Force fixed heap size when using NoGC (#1264)

The dynamic heap size trigger needs GC to adjust the heap size, but NoGC
can't do GC. So it doesn't make sense to use dynamic heap size with
NoGC. Currently, if the user selects the NoGC plan and the dynamic heap
size trigger, it will trigger GC at the minimum heap size and then panic
immediately.

With this change, MMTk will give a warning and use fixed heap size
trigger instead, using the maximum heap size specified in the dynamic
trigger as the heap size.

Author: wks

src/util/heap/gc_trigger.rs

@@ -40,10 +40,19 @@ impl<VM: VMBinding> GCTrigger<VM> {
                 GCTriggerSelector::FixedHeapSize(size) => Box::new(FixedHeapSizeTrigger {
                     total_pages: conversions::bytes_to_pages_up(size),
                 }),
-                GCTriggerSelector::DynamicHeapSize(min, max) => Box::new(MemBalancerTrigger::new(
-                    conversions::bytes_to_pages_up(min),
-                    conversions::bytes_to_pages_up(max),
-                )),
+                GCTriggerSelector::DynamicHeapSize(min, max) => 'dynamic_heap_size: {
+                    let min_pages = conversions::bytes_to_pages_up(min);
+                    let max_pages = conversions::bytes_to_pages_up(max);
+
+                    if *options.plan == crate::util::options::PlanSelector::NoGC {
+                        warn!("Cannot use dynamic heap size with NoGC.  Using fixed heap size trigger instead.");
+                        break 'dynamic_heap_size Box::new(FixedHeapSizeTrigger {
+                            total_pages: max_pages,
+                        });
+                    }
+
+                    Box::new(MemBalancerTrigger::new(min_pages, max_pages))
+                }
                 GCTriggerSelector::Delegated => {
                     <VM::VMCollection as crate::vm::Collection<VM>>::create_gc_trigger()
                 }