gh-141808: Don't remove the JIT stencils when building with PGO

[hroncok]

See https://discuss.python.org/t/building-the-jit-with-pre-built-stencils/91838/12

• Issue: Build with PGO needlesly generates JIT stencils twice (and prevents usage of prebuilt stencils) #141808

[hroncok]

I opened this as a draft, because keeping the files from being deleted when make clean is run might be surprising for somebody. An alternative solution for Fedora would be to add something like --with-jit-stencils PATH to configure, which would replace JIT building with copying. However, I still think that building the JIT files multiple times during a PGO build is wasting resources.

[emmatyping]

My understanding of clean-retain-profile is that it is meant to remove files that depend on PYTHON_FOR_REGEN, since that will change between the PGO run and the final build. If that is correct, then I would say the stencils should be rm'd in make clean and make distclean, but should not change with make clean-retain-profile, since, as I understand it, the JIT stencils do not depend on the Python binary used other than platform information (which should not change across executables for the same host and build).

So I would suggest making a new target clean-jit-stencils which will be a dep of distclean and clean but not clean-retain-profile

[hroncok]

However, when the stencils are removed in make clean, they are apparently also removed during the pgo task that calls it. I have not yet figured out why and where make clean is called.

[hroncok]

Ah:

cpython/Makefile.pre.in

Lines 838 to 841 in bc9e63d

	# Profile generation build must start from a clean tree.
	profile-clean-stamp:
	$(MAKE) clean
	touch $@

So the problem with my approach is that by dropping in the prebuilt stencils before this runs, they are deleted here. That's probably only problematic for my use case :/

[hroncok]

So, I modified the behavior to keep removing the stencils with make clean but not run make clean during the PTO build. I think this is cleaner.

[efimov-mikhail]

LGTM.
I can confirm that issue with double generation of JIT stencils for PGO build is fixed.
And Makefile.pre.in changes looks clean for me.