Re: [PATCH v2] libstdc++: Add support for POWER9 DARN instruction to std::random_device

[Jonathan Wakely <jwakely@redhat.com>]

Any feedback from POWER maintainers about this? I'd like to push it soon if
there's nothing wrong with it.

On Wed, 20 Oct 2021 at 14:00, Jonathan Wakely via Libstdc++ <
libstdc++@gcc.gnu.org> wrote:

> On 20/10/21 10:12 +0100, Jonathan Wakely wrote:
> >On 19/10/21 17:47 +0100, Jonathan Wakely wrote:
> >>The ISA-3.0 instruction set includes DARN ("deliver a random number")
> >>which can be used similar to the existing support for RDRAND and RDSEED.
> >>
> >>libstdc++-v3/ChangeLog:
> >>
> >>      * src/c++11/random.cc (USE_DARN): Define.
> >>      (__ppc_darn): New function to use POWER9 DARN instruction.
> >>      (Which): Add 'darn' enumerator.
> >>      (which_source): Check for __ppc_darn.
> >>      (random_device::_M_init): Support "darn" and "hw" tokens.
> >>      (random_device::_M_getentropy): Add darn to switch.
> >>      * testsuite/26_numerics/random/random_device/cons/token.cc:
> >>      Check "darn" token.
> >>      * testsuite/26_numerics/random/random_device/entropy.cc:
> >>      Likewise.
> >>
> >>Tested powerpc64le-linux (power8 and power9) and x86_64-linux.
> >>
> >>The new "darn" (power-specific) and "hw" (x86 and power)
> >>strings should be documented, but I'll do that if this gets committed.
> >>
> >>Most of this patch is just "more of the same", similar to the existing
> >>code for RDRAND and RDSEED on x86, but the parts of the patch I'd like
> >>more eyes on are:
> >>
> >>
> >>+#elif defined __powerpc__ && defined __BUILTIN_CPU_SUPPORTS__
> >>+# define USE_DARN 1
> >>#endif
> >
> >This means DARN can only be used when __builtin_cpu_supports is
> >available, which means glibc 2.23 ... is that acceptable? It means
> >RHEL 7 wouldn't be able to use DARN, but RHEL 8 would.
> >
> >There certainly are POWER9 machines running RHEL 7 and similar
> >vintages (the GCC compile farm has one) so if there's another way to
> >check for ISA 3.0 then I could use that.
> >
> >If __POWER9_VECTOR__ is defined when building libstdc++, presumably
> >that means the whole library can only be run on POWER9 hardware. So
> >would that mean we don't need to check __builtin_cpu_supports("darn")
> >when __POWER9_VECTOR__ is defined? Or is it possible to build with
> >-mcpu=power8 -mpower9-vector and run it on h/w without the DARN
> >instruction?
> >
> >Also, I forgot to add a configure check that the assembler supports
> >darn, which is another prerequisite for using it here.
> >
> >>@@ -135,6 +137,15 @@ namespace std _GLIBCXX_VISIBILITY(default)
> >>#endif
> >>#endif
> >>
> >>+#ifdef USE_DARN
> >>+    unsigned int
> >>+    __attribute__((target("power9")))
> >
> >Oops, that should be "cpu=power9".
> >
> >With that change it works on a POWER9 machine (9009-42A) with glibc
> >2.34 and binutils 2.35.
> >
>
> Here's the updated patch with a configure check for assembler support,
> and the target attribute fixed.
>
> This still requires Glibc 2.23 for __builtin_cpu_supports, which I'm
> assuming is acceptable.
>
>
>