1.1 --- a/src/share/vm/gc_implementation/g1/g1CollectedHeap.hpp Wed Oct 07 19:01:55 2009 -0400
1.2 +++ b/src/share/vm/gc_implementation/g1/g1CollectedHeap.hpp Fri Oct 16 02:05:46 2009 -0700
1.3 @@ -992,11 +992,39 @@
1.4
1.5 // Can a compiler initialize a new object without store barriers?
1.6 // This permission only extends from the creation of a new object
1.7 - // via a TLAB up to the first subsequent safepoint.
1.8 + // via a TLAB up to the first subsequent safepoint. If such permission
1.9 + // is granted for this heap type, the compiler promises to call
1.10 + // defer_store_barrier() below on any slow path allocation of
1.11 + // a new object for which such initializing store barriers will
1.12 + // have been elided. G1, like CMS, allows this, but should be
1.13 + // ready to provide a compensating write barrier as necessary
1.14 + // if that storage came out of a non-young region. The efficiency
1.15 + // of this implementation depends crucially on being able to
1.16 + // answer very efficiently in constant time whether a piece of
1.17 + // storage in the heap comes from a young region or not.
1.18 + // See ReduceInitialCardMarks.
1.19 virtual bool can_elide_tlab_store_barriers() const {
1.20 - // Since G1's TLAB's may, on occasion, come from non-young regions
1.21 - // as well. (Is there a flag controlling that? XXX)
1.22 - return false;
1.23 + return true;
1.24 + }
1.25 +
1.26 + bool is_in_young(oop obj) {
1.27 + HeapRegion* hr = heap_region_containing(obj);
1.28 + return hr != NULL && hr->is_young();
1.29 + }
1.30 +
1.31 + // We don't need barriers for initializing stores to objects
1.32 + // in the young gen: for the SATB pre-barrier, there is no
1.33 + // pre-value that needs to be remembered; for the remembered-set
1.34 + // update logging post-barrier, we don't maintain remembered set
1.35 + // information for young gen objects. Note that non-generational
1.36 + // G1 does not have any "young" objects, should not elide
1.37 + // the rs logging barrier and so should always answer false below.
1.38 + // However, non-generational G1 (-XX:-G1Gen) appears to have
1.39 + // bit-rotted so was not tested below.
1.40 + virtual bool can_elide_initializing_store_barrier(oop new_obj) {
1.41 + assert(G1Gen || !is_in_young(new_obj),
1.42 + "Non-generational G1 should never return true below");
1.43 + return is_in_young(new_obj);
1.44 }
1.45
1.46 // Can a compiler elide a store barrier when it writes
