[10] RFR(L): 8185979: PPC64: Implement SHA2 intrinsic

[Doerr, Martin]

Hi Gustavo R,

I guess you're right. vrsave is already set to -1, so all Vector Registers get saved.
It'd be good to know where it is set (OS, Flag in ELF header, ???) and if this is guaranteed.
I don't want to risk getting sporadic errors on some OS versions.

I'd like to enable SHA intrinsics on linux BE as well. I already managed to get the 256 bit version working (was quite some work!).

Thanks and best regards,
Martin


-----Original Message-----
From: Gustavo Romero [mailto:gromero at linux.vnet.ibm.com] 
Sent: Freitag, 25. August 2017 22:35
To: Doerr, Martin <martin.doerr at sap.com>
Cc: Gustavo Serra Scalet <gustavo.scalet at eldorado.org.br>; 'hotspot-compiler-dev at openjdk.java.net' <hotspot-compiler-dev at openjdk.java.net>; ppc-aix-port-dev at openjdk.java.net
Subject: Re: [10] RFR(L): 8185979: PPC64: Implement SHA2 intrinsic

Hi Martin,

On 25-08-2017 13:18, Doerr, Martin wrote:
> I think you didn't get my point about AIX.
> Your current version doesn't break AIX, but it lacks SHA2 acceleration for AIX on Power 8 and newer, which is still relevant.
> So I'd like to ask you kindly to take a look if Big Endian support for the stub could be added without high effort. AIX doesn't need VRSAVE handling (like Little Endian linux, unlike Big Endian linux), so a few lines in the stub could possibly be enough. I can assist with testing.

I don't think that VRSAVE is handled on Linux, even on BE. Although BE ABI [1]
says:

"Functions must ensure that the appropriate bits in the vrsave register are set for any vector registers they use"

and LE ABI does not say that, even on Linux BE VRSAVE is not in effect
used to determine which vector registers (VMX/Altivec) should be saved/restored.
No application uses it on Linux, so I would say that VRSAVE is ignored on Linux
completely both on BE and LE. save/restore library interfaces don't pay
attention to it in glibc: VRSAVE is just saved/restored completely in mechanisms
of swap/get/setcontext(), set/longjump(), and dl-trampoline() and that's all. I
checked that with toolchain folks and they agree. We've already discussed that a
long time ago but at that time I was just using the vector-scalar registers [2]
and at that time I agreed that if VMX/Altivec was in use instead of the VSX so
VRSAVE should be handled accordingly. But I have a different opinion now...

I'm wondering if something would really break on Linux BE if we forget about
VRSAVE at all in the JVM. If not, we could forget about VRSAVE forever on Linux.
Looks like VRSAVE was sort of born to the oblivion... ?


Kind regards,
Gustavo

[1] http://refspecs.linuxfoundation.org/ELF/ppc64/PPC-elf64abi-1.9.html
[2] http://mail.openjdk.java.net/pipermail/ppc-aix-port-dev/2016-May/002508.html