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.