RFR (XS): JDK-8076614: Add comment to ClearNoncleanCardWrapper::do_MemRegion()

[Kim Barrett]

On Apr 2, 2015, at 4:53 PM, Bengt Rutisson <bengt.rutisson at oracle.com> wrote:
> 
> 
> Hi all,
> 
> Can I have a couple of reviews to add this comment to cardTableRS.cpp?
> 
> https://bugs.openjdk.java.net/browse/JDK-8076614
> 
> This was discussed during the review of "JDK-8076241: Remove unused methods mod_card_iterate()    and non_clean_card_iterate_serial()", see:
> 
> http://mail.openjdk.java.net/pipermail/hotspot-gc-dev/2015-April/012661.html
> 
> For JDK-8076241 I removed this comment because it didn't make sense anymore at the place where it used to be. We now agree that a better place for
> it is close to the ClearNoncleanCardWrapper::do_MemRegion() method because that still does backwards iteration of the card table.
> 
> Since I am only adding a comment I think a diff is easier to review than a webrev:
> 
> diff --git a/src/share/vm/memory/cardTableRS.cpp b/src/share/vm/memory/cardTableRS.cpp
> --- a/src/share/vm/memory/cardTableRS.cpp
> +++ b/src/share/vm/memory/cardTableRS.cpp
> @@ -176,7 +176,10 @@
> bool ClearNoncleanCardWrapper::is_word_aligned(jbyte* entry) {
> return (((intptr_t)entry) & (BytesPerWord-1)) == 0;
> }
> -
> +// The regions are visited in *decreasing* address order.
> +// This order aids with imprecise card marking, where a dirty
> +// card may cause scanning, and summarization marking, of objects
> +// that extend onto subsequent cards.
> void ClearNoncleanCardWrapper::do_MemRegion(MemRegion mr) {
> assert(mr.word_size() > 0, "Error");
> assert(_ct->is_aligned(mr.start()), "mr.start() should be card aligned”);

Looks good.