RFR: 8367325: [s390x] build failure due to JDK-8361376

[Martin Doerr]

On Thu, 11 Sep 2025 20:17:24 GMT, Dean Long <dlong at openjdk.org> wrote:

>> Fixes the SIGLL caused after [JDK-8361376](https://bugs.openjdk.org/browse/JDK-8361376). 
>> 
>> Issue was with `cs` instruction, which requires the address to be aligned. And after this change address at which operating was not aligned.
>> 
>> Wisdom from Principle of Z Operations: 
>> 
>> Sytax: CS R<sub>1</sub>,R<sub>3</sub>,D<sub>2</sub>(B<sub>2</sub>)
>> 
>> Sytax: CSY R<sub>1</sub>,R<sub>3</sub>,D<sub>2</sub>(B<sub>2</sub>)
>> 
>> `
>> The second operand of COMPARE AND SWAP (CS, CSY) must be designated on a word boundary.
>> `
>> 
>> 
>> 
>> => 0x3fffc3149ba <_ZN17BarrierSetNMethod15set_guard_valueEP7nmethodii+266>: cs %r1,%r4,0(%r3)
>> (gdb) i r r3
>> r3             0x3ffe500017a       4397593526650
>> (gdb) p ($r3 % 8) 
>> $5 = 2
>> (gdb) si 
>> 
>> Thread 16 "C1 CompilerThre" received signal SIGILL, Illegal instruction.
>> NativeMethodBarrier::set_guard_value (bit_mask=2147483647, value=1, this=0x3ffe5000166)
>>     at /home/amit/jdk/src/hotspot/cpu/s390/gc/shared/barrierSetNMethod_s390.cpp:74
>> 74         if (v == old_value) break;
>
> src/hotspot/cpu/s390/gc/shared/barrierSetAssembler_s390.cpp line 174:
> 
>> 172: void BarrierSetAssembler::nmethod_entry_barrier(MacroAssembler* masm) {
>> 173:   BarrierSetNMethod* bs_nm = BarrierSet::barrier_set()->barrier_set_nmethod();
>> 174:   __ align(8); // must align the following block which requires atomic updates
> 
> Thanks for fixing this.  Rather than aligning at the start, wouldn't it be more maintainable to do
> `__ align(4, offset() + 2);`
> before the z_cfi?

That would require much more changes. The barrier sequence is expected to have a fixed size.

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PR Review Comment: https://git.openjdk.org/jdk/pull/27213#discussion_r2342448153