg.dir\src\mypkg.cpp.obj.d 
-o CMakeFiles/mypkg.dir/src/mypkg.cpp.obj -c C:/Users/conanws/kk/src/mypkg.cpp

The -MD and -MT in clang have different meaning https://clang.llvm.org/docs/ClangCommandLineReference.html

[jijjijj]

So, is this a bug in AutotoolsToolchain?

[memsharded]

Yes, yes, it looks there is a gap, maybe not only in AutotoolsToolchain, but lack of model for clang/clang-cl differences, as the current model is more focused on the binary compatibility than on the compiler frontend input args.