For the JDK 8 build vmStructs.o is compiled with default optimization level, which is -O3 for release. In JDK 9+ it's being compiled with -O0. It appears this was a porting artifact of this change:

http://hg.openjdk.java.net/jdk9/jdk9/hotspot/rev/e796d52ca85b

The intention was to add -fno-var-tracking-assignments for vmStructs's compilation. But since the old build overwrote default opt flags when per-file overrides get added the port of the change to the JDK 9+ build system added explicit -O0. The former seems intentional the latter not so much.

More info here:
http://mail.openjdk.java.net/pipermail/build-dev/2018-September/023216.html