Graal now explicitly whitelists the GCs it supports[1]. This involves looping over all the -XX:Use*GC selector flags. The flag values are read from HotSpotVMConfigAccess.readFlag[2] which needs to make a VM call for each flag not in the predefined set of flags included when initializing JVMCI. By including all the GC selector flags in the predefined set, these VM calls can be avoided.

[1] https://github.com/oracle/graal/blob/3ff06d1617240c5e6b7747d1305a060fd5c69369/compiler/src/org.graalvm.compiler.hotspot/src/org/graalvm/compiler/hotspot/HotSpotGraalRuntime.java#L157
[2] http://hg.openjdk.java.net/jdk/jdk/file/0fb45c3b185e/src/jdk.internal.vm.ci/share/classes/jdk.vm.ci.hotspot/src/jdk/vm/ci/hotspot/HotSpotVMConfigAccess.java#l277