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

[Jonathan Wakely <jwakely@redhat.com>]

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.


>+    __ppc_darn(void*)
>+    {
>+      return __builtin_darn_32();
>+    }
>+#endif