[llvm-dev] Change default config of clang on Windows

[Fangrui Song via llvm-dev]

On 2020-02-26, Reid Kleckner via llvm-dev wrote:
>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.
>
>On Sun, Feb 23, 2020 at 12:37 PM Максим Власов via llvm-dev <
>llvm-dev at lists.llvm.org> wrote:
>
>> 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?

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

https://gcc.gnu.org/onlinedocs/gcc/Spec-Files.html