ZGC reserves a virtual address range for its heap with one high order bit set which is referred to as the heap base. Internally we then often represent heap addresses as offset from this heap base.

Currently we select one specific heap base at the start based on MaxHeapSize and the current system properties.

With instrumented builds, or custom launchers it may be that we are unable to reserve a usable address range using that heap base. Currently we just give up if this happens and exits the VM.

This is problematic when using instrumented builds such as ASAN where there are certain address ranges it uses which often clash with the default ZGC heap base.

I propose that we are more flexible when selecting the heap base, and we start as we do today at our preferred location, but are able to retry other compatible heap bases within some broader limits.