RFR: 8241825: Make compressed oops and compressed class pointers independent on x86_64 (closed)

[coleen.phillimore at oracle.com]

Sorry, this went to the wrong list.

Coleen

On 4/24/20 11:17 AM, coleen.phillimore at oracle.com wrote:
>
> Hi Erik, well the interpreter code isn't as bad as I thought it would 
> be.  We tied UseCompressedKlassPointers to UseCompressedOops because 
> we couldn't imagine why someone would care about 4 bytes per object 
> with all 8 byte pointers to oops.  But maybe you do care.
>
> http://cr.openjdk.java.net/~eosterlund/8241825/webrev.00/src/hotspot/cpu/x86/interp_masm_x86.cpp.udiff.html
>
>        if (UseBiasedLocking) {
> - biased_locking_enter(lock_reg, obj_reg, swap_reg, tmp_reg, false, 
> done, &slow_case);
> + Register rklass_decode_tmp = LP64_ONLY(rscratch1) NOT_LP64(noreg);
> + biased_locking_enter(lock_reg, obj_reg, swap_reg, tmp_reg, 
> rscratch1, false, done, &slow_case);
>        }
> You didn't use rklass_decode_tmp here.
>
> http://cr.openjdk.java.net/~eosterlund/8241825/webrev.00/src/hotspot/cpu/x86/templateTable_x86.cpp.udiff.html
>
>
> + Register tmp_load_klass = LP64_ONLY(rscratch1) NOT_LP64(noreg);
>
> I wonder if you couldn't just pass rscratch1 into load_klass and 
> handle ignoring it inside the load_klass function, ie. fix the 
> asserts.  This gets repetitive in the templateTable and other code.  
> Maybe people may have the opposite opinion though.
>
> This is quite nice that UseCompressedKlassPointers doesn't have to 
> reinit_heapbase.  The interpreter code looks good.
>
> http://cr.openjdk.java.net/~eosterlund/8241825/webrev.00/src/hotspot/share/memory/metaspace.cpp.frames.html
>
> I don't know if base = HeapBaseMinAddress is right for the compressed 
> class space.  It's reserved but not committed as 1G. Maybe it makes 
> sense since the heap without UseCompressedOops should be above the 
> class space.
>
> Coleen
>
> On 4/24/20 4:25 AM, Erik Österlund wrote:
>> Hi,
>>
>> This is the closed version of the mostly open change. There are some 
>> @Contended tests in closed
>> that needed some love with my change to make compressed class 
>> pointers usable with compressed
>> oops disabled. In particular, these tests perform calculations to 
>> ensure that things are padded
>> as expected. Part of those calculations makes assumptions about how 
>> large the object header is,
>> presuming that the number is a constant that is the same for all 
>> objects, and is determinable
>> by checking the offset of the first field. However, with my changes, 
>> the first field will end
>> up either in the klass gap, or after, depending on the field size. I 
>> updated the tests to consider
>> different header sizes depending on the type of fields being used. It 
>> still isn't perfect, but
>> it is clever enough for the purposes of the tests; they pass with 
>> these changes.
>>
>> Bug:
>> https://bugs.openjdk.java.net/browse/JDK-8241825
>>
>> Webrev:
>> http://wikifiles.se.oracle.com/dev/eosterlund/8241825/webrev.00/
>>
>> Thanks,
>> /Erik
>