Change default config of clang on Windows

I’m getting trouble using clang with compiled libc++ on MSVC.

  1. why clang chooses link over lld-link by default? I don’t really wish to write -fuse-ld=lld-link every single time I build something.

  2. why lld-link doesn’t search inside default lib directory? I need to manually show him where it is although I put that binary in <LLVM root>\lib\

  3. why clang invokes linker with -defaultlib:libcmt? I mean libc++ build guide under MSVC produces -MD version of either libc++.lib or c++.lib

  4. Since (3), it’s not possible to build anything without splitting compiler and linker calls - clang by default produces -MT versions of objects and linker fails to compose it with -MD library. No, -dynamic flag does not help either.

Any fixes to these? Can I config clang somehow except --config option?

I think “legacy” is the answer to most of your questions.

  1. I think clang still uses ld by default, so clang-cl is similar. Besides, every build system we encountered migrating from MSVC calls link directly, and needed to be updated to call lld-link.

  2. I think we should probably configure lld to find the clang resource directory. At the very least, clang-cl -fuse-ld=lld should add it.

  3. I think this is because there is no clang spelling for the /MD & /MT flags, so we have to pick some CRT so that hello world links. You can work around it with -nodefaultlib and passing your own.

  4. That’s true, but that’s been the case for every build system we’ve encountered.

So, I think there is room for improvement and better defaults.

Most options in clang are controlled by flags, not configure-time cmake options. From the clang developer perspective, this is good, clang is always a cross compiler and can do anything anywhere at runtime. But from a user or packager perspective, this is often painful. Users often write compiler wrapper scripts to fill in their preferred default options: default target, default include paths, default library paths, etc.

May I rephrase the question as asking about the feasibility of GCC
-specs= ?

We have that already.