Linus has added a nice optimization for those of us with lots of CPU cores/threads/ALUs, but testing reveals that it tends to express the (known) race condition in GNU make 4.2.1. As such, an ebuild warning, uname -r check, or DEPEND restriction should probably be added to sys-devel/make. Or maybe ">=sys-devel/make-4.3" added to gentoo-sources-5.6? It's currently at RC1. https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=0ddad21d3e99c743a3aa473121dc5561679e26bb
I'm pretty sure we already have the necessary patch backported to make-4.2.1. See bug 664632.