Make parallel builds break on APFS filesystem

[FX]

Hello,

There is is a bug that occurs intermittently when doing parallel builds of GCC 
7.1.0 on macOS 10.13, with Apple’s new APFS filesystem. I reported it to GCC 
(https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81797), but it is looking more 
and more like a make issue, and I’m trying to debug it or narrow it down. The 
facts are:

- It occurs about 75% of the time in “make -j4” builds, where rules are 
executed out of order
- It occurs with both make 3.81 and make 4.2.1
- It occurs only if the file system is APFS, but not if the filesystem is HFS+

The notable differences between the two filesystems are:

- APFS has timestamp granularity of 1 ns, vs. 1 second for HFS+
- File ordering: “Calling readdir(2) on a directory in APFS returns filenames 
in hash order, whereas HFS+ returns filenames in lexicographical order.”

Are you alreayd aware of such an issue? (a web search and look into the mail 
archives revealed nothing) How should I go about debugging this further?

Thanks for any help,
FX