[PATCH] D99621: [CMake][Compiler-rt] Make it possible to configure standalone compiler-rt without `LLVMConfig.cmake`.

[Dan Liew via Phabricator via llvm-commits]

delcypher added a comment.

In D99621#2667768 <https://reviews.llvm.org/D99621#2667768>, @mstorsjo wrote:

> Thanks, this seems to work fine in my setups now!
>
> When building via llvm/runtimes (which I don't use so I don't have a test setup handy), I believe that sets `TARGET_TRIPLE` somehow. Does that end up trying to probe the triple from the compiler in that case? I guess it'd always be correct, but if we'd avoid the whole probing in that case too, that'd be great. Maybe it already does that - or the runtimes build does thing differently somehow? (I've just tried to read llvm/runtimes/CMakeLists.txt.) Or maybe we end up in a case where these routines aren't called at all in such a build?
>
> I might be able to try to set up such a build configuration in a couple days to try that out though...

I'm not sure. Reading that code is a little confusing...

So there are external project invocations that pass

1. `LLVM_DEFAULT_TARGET_TRIPLE`. That looks like it's only used in compiler-rt to help with finding GNU gold/ld. So that probably doesn't matter here.
2. `TARGET_TRIPLE` as an argument to the `llvm_ExternalProject_Add()` function

The second one is a bit confusing.

There's this code inside `llvm_ExternalProject_Add`

  if(NOT ARG_TARGET_TRIPLE)
    set(target_triple ${LLVM_DEFAULT_TARGET_TRIPLE})
  else()
    set(target_triple ${ARG_TARGET_TRIPLE})
  endif()
  
  is_msvc_triple(is_msvc_target ${target_triple})

but then `target_triple` doesn't get used again... which makes no sense. (@phosek @beanz is this a bug?).

Then later on I do see

  if(ARG_TARGET_TRIPLE)
    list(APPEND compiler_args -DCMAKE_C_COMPILER_TARGET=${ARG_TARGET_TRIPLE})
    list(APPEND compiler_args -DCMAKE_CXX_COMPILER_TARGET=${ARG_TARGET_TRIPLE})
    list(APPEND compiler_args -DCMAKE_ASM_COMPILER_TARGET=${ARG_TARGET_TRIPLE})
  endif()

So that does suggest Compiler-RT will have `CMAKE_C_COMPILER_TARGET` passed. The builtins and other runtime builds in `llvm-project/llvm/runtimes` look like they pass `-DCOMPILER_RT_DEFAULT_TARGET_ONLY=ON`.

So that would suggest that both `CMAKE_C_COMPILER_TARGET` and `COMPILER_RT_DEFAULT_TARGET_ONLY` get set.

**If I've read this correctly** that means this patch doesn't need to change because it already checks for `CMAKE_C_COMPILER_TARGET` and `COMPILER_RT_DEFAULT_TARGET_ONLY` being set and defers to using `CMAKE_C_COMPILER_TARGET` to set `TARGET_TRIPLE` in that case.

I don't use the `llvm-project/llvm/runtimes` directory either so I don't know if it works on Apple platforms. I'd be a bit happier if someone who does use it checked that this patch didn't break anything.


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https://reviews.llvm.org/D99621