The C2 load barrier code for ZGC assumes that r12 is not used as a general purpose register. However, JDK-8217909 might change that, so ZGC should not be making this assumption. Bug: https://bugs.openjdk.java.net/browse/JDK-8218767 Webrev: http://cr.openjdk.java.net/~pliden/8218767/webrev.0 /Per