On Wed, 3 Nov 2021 at 15:01, Jonathan Wakely wrote:

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

With the updated patch attached again this time ...



> 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.
>>
>>
>>