RFR(S/M): 7132678: G1: verify that the marking bitmaps have no marks for objects over TAMS

[Bengt Rutisson]

Hi all,

Picking up an over 1 year old task...

Tony created this patch at some point and passed it on to John 
Cuthbertson. John sent out a review request and there were some comments 
(http://mail.openjdk.java.net/pipermail/hotspot-gc-dev/2013-May/thread.html#7202 
and 
http://mail.openjdk.java.net/pipermail/hotspot-gc-dev/2013-May/007279.html), 
but the patch never got finalized or pushed. I thought I'd try to get 
this pushed now.

Just like John suggested I would like to do the push as contributed-by 
Tony unless there are major changes due to comments.

Here is a new webrev:
http://cr.openjdk.java.net/~brutisso/7132678/webrev.1/

Here is the diff compared to the one John sent out 15 months ago:
http://cr.openjdk.java.net/~brutisso/7132678/webrev.0-1.diff/

In the original patch there is a bug in the full GC case. We verify the 
"prev" bitmap after doing a full GC. But the full GC has compacted 
objects and reset tams without updating the prev bitmap. I think this is 
normally safe since the prev bitmap will be cleared once it becomes the 
next bitmap later on. However, the new verification code looks at stale 
data in the prev bitmap. There was already code added to clear the prev 
bitmap if verification was enabled. I moved that code to just before the 
verification. It might seem unnecessary to verify a bitmap that we just 
cleared, but that seemed easier than to propagate the state down to the 
verification code that it should not verify the prev bitmap for this 
particular call.

There is one more change that I made. I removed one guarantee that I 
think was unnecessary.

Thanks,
Bengt

On 2013-01-15 21:10, John Cuthbertson wrote:
> Hi Everyone,
>
> Can I have a couple of volunteers review the changes for this CR? The 
> webrev can be found at: 
> http://cr.openjdk.java.net/~johnc/7132678/webrev.0/
>
> Most of the changes come from a patch that Tony gave me before he left 
> and I had to tweak them slightly to remove a spurious failure. The 
> changes verify that the heap regions don't have any marks between 
> [TAMS, top) at strategic places: start and end of each GC, start and 
> end of remark and cleanup, and when allocating a region. Tony deserves 
> the bulk of the credit so, if possible and there are no objections, I 
> intend to list him as author of the change and include myself as a 
> reviewer.
>
> Testing:
> GC test suite with the both the new flags (separately and together) 
> and a low IHOP value.
> jprt with the new flags (+IgnoreUnrecognizedVMOptions so that product 
> test runs did not fail).
>
> Thanks,
>
> JohnC