RFR (XS): 8021823 G1: Concurrent marking crashes with -XX:ObjectAlignmentInBytes>=32 in 64bit VMs (patch for hs24+hs25)

[Vladimir Kozlov]

Thomas,

There is LogMinObjAlignment. Why you did not use it?

Thanks,
Vladimir

On 7/31/13 5:48 AM, Thomas Schatzl wrote:
> Hi all,
>
>    can I have reviews for the following fix? The change addresses wrong
> initialization of the mark bitmaps in G1 when ObjectAlignmentInBytes is
> larger than or equal to 32.
>
> The concurrent marking uses a shift value for mapping heap address to
> mark bitmap bits. This shift value has been calculated wrongly depending
> on ObjectAlignment (=ObjectAlignmentInBytes in HeapWords), so that only
> worked for ObjectAlignmentInBytes values <= 16 it has been correct.
>
> Instead of the correct value of "log2(ObjectAlignment)" the code used
> "ObjectAlignment-1".
>
> I.e. some table will show the problem (64 bit VM with sizeof(HeapWord)
> == 8):
>
> ObjectAlignment (OAInBytes)  Old Shift value  New Shift value
>         8 (64)                      7                3
>         4 (32)                      3                2
>         2 (16)                      1                1
>         1 (8)                       0                0
>
> I.e. when mapping HeapWords to bits, the code previously used a much too
> high value for ObjectAlignmentInBytes, resulting in truncating important
> bits from the heap address, resulting in wrong locations to look for and
> set bits.
>
> Coincidently the old calculation resulted in the same values for
> ObjectAlignmentInBytes<=16, so this issue went unnoticed.
>
> There is a new test that simply looks for these crashes for all
> supported object alignments by running a few concurrent markings.
>
> JBS:
> https://jbs.oracle.com/bugs/browse/JDK-8021823
>
> Bugs.sun:
> http://bugs.sun.com/view_bug.do?bug_id=8021823
>
> Webrev:
> hs24: http://cr.openjdk.java.net/~tschatzl/8021823/webrev.hs24/
> hs25: http://cr.openjdk.java.net/~tschatzl/8021823/webrev.hs25/
>
> Testing:
> manual testing of all failing tests with ObjectAlignmentInBytes from 8
> to 256, test case failing without the patch, succeeding with the patch,
> jprt.
>
> Testing:
>