Improve auto-linking behavior in lld to find runtime libraries automatically

[tru]

As discussed here: https://discourse.llvm.org/t/improve-autolinking-of-compiler-rt-and-libc-on-windows-with-lld-link

We want autolinking to builtins.

[llvmbot]

@llvm/issue-subscribers-lld-coff

[tru]

Started to look at auto-linking builtins now. There are some open questions:

• Do we always want to link clang_rt.builtins? Should we respect the --rtlib=compiler-rt flag in the MSVC toolchain?
• How do we know if we want to link clang_rt.builtins_x86_64.lib or <triple>/clang_rt.builtins_x86_64.lib? I know we talked about making LLVM_ENABLE_PER_TARGET_RUNTIME_DIR a compile time option. Do we want to do that first? Or do we want to probe in the MSVC toolchain for the two different libraries and then use the one we find?

Btw I noticed with the patches that already landed ubsan already works for the non PER_TARGET_RUNTIME option, I will look at adding support for the runtime separated version as well.

cc: @rnk @zmodem @petrhosek @mstorsjo

[zmodem]

Do we always want to link clang_rt.builtins? Should we respect the --rtlib=compiler-rt flag in the MSVC toolchain?

I'm not sure how --rtlib works for Windows today, but I'm inclined to say we should always link the compiler-rt builtins unless the user opts out (e.g. with /nodefaultlib)

How do we know if we want to link clang_rt.builtins_x86_64.lib or /clang_rt.builtins_x86_64.lib?

I think we should do probing for now, and once LLVM_ENABLE_PER_TARGET_RUNTIME_DIR is universal we could drop it.

[mstorsjo]

Started to look at auto-linking builtins now. There are some open questions:

• Do we always want to link clang_rt.builtins? Should we respect the --rtlib=compiler-rt flag in the MSVC toolchain?
• How do we know if we want to link clang_rt.builtins_x86_64.lib or <triple>/clang_rt.builtins_x86_64.lib? I know we talked about making LLVM_ENABLE_PER_TARGET_RUNTIME_DIR a compile time option. Do we want to do that first? Or do we want to probe in the MSVC toolchain for the two different libraries and then use the one we find?

One aspect that was mentioned before, and which I think is important to keep - is that building with clang targeting msvc currently works without having the builtins available. (Consider cross compiling; you unpack msvc/winsdk, you take a freshly built clang - currently that’s all you need to build for windows, for any of the 4 architectures supported. You don’t need to start out by building compiler-rt.) Currently, you can already build 98% (totally made up number) of all code in clang/msvc environments without compiler-rt builtins. Complicating that basic case would be a shame.

So potential options would be to either only do this if --rtlib was specified, or probe for whether they’re available. Probing is needed for knowing which name/path style to use anyway (although it’s not ideal to need to do it for all compilation jobs, as opposed to when driving the linker).

Making LLVM_ENABLE_PER_TARGET_RUNTIME_DIR a hardcoded option in clang helps a little (but that’s a possibly complicated thing to pull off anyway - I’m mildly sceptical about that in general), but that doesn’t help for knowing whether we should include at all or not anyway.