RFR: 8353500: [s390x] Intrinsify Unsafe::setMemory [v4]

[Andrew Haley]

On Wed, 23 Apr 2025 06:09:25 GMT, Amit Kumar <amitkumar at openjdk.org> wrote:

>> Unsafe::setMemory intrinsic implementation for s390x. 
>> 
>> Stub Code: 
>> 
>> 
>> StubRoutines::unsafe_setmemory [0x000003ffb04b63c0, 0x000003ffb04b64d0] (272 bytes)
>> --------------------------------------------------------------------------------
>>   0x000003ffb04b63c0:   ogrk	%r1,%r2,%r3
>>   0x000003ffb04b63c4:   nill	%r1,7
>>   0x000003ffb04b63c8:   je	0x000003ffb04b6410
>>   0x000003ffb04b63cc:   nill	%r1,3
>>   0x000003ffb04b63d0:   je	0x000003ffb04b6460
>>   0x000003ffb04b63d4:   nill	%r1,1
>>   0x000003ffb04b63d8:   jlh	0x000003ffb04b64a0
>>   0x000003ffb04b63dc:   risbg	%r4,%r4,48,55,8
>>   0x000003ffb04b63e2:   risbgz	%r1,%r3,32,63,62
>>   0x000003ffb04b63e8:   je	0x000003ffb04b6402
>>   0x000003ffb04b63ec:   nopr
>>   0x000003ffb04b63ee:   nopr
>>   0x000003ffb04b63f0:   sth	%r4,0(%r2)
>>   0x000003ffb04b63f4:   sth	%r4,2(%r2)
>>   0x000003ffb04b63f8:   agfi	%r2,4
>>   0x000003ffb04b63fe:   brct	%r1,0x000003ffb04b63f0
>>   0x000003ffb04b6402:   nilf	%r3,2
>>   0x000003ffb04b6408:   ber	%r14
>>   0x000003ffb04b640a:   sth	%r4,0(%r2)
>>   0x000003ffb04b640e:   br	%r14
>>   0x000003ffb04b6410:   risbg	%r4,%r4,48,55,8
>>   0x000003ffb04b6416:   risbg	%r4,%r4,32,47,16
>>   0x000003ffb04b641c:   risbg	%r4,%r4,0,31,32
>>   0x000003ffb04b6422:   risbgz	%r1,%r3,32,63,60
>>   0x000003ffb04b6428:   je	0x000003ffb04b6446
>>   0x000003ffb04b642c:   nopr
>>   0x000003ffb04b642e:   nopr
>>   0x000003ffb04b6430:   stg	%r4,0(%r2)
>>   0x000003ffb04b6436:   stg	%r4,8(%r2)
>>   0x000003ffb04b643c:   agfi	%r2,16
>>   0x000003ffb04b6442:   brct	%r1,0x000003ffb04b6430
>>   0x000003ffb04b6446:   nilf	%r3,8
>>   0x000003ffb04b644c:   ber	%r14
>>   0x000003ffb04b644e:   stg	%r4,0(%r2)
>>   0x000003ffb04b6454:   br	%r14
>>   0x000003ffb04b6456:   nopr
>>   0x000003ffb04b6458:   nopr
>>   0x000003ffb04b645a:   nopr
>>   0x000003ffb04b645c:   nopr
>>   0x000003ffb04b645e:   nopr
>>   0x000003ffb04b6460:   risbg	%r4,%r4,48,55,8
>>   0x000003ffb04b6466:   risbg	%r4,%r4,32,47,16
>>   0x000003ffb04b646c:   risbgz	%r1,%r3,32,63,61
>>   0x000003ffb04b6472:   je	0x000003ffb04b6492
>>   0x000003ffb04b6476:   nopr
>>   0x000003ffb04b6478:   nopr
>>   0x000003ffb04b647a:   nopr
>>   0x000003ffb04b647c:   nopr
>>   0x000003ffb04b647e:   nopr
>>   0x000003ffb04b6480:   st	%r4,0(%r2)
>>   0x000003ffb04b6484:   st	%r4,4(%r2)
>>   0x000003ffb04b6488:   agfi	%r2,8
>>   0x000003ffb04b648e:   brct	%r1,0x000003ffb04b6480
>>   0x000003ffb04b6492:   nilf	%r3,4
>>   0x000003ffb04b6498:   ber	%r14
>>   0x000003ffb04b649a:   st	%r4,0(%r2)
>>   0x0000...
>
> Amit Kumar has updated the pull request incrementally with one additional commit since the last revision:
> 
>   improved mvc implementation

There's a lot of confusion about this. There is no requirement that all bytes before the non-writable address have been written when hitting a signal. Behaving nicely when writing beyond allocated memory is "best effort" only: we're trying to be nice, that's all.

The atomicity requirement is here , in the specification of `Unsafe::SetMemory`:


     * <p>The stores are in coherent (atomic) units of a size determined
     * by the address and length parameters.  If the effective address and
     * length are all even modulo 8, the stores take place in 'long' units.
     * If the effective address and length are (resp.) even modulo 4 or 2,
     * the stores take place in units of 'int' or 'short'.

-------------

PR Comment: https://git.openjdk.org/jdk/pull/24480#issuecomment-2900591094