[PATCH] D106305: [compiler-rt][CMake] create aarch64 helper asm during configure time

Tue Jul 20 12:28:01 PDT 2021

tambre added a comment.

In D106305#2890733 <https://reviews.llvm.org/D106305#2890733>, @steven_wu wrote:

> The configured directory is rather large so let me give some more detail from the makefile generated. The custom target is generated in two of the makefiles I mentioned above, like following:
>
>   projects/compiler-rt/lib/builtins/outline_atomic_helpers.dir/outline_atomic_cas1_1.S:
>           @$(CMAKE_COMMAND) -E cmake_echo_color --switch=$(COLOR) --blue --bold --progress-dir=/Users/steven/makefile_build/CMakeFiles --progress-num=$(CMAKE_PROGRESS_1) "Generating outline_atomic_helpers.dir/outline_atomic_cas1_1.S"
>           cd /Users/steven/makefile_build/projects/compiler-rt/lib/builtins && /usr/local/bin/cmake -E create_symlink /Users/steven/llvm-project/compiler-rt/lib/builtins/aarch64/lse.S /Users/steven/makefile_build/projects/compiler-rt/lib/builtins/outline_atomic_helpers.dir/outline_atomic_cas1_1.S
>
> When you build the clang builtin library, it does:
>
>   $ make -j 32 clang_rt.osx VERBOSE=1
>   ...
>   /usr/bin/make  -f projects/compiler-rt/lib/builtins/CMakeFiles/clang_rt.builtins_arm64_osx.dir/build.make projects/compiler-rt/lib/        builtins/CMakeFiles/clang_rt.builtins_arm64_osx.dir/depend
>   /usr/bin/make  -f projects/compiler-rt/lib/builtins/CMakeFiles/clang_rt.builtins_arm64e_osx.dir/build.make projects/compiler-rt/lib/       builtins/CMakeFiles/clang_rt.builtins_arm64e_osx.dir/depend
>   ...
>   CMake Error: failed to create symbolic link '/Users/steven/makefile_build/projects/compiler-rt/lib/builtins/outline_atomic_helpers.dir/outline_atomic_ldset1_3.S': file already exists
>   ``

Thanks, I now understand why this issue happens. With Make each target gets a Make subinvocation, while in Ninja they're generated into a single buildfile and can be correctly scheduled.
This issue only happens with targets that can build multiple sub-architectures at the same time (ARM32, MIPS, PPC64) possibly causing the symlinking to run at the sametime. On Apple this includes ARM64 and x86-64, making this a bit easier to hit.

The `add_custom_target()` solution noted in the mailing list seems the most appropriate to force ordering but still keep parallelism as noted by @thakis. Let's go with that.

Repository:
  rG LLVM Github Monorepo

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https://reviews.llvm.org/D106305