(related to issue #24915)
I discovered that `make profile-opt` does not use the profile information for the builtin-modules (e.g. arraymodule or _pickle) because in the `profile-opt` target there is the following sequence of actions:
…
$(MAKE) build_all_merge_profile
@echo "Rebuilding with profile guided optimizations:"
$(MAKE) clean
$(MAKE) build_all_use_profile
…
The action `$(MAKE) clean` performs an `rm -rf build`, destroying among other things all the *.gcda files generated for the built-in modules.
On my Linux system with gcc, a kludge to `Makefile.pre.in` that works is:
…
@echo "Rebuilding with profile guided optimizations:"
find build -name \*.gcda -print | cpio -o >_modules.gcda.cpio # XXX
$(MAKE) clean
cpio -id <_modules.gcda.cpio # XXX
$(MAKE) build_all_use_profile
…
but, like I said, it's a kludge and it's POSIX-only (-print0 can be avoided since I believe it's guaranteed that there will be no whitespace-containing-filenames).
Now, if this road is to be taken, I believe the most cross-platform method available to save the profile-generated files is to use a custom python script (at this point, the local profile-generating python executable is functional) and make a tar file, which will be untared back. However, I don't like it much.
I'm willing to provide any necessary patches if someone gives me a "proper" roadmap as to how to handle this issue.