llvm dev - Jul 2021 - Clang-12 fails build on MacOS

On 7/17/21, 09:11, "Raul Tambre" <raul at tambre.ee> wrote:

 
>    How does the build end up with two commands symlinking to the same
location?
>    Different targets through `LLVM_RUNTIME_TARGETS` should end up with
separate
>    build directories. arm64 and arm64e are different targets, right?
> 
>    Creating the symlinks during CMake configuration should be possible as
an
>    alternative.
 

Ken researched this problem, discovered its cause and suggested a workaround:

 

It appears that when running a custom_command, cmake does not use dependency
tracking and collisions can happen.

 

"Do not list the output in more than one independent target that may build
in parallel or the two instances of the rule may conflict (instead use the
add_custom_target() command to drive the command and make the other targets
depend on that one)."

 

​https://cmake.org/cmake/help/v3.19/command/add_custom_command.html 

 

The workaround is to add_custom_target a dependency on the output of the custom
command.

 

​https://gist.github.com/socantre/7ee63133a0a3a08f3990 

 

It would be great if LLVM maintainers incorporated this workaround, or, at the
very least, gave it a try.

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Thanks for the info, however I don't think we still understand the root
cause.
Why do we end up with two instances trying to create the symlink to the same 
location?

Per my thinking different targets end up with separate build directories thus 
this shouldn't happen. And since the different runtime builds are sub-builds
your proposed dependency tracking solution wouldn't work.

It appears that the proposed workaround had been tested and proven to work. 

Which is why I'm asking to give it a try. 

Perhaps doing that would help understanding the root cause too.

Regards,
Uri
> On Jul 18, 2021, at 14:27, Raul Tambre <raul at tambre.ee> wrote:
> 
> Thanks for the info, however I don't think we still understand the
root cause. Why do we end up with two instances trying to create the symlink to
the same location?
> 
> Per my thinking different targets end up with separate build directories
thus this shouldn't happen. And since the different runtime builds are
sub-builds your proposed dependency tracking solution wouldn't work.
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