RFR(s): 8013171: G1: C1 x86_64 barriers use 32-bit accesses to 64-bit PtrQueue::_index

Per Liden per.liden at oracle.com
Mon May 4 08:35:56 UTC 2015

On 2015-04-29 14:00, Per Liden wrote:
> Hi,
> On 2015-04-27 18:20, Christian Thalinger wrote:
>>> On Apr 23, 2015, at 9:40 AM, Per Liden <per.liden at oracle.com> wrote:
>>> Hi Thomas,
>>>> On 23 Apr 2015, at 13:16, Thomas Schatzl <thomas.schatzl at oracle.com>
>>>> wrote:
>>>> Hi,
>>>> On Thu, 2015-04-23 at 10:52 +0200, Per Liden wrote:
>>>>> Hi,
>>>>> (This change affects G1, but it's touching code in C1 so I'd like
>>>>> to ask
>>>>> someone from the compiler team to also reviewed this)
>>>>> Summary: The G1 barriers loads and updates the PrtQueue::_index field.
>>>>> This field is a size_t but the C1 version of these barriers aren't
>>>>> 64-bit clean. The bug has more details.
>>>>> In addition I've massaged the code a little bit, so that the 32-bit
>>>>> and
>>>>> 64-bit sections look more similar (and as a bonus I think we avoid an
>>>>> extra memory load on 32-bit).
>>>>> Webrev: http://cr.openjdk.java.net/~pliden/8013171/webrev.0/
>>>>> Bug: https://bugs.openjdk.java.net/browse/JDK-8013171
>>>>> Testing:
>>>>> * gc-test-suite on both 32 and 64-bit builds (with -XX:+UseG1GC
>>>>> -XX:+TieredCompilation -XX:TieredStopAtLevel=3 -XX:+VerifyAfterGC)
>>>>> * Passes jprt
>>>> Looks good, with the following caveats which should be decided by
>>>> somebody else if they are important as they are micro-opts:
>>>> - instead of using cmp to compare against zero in a register, it would
>>>> be better to use the test instruction (e.g. __ testX(tmp, tmp)) as
>>>> it saves
>>>> a byte of encoding per instruction with the same effect.
>> Tighter code is always better.  For barriers it might be important in
>> tight loops to better fit in the cache.
> I'll make it a testprt().
>>>> - post barrier stub: I would prefer if the 64 bit code did not
>>>> push/pop the rdx register to free tmp. There are explicit rscratch1/2
>>>> registers for temporaries available on that platform. At least
>>>> rscratch1
>>>> (=r8) seems to be used without save/restore in the original code
>>>> already.
>>>> This would also remove the need for 64 bit code to push/pop any
>>>> register it
>>>> seems to me.
>> Sounds like a good suggestion if it doesn’t complicate the code too much.
> I'd like to avoid reintroducing different code paths for 32 and 64-bit,
> which I think complicates the code. However, I can defer the pushing of
> tmp until it's actually needed, which essentially gets us to the same
> situation as before this change in terms of register usage for 64-bit.
> Updated webrev: http://cr.openjdk.java.net/~pliden/8013171/webrev.2/

Thomas/Roland, are you ok with the latest version?


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