Request for review JDK-8185591 guarantee(_byte_map[_guard_index] == last_card) failed: card table guard has been modified

[Kim Barrett]

> On Nov 21, 2017, at 5:18 PM, Alexander Harlap <alexander.harlap at oracle.com> wrote:
> 
> Please review change for JDK-8185591 <https://bugs.openjdk.java.net/browse/JDK-8185591> - guarantee(_byte_map[_guard_index] == last_card) failed: card table guard has been modified
> 
> Change is located at http://cr.openjdk.java.net/~aharlap/8185591/webrev.01/
> 
> Problem was in mishandling zero count in code generated by gen_write_ref_array_post_barrier().
> 
> Code is machine specific. Suggested fix for arm, sparc and x86_64.
> 
> No changes are  required for x86_32 - case zero count already is handled properly ( same as for s390 and ppc).
> 
> Aarch64 code have same problem as arm, sparc and x86_64, but I did not include this platform in suggested changeset.
> 
> I attached possiible change for aarch64 stubGenerator_aarch64.cpp.diff
> 
> Testing was done with JPRT.
> 
> Thank you,
> 
> Alex

Thanks for tracking this down.

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src/hotspot/cpu/x86/stubGenerator_x86_64.cpp
1270           __ testl(count, count);
1271           __ jcc(Assembler::zero, L_done); // zero count - nothing to do

Instead of that, might it be better to instead add something like this?

  1277           __ subptr(end, start); // end --> cards count
+ 1278           __ jcc(Assembler::less, L_done); // negative inclusive count - nothing to do

Similar question for all of the affected platforms.

I don't currently have a strong preference either way, but wonder if
there's a good reason to choose one over the other.

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