[flang] include flang by default under LLVM_ENABLE_PROJECTS

[h-vetinari]

This has been discussed numerous times, and generally was made dependent on the rename from flang-new to flang which now happened: 06eb10d

I've tried to find if there's an issue for this but couldn't find one (apologies if I missed something), so opening this.

[nikic]

I don't think that flang should become part of the default project list unless and until significant improvements are made to the memory usage of the flang build. A release build should not require more than 2GB of memory per thread, otherwise including it in the default build becomes a liability.

A possible alternative would be to include cmake logic that automatically reduces the parallelism of only the flang build to fit its unusual memory usage requirements.

[h-vetinari]

A possible alternative would be to include cmake logic that automatically reduces the parallelism of only the flang build to fit its unusual memory usage requirements.

I was under the impression that this happened already? I'm on a phone right now, but IIRC there was the introduction of a specific CMake variable for a lower default parallelism specifically for the flang build

[kiranchandramohan]

A possible alternative would be to include cmake logic that automatically reduces the parallelism of only the flang build to fit its unusual memory usage requirements.

I was under the impression that this happened already? I'm on a phone right now, but IIRC there was the introduction of a specific CMake variable for a lower default parallelism specifically for the flang build

That patch (#95672) was reverted (#96000) since there was a failure in one of the buildbots.

[h-vetinari]

Sounds like that patch should be reattempted/relanded then? :)

[Meinersbur]

Should the default of #95672 consider the host's available memory then? The default of "0" (infinity) will catch many users by surprise and crash their system, especially since it will override LLVM_PARALLEL_COMPILE_JOBS which is actually derived from LLVM_RAM_PER_COMPILE_JOB (Since #65274).

[tstellar]

Sounds like that patch should be reattempted/relanded then? :)

Here it is: #127364