RFR(S): 8212481: PPC64: Enable POWER9 CPU detection

[Gustavo Romero]

Hi Martin,

On 10/24/2018 05:20 AM, Doerr, Martin wrote:
> looks good. Thank you for cleaning things up and also for sharing the story about mftgpr. Interesting.
> Reviewed.

Thanks for reviewing!


Best regards,
Gustavo

> Best regards,
> Martin
> 
> 
> -----Original Message-----
> From: Gustavo Romero <gromero at linux.vnet.ibm.com>
> Sent: Dienstag, 23. Oktober 2018 23:46
> To: Doerr, Martin <martin.doerr at sap.com>; hotspot-dev at openjdk.java.net; ppc-aix-port-dev at openjdk.java.net
> Subject: Re: RFR(S): 8212481: PPC64: Enable POWER9 CPU detection
> 
> Hi Martin,
> 
> Thanks for your comments on fsqrt{,s} and mftgpr opcodes.
> 
> On 10/23/2018 01:51 PM, Doerr, Martin wrote:
>> Hi Gustavo,
>>
>>> I agree. So, if you don't mind, to reduce any future rework on that
>>> I removed the hardcoded L=1 and now it's just a default. I introduced
>>> l14() function that matches two bits in the proper field as I don't
>>> see any collision with any other one bit field at the same position
>>> (bit 14).
>> I like it, but the default value needs to get specified in the .hpp file.
> 
> Fixed.
> 
> 
>>> I just have one question regarding the feature-string. I see
>>> instrumentation for "fsqrts" feature but it's not currently
>>> advertised by the feature-string. On the other hand, ISA info
>>> looks to indicate that fsqrt and fsqrts are not aliases, because:
>>>
>>> fsqrt  P2  -> Instruction introduced in the POWER2 Architecture.
>>> fsqrts PPC -> Instruction introduced in the PowerPC Architecture prior to ISA v2.00
>>>
>>> so I'm wondering if just for completeness the "fsqrts" feature
>>> should be included in the feature-string. Besides that I don't
>>> see instrumentation to support has_mftgpr(), which is commented
>>> out, thus should we remove it? Like the following:
>>
>> This code is very old. If we are sure nobody can run the VM on a machine without these instructions they can get removed from the feature checking.
> 
> I'm not sure about it. Maybe somebody is using out there, in the community
> for example, so kept it.
> 
> 
>> The mftgpr was only designed for an extension of Power6 and the opcode was reused for something else later on if I remember correctly. I think you can remove the remaining comment.
> 
> I was not aware of that, thanks for the info.
> Just out of curiosity I asked the toolchain folks and they said that mftgpr
> is present in some POWER6 hardware but on the so-called raw mode (ie power6x,
> which I think it's the extension you mentioned), but not in the architected /
> normal mode (power6). So even if it's present in the hardware it's disabled
> unless you boot the system up in raw mode.
> 
> On that raw mode (power6x) one would see the following in AUXV, for instance:
> 
> $ LD_SHOW_AUXV=1 /bin/true | grep PLATFORM
> AT_PLATFORM:     power6x
> AT_BASE_PLATFORM:power6x
> 
> But probably theses machines w/ power6x never got out of IBM indeed.
> 
> Anyway, they said that power6x is unsupported. So, as you said, I removed the
> comment about mftgpr.
> 
> v3 webrev:
> 
> http://cr.openjdk.java.net/~gromero/8212481/v3/
> 
> 
> Thanks!
> 
> Best regards,
> Gustavo
>