8231779: crash HeapWord*ParallelScavengeHeap::failed_mem_allocate

Reviewed-by: dlong, tschatzl, pliden

src/hotspot/share/gc/shared/threadLocalAllocBuffer.cpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 1999, 2019, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 1999, 2020, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -69,7 +69,8 @@ void ThreadLocalAllocBuffer::accumulate_and_reset_statistics(ThreadLocalAllocSta
       // The result can be larger than 1.0 due to direct to old allocations.
       // These allocations should ideally not be counted but since it is not possible
       // to filter them out here we just cap the fraction to be at most 1.0.
-      double alloc_frac = MIN2(1.0, (double) allocated_since_last_gc / used);
+      // Keep alloc_frac as float and not double to avoid the double to float conversion
+      float alloc_frac = MIN2(1.0f, allocated_since_last_gc / (float) used);
       _allocation_fraction.sample(alloc_frac);
     }
 
@@ -187,7 +188,8 @@ void ThreadLocalAllocBuffer::initialize() {
   set_desired_size(initial_desired_size());
 
   size_t capacity = Universe::heap()->tlab_capacity(thread()) / HeapWordSize;
-  double alloc_frac = desired_size() * target_refills() / (double) capacity;
+  // Keep alloc_frac as float and not double to avoid the double to float conversion
+  float alloc_frac = desired_size() * target_refills() / (float) capacity;
   _allocation_fraction.sample(alloc_frac);
 
   set_refill_waste_limit(initial_refill_waste_limit());