src/share/vm/gc_implementation/g1/g1BlockOffsetTable.hpp

rev 7147 : 8059758: Startup benchmark performance and footprint regressions with JDK-8038423
Summary: Changes in JDK-8038423 always initialize (zeroes out) virtual memory used for auxiliary data structures. This causes a footprint and performance regression for G1 in startup benchmarks. This is because they do not touch that memory at all, so the operating system does not actually commit these pages. The fix is to, if the initialization value of the data structures matches the default value of just committed memory (=0), do not do anything.
Reviewed-by: tbd

*** 107,117 ****
    inline HeapWord* block_start_const(const void* addr) const;
  };
  
  class G1BlockOffsetSharedArrayMappingChangedListener : public G1MappingChangedListener {
   public:
!   virtual void on_commit(uint start_idx, size_t num_regions) {
      // Nothing to do. The BOT is hard-wired to be part of the HeapRegion, and we cannot
      // retrieve it here since this would cause firing of several asserts. The code
      // executed after commit of a region already needs to do some re-initialization of
      // the HeapRegion, so we combine that.
    }
--- 107,117 ----
    inline HeapWord* block_start_const(const void* addr) const;
  };
  
  class G1BlockOffsetSharedArrayMappingChangedListener : public G1MappingChangedListener {
   public:
!   virtual void on_commit(uint start_idx, size_t num_regions, bool zero_filled) {
      // Nothing to do. The BOT is hard-wired to be part of the HeapRegion, and we cannot
      // retrieve it here since this would cause firing of several asserts. The code
      // executed after commit of a region already needs to do some re-initialization of
      // the HeapRegion, so we combine that.
    }