Sometimes, when compiling with more than one job on my hifive unmatched board with gcc 9.2.0 I get an ICE (see attached build log):

malloc(): unaligned tcache chunk detected

So far it doesn't happen with all packages, but only with some, and even then it's doesn't always happen with each package where it does happen.
The only two packages I've noticed it happen every time are glibc and gcc itself.

My temporary workaround is to use ebuild /path/to/ebuild merge to finish emerging the failing package without starting over. I could set MAKEOPTS="-j1" for those packages, as well, but I would consider that to be the last resort if there's no fix to be found in the intermediate future.

My first idea would be to compile Gentoo's GCC 9.3.0 and see if it also occurs there.