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
