RFR(M): 8167184: [s390] Extend relocations for pc-relative instructions.

[Vladimir Kozlov]

On 10/6/16 8:48 AM, Lindenmaier, Goetz wrote:
> Hi Vladimir,
>
> your're right, adding a field to CodeBlob will blow up nmethods on all
> platforms, so I agree having an #ifdef makes sense.
> But for the relocation, it's just an extra class, should not
> matter too much, but it's much clearer than adding #ifdefs I think.

Okay, I am fine with new relocation class since it is not used on other 
platforms.

Thanks,
Vladimir

>
> I edited the bug to mention 'constant section".
>
> Best regards,
>   Goetz.
>
>> -----Original Message-----
>> From: Vladimir Kozlov [mailto:vladimir.kozlov at oracle.com]
>> Sent: Mittwoch, 5. Oktober 2016 20:27
>> To: Lindenmaier, Goetz <goetz.lindenmaier at sap.com>; hotspot-compiler-
>> dev at openjdk.java.net
>> Subject: Re: RFR(M): 8167184: [s390] Extend relocations for pc-relative
>> instructions.
>>
>> When you say "constant pool" do you mean "conatant section" in code
>> buffer? Or CP in class data? I assume first. It is confusing.
>>
>> Can you implement this as S390 code (#ifdef)? I am not comfortable that
>> code is executed on other platforms too (ctable setting).
>>
>> Can you add _offset under #ifdef to runtime_call_Relocation instead? I
>> see that s390 code use it this way:
>>
>> +    if (type() == relocInfo::runtime_call_type) {
>> +      toc_offset = ((runtime_call_Relocation
>> *)this)->get_constant_pool_offset();
>>
>> Thanks,
>> Vladimir
>>
>> On 10/5/16 8:16 AM, Lindenmaier, Goetz wrote:
>>> Hi,
>>>
>>>
>>>
>>> This extension to relocations is needed for the s390 port.
>>>
>>> Please review:
>>>
>>> http://cr.openjdk.java.net/~goetz/wr16/8167184-
>> s390_relocations/webrev.01/
>>>
>>>
>>> This change implements two extensions to relocation to support
>>> pc-relative instructions.
>>>
>>> On s390, there are call instructions that branch to an address relative
>>> to the current pc. If the offset encodable in the instruction does not
>>> suffice,  our port uses a pc-relative load to load the target address
>>> from the constant pool into a register that is then used as branch
>>> target. Relocations must support switching between these two
>>> implementations. For this we need a relocation that stores the offset of
>>> the address in the constant pool, as this value can not be read from the
>>> instruction.
>>>
>>> Further the pc-relative instructions encode the offset from the
>>> instruction to the constant. If the constant pool is resized, or the
>>> instruction is moved (as during shorten branches), this offset must be
>>> adapted. To compute the new offset, the change in offset must be known.
>>> To compute this we store the position of the constant pool in the
>> CodeBlob.
>>>
>>>
>>>
>>> Best regards,
>>>
>>>   Goetz.
>>>