[libcxx-commits] [PATCH] D101194: [Driver] Push multiarch path setup to individual drivers

[Fangrui Song via Phabricator via libcxx-commits]

MaskRay added a comment.

In D101194#2721597 <https://reviews.llvm.org/D101194#2721597>, @phosek wrote:

> In D101194#2721390 <https://reviews.llvm.org/D101194#2721390>, @MaskRay wrote:
>
>> I thought this was NFC:(
>>
>> Yeah, personally I think the previous behavior (`lib/clang/13.0.0/lib/x86_64-unknown-linux-gnu/libclang_rt.asan.a`) is slightly better than the new `lib/clang/13.0.0/lib/x86_64-linux-gnu/libclang_rt.asan.a`.
>> multiarch paths are suitable for GCC specific directories (`lib/$triple`, libstdc++ `aarch64-linux-gnu/include/c++/10`, etc) but llvm specific libraries don't necessarily use that.
>
> I should have been clear, sorry about that. I think there are three separate issues:
>
> 1. Rather than trying various triple versions, we should use one canonical spelling when searching for Clang's standard and runtime libraries. That means less system calls and better error messages, this is what this change was primarily intended to do.
> 2. Following from the previous point, we need to decide which canonical spelling to use. Since we're trying to use the multiarch layout, I was following the Debian documentation <https://wiki.debian.org/Multiarch/Tuples> which is closest thing to a spec I'm aware of. The alternative would be to use the normalized target, the main disadvantage being that we would be diverging from existing quasi standard.
> 3. `-DLLVM_ENABLE_PER_TARGET_RUNTIME_DIR=ON` gives you a different result depending on how you spell the targets in `-DLLVM_BUILTIN_TARGETS=` and `-DLLVM_RUNTIME_TARGETS=`, this is what @bjope ran into and I'd consider that a bug since the directory layout is an implementation detail and we should be able to change it without users having to change their build.
>
> I'm working on a solution for #3 and I should have a patch ready soon. I'm curious about your thoughts on #2.

multiarch is not consistent within GCC, either (also note that not all GCC installations are configured with multiarch.).
I have tried some examples, see https://maskray.me/blog/2021-03-28-compiler-driver-and-cross-compilation : even if the multiarch triple for x86_64 Linux is `x86_64-linux-gnu`, there are a few `x86_64-pc-linux-gnu` paths. (You can configure and get `x86_64-pc-linux-gnu-gcc` or `x86_64-unknown-linux-gnu-gcc`)
For musl systems (`*-linux-musl`), multiarch doesn't make sense. If we use multiarch, we'll need to invent `*-linux-musl` for them.

For some GCC related paths, we have no choice but to detect their paths. For entirely clang internal stuff (runtime library paths), I actually appreciate that we have a useful, un-normalized 'vendor' part.
It gives users a choice if they want to have `x86_64-pc[12345]-linux-gnu`. This argument is weak, but the other arguments are probably stronger.

---

I haven't figured out how to do a proper runtime build...

`-DLLVM_ENABLE_PER_TARGET_RUNTIME_DIR=on -DLLVM_RUNTIME_TARGETS=default -DLLVM_ENABLE_RUNTIMES='compiler-rt;libcxx;libunwind' ` gives me

  CMake Error at /usr/share/cmake-3.18/Modules/ExternalProject.cmake:3152 (add_custom_target):                                                                                                                                                                                                
    add_custom_target cannot create target "builtins" because another target                                                                                                                                                                                                                  
    with the same name already exists.  The existing target is a custom target                                                                                                                                                                                                                
    created in source directory                                                                                                                                                                                                                                                               
    "/home/maskray/llvm/compiler-rt/lib/builtins".  See                                                                                                                                                                                                                      
    documentation for policy CMP0002 for more details.                                                                                                                                                                                                                                        
  Call Stack (most recent call first):                                                                                                                                                                                                                                                        
    cmake/modules/LLVMExternalProjectUtils.cmake:283 (ExternalProject_Add)                                                                                                                                                                                                                    
    runtimes/CMakeLists.txt:81 (llvm_ExternalProject_Add)                                                                                                                                                                                                                                     
    runtimes/CMakeLists.txt:134 (builtin_default_target)


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D101194/new/

https://reviews.llvm.org/D101194