make -j unsafe

[barak]

The Makefile is not parallel-build safe when using the default target. In other words, make -j8 results in multiple processes stepping on each others' toes.

[attila-lendvai]

the situation is that make -j works, and is a nice speedup when building the current version, but the chain of the bootstrapping process is not thread safe (i.e. the first invocation of the build after a make distclean).

besides making the bootstrap threadsafe, we could disable parallelism alltogether, but then building during normal development would be slower.

[barak]

Sure. Making the bootstrap safe is the "right" solution, but also the trickiest, of course.
For now, maybe have the bootstrap do a $(MAKE) -j1 actual-bootstrap-target, to keep it safe?
Or put it in another Makefile with .NOPARALLEL: ?

[attila-lendvai]

i tried to play with it but i can't even build it on guix at the moment... :/

see the latest commit for commentary, and/or the llvm crash: llvm/llvm-project#60952