# HG changeset patch
# User tschatzl
# Date 1472541469 -7200
# Node ID f09b053be364e073c463859913b30d433f6638fe
# Parent  8d35e19f55484f6ccc2ca19114e941041474323c
8155917: Memory access in free regions during G1 full gc causes regressions in SPECjvm2008 scimark.fft,lu,sor,sparse with 9+116 on Linux-x64
Summary: Do not unnecessarily touch the memory of free regions during the compaction phase in G1 full gc causing some OSes to allocate physical memory for them, decreasing performance in some situations.
Reviewed-by: mgerdin, jmasa

diff -r 8d35e19f5548 -r f09b053be364 hotspot/src/share/vm/gc/shared/space.inline.hpp
--- a/hotspot/src/share/vm/gc/shared/space.inline.hpp	Mon Aug 29 20:13:45 2016 -0400
+++ b/hotspot/src/share/vm/gc/shared/space.inline.hpp	Tue Aug 30 09:17:49 2016 +0200
@@ -293,10 +293,11 @@
 
   verify_up_to_first_dead(space);
 
+  HeapWord* const bottom = space->bottom();
   HeapWord* const end_of_live = space->_end_of_live;
 
   assert(space->_first_dead <= end_of_live, "Invariant. _first_dead: " PTR_FORMAT " <= end_of_live: " PTR_FORMAT, p2i(space->_first_dead), p2i(end_of_live));
-  if (space->_first_dead == end_of_live && !oop(space->bottom())->is_gc_marked()) {
+  if (space->_first_dead == end_of_live && (bottom == end_of_live || !oop(bottom)->is_gc_marked())) {
     // Nothing to compact. The space is either empty or all live object should be left in place.
     clear_empty_region(space);
     return;
@@ -305,8 +306,8 @@
   const intx scan_interval = PrefetchScanIntervalInBytes;
   const intx copy_interval = PrefetchCopyIntervalInBytes;
 
-  assert(space->bottom() < end_of_live, "bottom: " PTR_FORMAT " should be < end_of_live: " PTR_FORMAT, p2i(space->bottom()), p2i(end_of_live));
-  HeapWord* cur_obj = space->bottom();
+  assert(bottom < end_of_live, "bottom: " PTR_FORMAT " should be < end_of_live: " PTR_FORMAT, p2i(bottom), p2i(end_of_live));
+  HeapWord* cur_obj = bottom;
   if (space->_first_dead > cur_obj && !oop(cur_obj)->is_gc_marked()) {
     // All object before _first_dead can be skipped. They should not be moved.
     // A pointer to the first live object is stored at the memory location for _first_dead.