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

[Lindenmaier, Goetz]

Hi Vladimir, 

I added S390_ONLY macros around the field declaration and the two 
methods so that set_ctable_begin should be inlined as empty method
and optimized away on non-s390 platforms. 
http://cr.openjdk.java.net/~goetz/wr16/8167184-s390_relocations/webrev.02/

Best regards,
  Goetz.


> -----Original Message-----
> From: Vladimir Kozlov [mailto:vladimir.kozlov at oracle.com]
> Sent: Donnerstag, 6. Oktober 2016 18:34
> 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.
> 
> 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.
> >>>